Synthesis and Structural Characterization of Natural Benzofuranoids

Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Sakyo-ku, Kyoto 606-8522, Japan. J. Nat. Prod. , 2015, 78 (5), pp 10...
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Synthesis and Structural Characterization of Natural Benzofuranoids Kouji Kuramochi* and Kazunori Tsubaki Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Sakyo-ku, Kyoto 606-8522, Japan S Supporting Information *

ABSTRACT: The synthesis of ustusoranes A, B, and E, pergillin, dihydropergillin, (±)-penicisochroman A, and (−)-brassicadiol, starting from optically active (R)-benzolactone, is described. The synthesis of ustusoranes A and E established the absolute configurations of these natural products. The synthesis of pergillin and (±)-penicisochroman A led to the structural revision of aspergiones E and F. This study clearly indicates that (R)-benzolactone is a potential intermediate in the synthesis of natural benzofuranoids.

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determined by X-ray crystallography. Both of these compounds have been reported to significantly inhibit the growth of wheat coleoptile at concentrations of 10−3 and 10−4 M. Penicisochroman A (5), which is the methyl acetal derivative of pergillin, was isolated from Penicillium sp. PSU-F40 by Trisuwan et al.5 Brassicadiol (7), which is a 2,3-dihydrobenzofuran, was originally isolated from Alternaria brassicae by Ayer et al.,6 although the specific rotation and the absolute configuration of this compound have not yet been reported. The related natural product annullatin B (8) was recently isolated by Asai et al.,7 and the absolute configuration at its C-2 position was determined to be R according to the vibrational circular dichroism (VCD) method using density functional theory (DFT) calculations. This compound was found to display agonistic activity toward the cannabinoid CB1 receptor and inverse agonistic activity toward the cannabinoid CB2 receptor. We previously reported the total synthesis of (+)-pseudodeflectusin (9),8 (+)-ustusorane C (10),2 (−)-ustusorane D (11),2 and (+)-penicisochromans B (12)5 from the optically active (R)-benzolactone 13 (Scheme 1).9−11 Chemoselective reduction of 13 by DIBAL-H in THF gave (+)-9, which was converted to (+)-10 by acetalization with MeOH.9,10 Hydrogenation of 13 gave alcohol 14, which was converted to (−)-11 and (+)-12 in four and six steps, respectively.11 Inspired by these results, it was envisioned that compound 13 could be used as a common intermediate for the synthesis of several natural benzofuranoids. Given that the NMR chemical shifts of benzofuranoids can be affected by differences in the sample conditions, the structures of several benzofuranoids have been misassigned or assigned in an ambiguous manner.11 For example, the original structures for aspergiones A (15) and B (16) and aspergillitine

here have been several reports on the isolation and biological activities of natural benzofuranoids.1 Ustusoranes A (1), B (2), and E (3) were isolated from Aspergillus ustus 094102 by Lu et al. (Figure 1).2 Ustusorane E showed growth inhibition against human leukemia HL-60 cells with an IC50 value of 0.13 μM. However, the absolute configurations of 1 and 3 have not yet been determined. Pergillin (4)3 and dihydropergillin (5)4 were isolated from Aspergillus ustus by Cutler et al., and the structures of these compounds were

Figure 1. Structures of ustusoranes A (1), B (2), and E (3), pergillin (4), dihydropergillin (5), penicisochroman A (6), brassicadiol (7), and annullatin B (8). © XXXX American Chemical Society and American Society of Pharmacognosy

Received: December 24, 2014

A

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Scheme 1. Synthetic Routes for Pseudodeflectusin, Ustusoranes C and D, and Penicisochroman B

(17) were assigned as 2,3-dimethylchromones,12 but later corrected to 3-isopropylidene-3H-furan-2-ones by our group9,10 and Kaufman’s group13 (Figure 2A). Aspergiones A and B and aspergillitine are in fact pseudodeflectusin (9),8 ustusorane C (10),2 and TMC-120B (18),14 respectively. The 13C NMR spectra of the synthetic 2,3-dimethylchromones 15−17 were found to be different from those of the 3-isopropylidene-3Hfuran-2-ones 9, 10, and 18, respectively, especially in terms of the following key features: (i) peaks derived from the carbonyl carbon of the 2,3-dimethylchromones were shifted upfield compared with those of the 3-isopropylidene-3H-furan-2-ones; and (ii) peaks derived from the methyl group at C-3 of the 2,3dimethylchromones were shifted upfield compared with the peaks derived from both of the methyl groups of the 3isopropylidene-3H-furan-2-ones. On the basis of these observations, it is possible that aspergiones E (19) and F (20) could be penicisochroman A (6) and pergillin (4), respectively (Figure 2B). The chemical synthesis of natural benzofuranoids has therefore played an important role in confirming the structures and absolute configuration of these compounds. In this study, we have completed the synthesis of ustusoranes A, B, and E, pergillin, dihydropergillin, (±)-penicisochroman A, and (−)-brassicadiol, starting from (R)-benzolactone 13. The absolute configurations of ustusoranes A and E, structural revisions for aspergiones E and F, and an investigation of the structure of dihydropergillin have also been reported.

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RESULTS AND DISCUSSION The synthesis of ustusorane A (1) was achieved in six steps from 1411 (Scheme 2). Compound 14 was dehydrated under acidic conditions to give 22, which was reduced with lithium triethylborohydride to afford diol 23. Treatment of 23 with an excess of TBSCl and imidazole gave 24 and 25 in 65% and 35% yields, respectively. Mono-TBS ether 25 was converted to bisTBS ether 24 by treatment with TBSOTf and 2,6-lutidine. Oxidation of 24 with a stoichiometric amount of OsO4 in pyridine, followed by the treatment of the resulting intermediate with an aqueous Na2SO3 solution, afforded keto alcohol 26 through air-oxidation under basic conditions.15 Dehydration of alcohol 26 with Burgess reagent16 gave unsaturated ketone 27. Subsequent acid-mediated deprotection of the TBS groups in 27 yielded (R)-1. The 1H and 13C NMR spectra of (R)-1 were identical to those of natural ustusorane A (Table S1 in the Supporting Information). The specific rotation of (R)-1 was determined to be [α]21D −4.3 (c 0.1, MeOH) and

Figure 2. Structures and selected 13C NMR chemical shifts of 2,3dimethylchromones and 3-isopropylidene-3H-furan-2-ones. (A) Structures and selected 13C NMR chemical shifts for aspergiones A (15) and B (16) and aspergillitine (17) (left: the structures and chemical shifts reported for 15−17; middle: the chemical shifts of synthetic 15− 17; right: the revised structures and chemical shifts of 15−17). (B) Structures and selected 13C NMR chemical shifts for aspergiones E (19) and F (20) (left: the structures and chemical shifts reported for 19 and 20; right: the structures and chemical shifts reported for penicisochroman A (6) and pergillin (4), which are the candidates for the revised structures of 19 and 20).

was higher than that of natural ustusorane A, which has been reported to be [α]20D −28 (c 0.1, MeOH).2 However, the high optical purity of (R)-1 was confirmed by conversion of (R)-1 to (R)-13 and comparison of the specific rotation of the resultant (R)-13 with that of (R)-13 (with 99% ee), which was used as the starting material in this study.17 The sign of the specific B

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Scheme 2. Synthesis of Ustusorane A (1)

Scheme 3. Synthesis of Pergillin (4) and Dihydropergillin (5)

were determined by DFT calculations at the B3LYP/631G(d,p) level. These calculations indicated that the lowest energy conformation of 5′ was almost equal to that of 5. Indeed, this prediction was consistent with the fact that 5 and 5′ were obtained as a 1:1 ratio.19 (±)-Penicisochroman A (6) and ustusorane B (2) were obtained by the treatment of 29 with TsOH·H2O (1.1 equiv) in MeOH in 65% and 12% yields, respectively (Scheme 5). The Scheme 4. Possible Tautomerization between Compounds 5 and 5′a rotation of (R)-1 was identical to that of natural 1. The structurally related compounds pseudodeflectusin and ustusorane C had the 7R-configuration.10 Taken together, these results show that the absolute configuration of natural ustusorane A must be R. Pergillin (4) and dihydropergillin (5) were prepared from (R)-1 (Scheme 3). The mono TBS protection of the primary alcohol in (R)-1 gave 28. Subsequent oxidation of the secondary alcohol in 28, followed by deprotection of the TBS group in 29, afforded pergillin (4). The 1H and 13C NMR spectra of 4 in a 1:1 mixture of CDCl3 and DMSO-d6 were identical to those reported for natural pergillin (4) (Table S2 in the Supporting Information).3 Interestingly, however, the 1H and 13C NMR spectra of 4 in DMSO-d6 matched those reported for natural aspergione F (20) (Table S3 in the Supporting Information).12 Thus, pergillin and aspergione F were therefore determined to be the same compound, which meant that the proposed structure for aspergione F (20) was incorrect. The catalytic hydrogenation of 4 gave an inseparable 1:1 mixture of dihydropergillin (5) and its 7-epimer (5′). The 1 H and 13C NMR spectra of 5 and 5′ were similar to each other, but displayed only two sets of signals for 5 and 5′.18 Generally, the cyclic hemiketal forms of these compounds can interconvert in solution via the corresponding open-chain form. Given that the hemiketal formation between 5 and 5′ is a reversible process (Scheme 3), the thermodynamically more stable product should be obtained. The lowest conformations and the sum of electronic and zero-point energies of 5 and 5′

a

The differences in the sum of electronic and zero-point energies between 5 and 5′ are shown in kcal/mol.

Scheme 5. Synthesis of (±)-Penicisochroman A (6) and Ustusorane B (2)

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Table 1. NMR Spectroscopic Data (CDCl3) for Natural Penicisochroman A (6)5 (300 MHz) and Synthetic (±)-6 (400 MHz)

(±)-6

6 position 2 3 3a 4 5 5a 6 7 9 9a 9b 10 11 12 13 OMe

δC, type 145.3, 183.4, 121.2, 121.9, 123.0, 140.7, 39.3,

δH (J in Hz)

C C C CH CH C CH2

145.2, 183.4, 121.2, 121.9, 123.0, 140.7, 39.3,

7.46 d (7.8) 6.77 d (7.8) 2.94 d (16.2) 2.85 d (16.2)

97.4, C 57.8, CH2 118.3, 160.4, 23.0, 131.5, 20.2, 17.4, 49.0,

δC, type C C C CH CH C CH2

7.53 d (7.6) 6.84 d (7.6) 3.00 d (17.2) 2.92 d (17.2)

97.4, C 57.7, CH2

4.86 d (16.2) 4.66 d (16.2)

C C CH3 C CH3 CH3 CH3

δH (J in Hz)

118.2, 160.4, 23.0, 131.6, 20.2, 17.4, 49.0,

1.46 s 2.02 s 2.29 s 3.27 s

4.93 d (17.2) 4.73 d (17.2)

C C CH3 C CH3 CH3 CH3

1.53 s 2.09 s 2.36 s 3.35 s

ΔδC (ppm)

ΔδH (ppm)

+0.1 0.0 0.0 0.0 0.0 0.0 0.0

−0.07 −0.07 −0.06 −0.07

0.0 +0.1

−0.07 −0.07

+0.1 0.0 0.0 +0.1 0.0 0.0 0.0

−0.07 −0.07 −0.07 −0.08

Table 2. NMR Spectroscopic Data (CD3OD) for Natural Aspergione E (19),12 Synthetic (±)-6 (400 MHz), and Synthetic Pergillin (4) (400 MHz)

(±)-6a

19 positionb

δC

2 4 5 6 7 8 12

146.5 182.5 121.9 122.5c 124.8c 143.6 40.1

13 11

95.4 58.0

9 10 14 3 15 16 OMe

120.1 160.9 28.8 134.3 20.2 17.5 49.6d

δH (J in Hz)

7.55 d (8.0) 6.99 d (8.0) 3.07 d (17.3) 2.97d (17.3) 5.05d (16.0) 4.94d (16.0)

1.58 s 2.17 s 2.39 s 3.35, sd

δC

positionb 2 3 3a 4 5 5a 6

146.5 184.9 122.0 122.5 124.6 143.2 40.1

7 9

98.8 58.4

9a 9b 10 11 12 13 OMe

120.0 161.7 23.2 134.3 20.1 17.5 49.3

δH (J in Hz)

7.48 d (8.0) 6.92 d (8.0) 3.00 d (17.6) 2.91 d (17.6) 4.89 d (15.6) 4.69 d (15.6)

1.50 s 2.12 s 2.35 s 3.32 s

4a δC 146.5 185.0 122.0 122.5 124.9 143.7 40.1 95.5 58.0 120.1 161.9 28.9 134.3 20.2 17.5

δH (J in Hz)

7.49 d (8.0) 6.94 d (8.0) 2.99 d (17.6) 2.89 d (17.6) 4.97 d (16.0) 4.87 d (16.0)

1.54 s 2.12 s 2.35 s

a

The residual solvent peak of CD3OD was used as an internal standard (3.30 ppm for 1H NMR and 49.0 ppm for 13C NMR). bThe positions of the atoms in 19 and 6 have been numbered according to refs and 5, respectively. cThe chemical shifts reported by Lin et al.12 for C-6 and C-7 were incorrectly assigned. dAccording to ref 12, the peaks were obscured by the solvent signal.

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major product, with the anti-isomer as the minor product. Reduction of 31 and 32 with lithium triethylborohydride, followed by deprotection of the TMS group with tetrabutylammonium fluoride (TBAF), afforded ustusorane E (3) and 2epi-3 in 71% and 79% yields over the two steps, respectively. The NMR spectrum of synthetic 3 was identical to that reported for natural 3 (Table S5 in the Supporting Information).2 The specific rotation of 3 was determined to be [α]21D −6.7 (c 0.1, MeOH)22 and was similar to the value of [α]20D −11 (c 0.1, MeOH) reported for natural ustusorane E.2 On the basis of these results, the absolute configuration of natural ustusorane E was determined to be 2R,7R. The NMR spectrum of 2-epi-3 was similar, but not identical, to that reported for natural 3. The 1H NMR signals derived from the 7- and 9-OH groups in 2-epi-3 gave δ values of 4.72 (1H, d, J = 4.4 Hz) and 4.69 (1H, t, J = 5.2 Hz), respectively, and partially overlapped with each other. However, the signals derived from the 7- and 9-OH groups in natural 3 appeared at δ values of 4.78 (1H, d, J = 5.8 Hz) and 4.72 (1H, dd, J = 4.8, 4.0 Hz), respectively, as separate peaks. Furthermore, the specific rotation of 2-epi-3 was [α]22D +3.4 (c 0.10, MeOH), which was different from that of natural 3 both in sign and in magnitude. The synthesis of (−)-brassicadiol (7) is depicted in Scheme 7. Reduction of 31 with lithium triethylborohydride, followed

(±)-penicisochroman A (6) material obtained in this way was converted to ustusorane B (2) in quantitative yield by treatment with (±)-10-camphorsulfonic acid (CSA) in toluene at 50 °C. Although all of the 1H NMR signals of (±)-6 were shifted downfield with differences in their chemical shift values (ΔδH) of 0.06−0.08 ppm relative to those reported for natural 6, the 13C NMR signals of (±)-6 were identical to those reported for 6 (Table 1).5 The NMR spectra for (±)-6 were different from those reported for aspergione E (19) (Table 2).12 However, we found that the NMR data of natural aspergione E (19) in CD3OD were similar to those of synthetic pergillin (4) in CD3OD except for the signal derived from the methoxy group in 19. These results therefore suggest that the sample of natural aspergione E could be a 1:1 mixture of pergillin and methanol. The 1H and 13C NMR spectra for synthetic ustusorane B (2) were identical to those reported for natural 2 (Table S4 in the Supporting Information).2 The route used for the synthesis of ustusorane E is shown in Scheme 6. The reduction of 13 with NaBH4 in the presence of Scheme 6. Synthesis of Ustusorane E (3)

Scheme 7. Synthesis of (−)-Brassicadiol (7)

by protection of the primary alcohol as a TBS ether, gave 33. Dehydration of the secondary alcohol in 33 with Martin’s sulfurane,23 followed by the sequential catalytic hydrogenation of the resulting alkene and deprotection of the silyl groups with TBAF, gave (−)-brassicadiol (7). The NMR spectra of (−)-7 were identical to those reported for natural 7 (Table S6 in the Supporting Information). The specific rotation of (−)-7 was determined to be [α]24D −17.5 (c 0.35, CHCl3). Unfortunately, the specific rotation value for natural 7 has not been reported in the literature. However, the specific rotation value for natural annullatin B (8) was reported to be [α]27D −13.5 (c 0.39, CHCl3).7 On the basis of the structural similarities between 7 and 8, these data indicated that the absolute configuration of our synthetic (−)-7 was R. In conclusion, we have successfully synthesized ustusoranes A, B, and E, pergillin, dihydropergillin, (±)-penicisochroman A, and (−)-brassicadiol starting from (R)-benzolactone 13. The absolute configuration of natural ustusorane A was determined to be R through a comparison of the specific rotation of our synthetic ustusorane A with that of the natural product.

CeCl320 in a mixture of THF and MeOH, followed by acidic isomerization21 with aqueous H2SO4 in 1,2-dimethoxyethane (DME), gave benzofuran 30. Catalytic hydrogenation of 30, followed by protection of the tertiary alcohol as the corresponding trimethylsilyl (TMS) ether, afforded compounds 31 and 32 in 72% and 12% yields, respectively. Given that the hydrogenation of 13 occurred preferentially from the opposite side of the methyl group at C-7 to give 11 (Scheme 1),11 it was envisaged that the hydrogenation of 30 would occur from the opposite side of the methyl group to give the syn-isomer as the E

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(MeOH) λmax (log ε) 229 (4.25), 270 (3.72), 3.14 (3.53) nm; IR (neat) νmax 3016, 2974, 2935, 2877, 1723, 1618, 1587, 1464, 1429, 1385, 1350, 1286, 1223, 1203, 1157, 1120, 1060, 1027 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.62 (1H, d, J = 7.6 Hz, H-4), 7.03 (1H, d, J = 7.6 Hz, H-5), 6.39 (1H, d, J = 0.8 Hz, H-3), 4.70 (1H, m, H-7), 3.19 (1H, sept, J = 6.8 Hz, H-11), 3.04 (1H, dd, J = 16.4 Hz, 10.4 Hz, H-6), 2.97 (1H, dd, J = 16.4 Hz, 4.0 Hz, H-6), 1.53 (3H, d, J = 6.4 Hz, H10), 1.38 (3H, d, J = 6.8 Hz, H-12), 1.37 (3H, d, J = 6.8 Hz, H-13); 13 C NMR (CDCl3, 100 MHz) δ 166.8 (C, C-9), 162.7 (C, C-2), 153.5 (C, C-9b), 135.1 (C, C-5a), 129.8 (C, C-9a), 125.7 (CH, C-5), 121.4 (CH, C-4), 109.6 (C, C-3a), 99.4 (CH, C-3), 75.2 (CH, C-7), 35.1 (CH2, C-6), 28.1 (CH, C-11), 21.1 (CH3, C-10), 20.8 (CH3, C-12), 20.8 (CH3, C-13); HRESIMS m/z 267.1014 (calcd for C15H16O3Na, 267.0992). (R)-1-[7-(Hydroxymethyl)-2-isopropylbenzofuran-6-yl]propan-2ol (23). LiBHEt3 (1.1 mL of 1.0 M solution in THF, 0.60 mmol) was added to a stirred solution of 22 (103.5 mg, 0.42 mmol) in THF (4 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 30 min. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 2:1, v/v) to give 23 (105.2 mg, quant.) as a colorless oil: [α]22D +10.9 (c 0.42, MeOH); UV (MeOH) λmax (log ε) 211 (4.30), 252 (4.05), 284 (3.46) nm; IR (neat) νmax 3303, 2968, 2929, 2875, 1587, 1462, 1421, 1342, 1300, 1250, 1219, 1184, 1153, 1122, 1070, 1045, 1024 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.38 (1H, d, J = 8.0 Hz, H-4), 7.02 (1H, d, J = 8.0 Hz, H-5), 6.33 (1H, d, J = 1.2 Hz, H-3), 5.08 (1H, d, J = 12.0 Hz, H-9), 4.87 (1H, d, J = 12.0 Hz, H-9), 4.04 (1H, m, H-7), 3.08 (1H, septd, J = 6.8 Hz, 1.2 Hz, H-11), 2.94 (1H, dd, J = 14.0 Hz, 3.6 Hz, H6), 2.88 (1H, dd, J = 14.0 Hz, 8.4 Hz, H-6), 1.34 (6H, d, J = 6.8 Hz, H12 and H-13), 1.33 (3H, d, J = 7.2 Hz, H-10); 13C NMR (CDCl3, 100 MHz) δ 165.0 (C, C-2), 153.5 (C, C-9b), 132.8 (C, C-5a), 127.3 (C, C-3a), 124.8 (CH, C-5), 122.9 (C, C-9a), 119.9 (CH, C-4), 99.8 (C, C-3), 69.4 (CH, C-7), 55.7 (CH2, C-9), 41.2 (CH2, C-6), 28.2 (CH, C-11), 23.8 (CH3, C-10), 21.0 (CH3, C-12), 20.9 (CH3, C-13); HRESIMS m/z 271.1326 (calcd for C15H20O3Na, 271.1305). 7-(tert-Butyldimethylsilyloxymethyl)-6-{(R)-2-(tertbutyldimethylsilyloxy)propyl}-2-isopropylbenzofuran (24) and (R)1-[7-(tert-Butyldimethylsilyloxymethyl)-2-isopropylbenzofuran-6yl]propan-2-ol (25). tert-Butyldimethylsilyl chloride (201 mg, 2.55 mmol) was added to a stirred solution of 23 (105 mg, 0.42 mmol), imidazole (173 mg, 2.55 mmol), and DMAP (13 mg, 0.11 mmol) in CH2Cl2 (5 mL) at rt, and the resulting mixture was stirred at rt for 6 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with CHCl3 to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 40:1 to 8:1, v/v) to give 24 (131 mg, 65%) as a colorless oil and 25 (54 mg, 35%) as a colorless oil. 24: [α]22D −24.3 (c 0.86, CHCl3); UV (CHCl3) λmax (log ε) 254 (4.16), 277 (3.67) nm; IR (neat) νmax 3034, 2958, 2931, 2858, 1587, 1467, 1423, 1381, 1365, 1296, 1252, 1188, 1153, 1126, 1077, 1028, 1001 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.29 (1H, d, J = 8.0 Hz, H-4), 7.02 (1H, d, J = 8.0 Hz, H-5), 6.29 (1H, d, J = 0.8 Hz, H-3), 5.05 (1H, d, J = 11.6 Hz, H-9), 4.99 (1H, d, J = 11.6 Hz, H-9), 4.08 (1H, m, H-7), 3.06 (1H, septd, J = 6.8 Hz, 0.8 Hz, H-11), 2.98 (1H, dd, J = 13.6 Hz, 7.6 Hz, H-6), 2.88 (1H, dd, J = 13.6 Hz, 7.6 Hz, H-6), 1.34 (6H, d, J = 6.8 Hz, H-12 and H-13), 1.17 (3H, d, J = 7.2 Hz, H10), 0.90 (9H, s, C(CH3)3), 0.83 (9H, s, C(CH3)3), 0.10 (3H, s, SiCH3), 0.06 (3H, s, SiCH3), −0.12 (3H, s, SiCH3), −0.26 (3H, s, SiCH3); 13C NMR (CDCl3, 100 MHz) δ 164.2 (C, C-2), 153.6 (C, C9b), 134.2 (C, C-5a), 126.8 (C, C-3a), 125.8 (CH, C-5), 122.2 (C, C9a), 118.9 (CH, C-4), 99.7 (C, C-3), 70.3 (CH, C-7), 56.9 (CH2, C9), 42.4 (CH2, C-6), 28.2 (CH, C-11), 26.0 (3 x CH3, SiC(CH3)3), 25.9 (3 × CH3, SiC(CH3)3), 23.9 (CH3, C-10), 21.0 (CH3, C-12), 20.9 (CH3, C-13), 18.4 (C, SiC(CH3)3), 18.0 (C, SiC(CH3)3), −5.0 (CH3, SiCH3), −5.1 (CH3, SiCH3), −5.2 (CH3, SiCH3), −5.3 (CH3, SiCH3); HRESIMS m/z 499.3013 (calcd for C27H48O3Si2Na,

Pergillin, dihydropergillin, ustusorane B, and (±)-penicisochroman A were synthesized from ustusorane A. As a result of our synthesis of pergillin, the structure proposed for aspergione F has been revised because pergillin and aspergione F have been revealed to be the same compound. Dihydropergillin was obtained as an inseparable 1:1 mixture with 7-epi-dihydropergillin. The calculated sum of the electronic and zero-point energies of dihydropergillin and 7-epi-dihydropergillin was only 0.06 kcal/mol. Given that the hemiketal formation between dihydropergillin and 7-epi-dihydropergillin is a reversible process, the small energy difference is consistent with the fact that two diastereomers were obtained in a 1:1 ratio. The NMR spectrum of (±)-penicisochroman A matched that of the natural penicisochroman A, but not the spectrum reported for aspergione E. The 1H and 13C NMR spectra of pergillin in CD3OD were similar to those reported for aspergione E, except for the signal derived from the methoxy group, which suggested that the sample of natural aspergione E could be a 1:1 mixture of pergillin and methanol. The 1H and 13C NMR spectra of synthetic ustusorane E were identical to those of the natural product. Given that the specific rotation of natural ustusorane E was similar to that of our synthetic product, the absolute configuration of natural ustusorane E was determined to be 2R,7R. (−)-Brassicadiol was prepared from the same intermediate as that used for the preparation of ustusorane E. This study clearly indicates that (R)-benzolactone 13 could potentially be a key intermediate for the synthesis of natural benzofuranoids. Further synthetic studies toward related natural products as well as biological evaluation of their synthetic derivatives are currently ongoing in our laboratory.

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EXPERIMENTAL SECTION

General Experimental Procedures. Melting point data were determined with a Yanaco MP-3S instrument (Kyoto, Japan) and were uncorrected. Optical rotations were recorded on a JASCO P-2000 digital polarimeter (Tokyo, Japan) and were recorded as [α]D values (concentration in g/100 mL). UV spectra were measured on a JASCO UV V-650 spectrophotometer (Tokyo, Japan). IR spectra were recorded on a Horiba FT210 spectrometer (Kyoto, Japan), using NaCl (neat) or KBr pellets (solid). 1H and 13C NMR spectra were recorded on a Bruker Biospin Avance 400 (400 and 100 MHz, respectively) spectrometer (Rheinstetten, Germany) using CDCl3, CD3OD, DMSO-d6, or acetone-d6 as the solvent. Chemical shift values are expressed in δ (ppm) relative to tetramethylsilane or the residual solvent resonance (CDCl3: δ 77.0 for 13C NMR; CD3OD: δ 3.30 for 1 H NMR; CD3OD: δ 49.0 for 13C NMR; DMSO-d6: δ 2.49 for 1H NMR; DMSO-d6: δ 39.7 for 13C NMR; acetone-d6: δ 2.04 for 1H NMR; acetone-d6: δ 29.8 for 13C NMR). The NMR data have been reported as follows: chemical shifts, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, septd = septet of doublet, br = broad, m = multiplet); coupling constants (J in Hz); and integration. MS spectra were obtained on a Fourier transformation-ion cyclotron resonance-mass spectrometer, Bruker solariX (FT-ICR-MS), by using ESI and laser desorption ionization (LDI) techniques. Analytical TLC was performed on silica gel 60 F254 plates (0.5 mm, Merck). Flash column chromatography was performed on a SilicaFlash F60 column (230−400 mesh). (R)-2-Isopropyl-7-methyl-6,7-dihydro-9H-furo[3,2-h]isochromen9-one (22). A solution of 1411 (19.8 mg, 75.5 μmol) and p-TsOH· H2O (4.6 mg, 24.2 μmol) in CH2Cl2 (3 mL) was stirred at rt for 30 min. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 4:1, v/v) to give 22 (15.7 mg, 85%) as colorless oil: [α]22D −133.8 (c 1.05, CHCl3); UV F

DOI: 10.1021/np5010483 J. Nat. Prod. XXXX, XXX, XXX−XXX

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499.3034). 25: [α]22D −7.9 (c 0.55, CHCl3); UV (CHCl3) λmax (log ε) 255 (4.04), 277 (3.48) nm; IR (neat) vmax 3446, 2962, 2929, 2885, 2857, 1587, 1466, 1421, 1385, 1365, 1298, 1254, 1182, 1153, 1122, 1072, 1047, 1008 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.39 (1H, d, J = 8.0 Hz, H-4), 7.06 (1H, d, J = 8.0 Hz, H-5), 6.33 (1H, d, J = 1.2 Hz, H-3), 5.12 (1H, d, J = 11.6 Hz, H-9), 4.97 (1H, d, J = 11.6 Hz, H-9), 4.03 (1H, m, H-7), 3.05 (1H, septd, J = 6.8 Hz, 1.2 Hz, H-11), 3.01 (1H, dd, J = 14.0 Hz, 3.6 Hz, H-6), 2.87 (1H, dd, J = 14.0 Hz, 8.8 Hz, H-6), 1.34 (6H, d, J = 6.8 Hz, H-12 and H-13), 1.30 (3H, d, J = 6.0 Hz, H-10), 0.94 (9H, s, C(CH3)3), 0.16 (3H, s, SiCH3), 0.14 (3H, s, SiCH3); 13C NMR (CDCl3, 100 MHz) δ 164.5 (C, C-2), 153.4 (C, C9b), 133.7 (C, C-5a), 127.1 (C, C-3a), 125.0 (CH, C-5), 121.9 (C, C9a), 119.9 (CH, C-4), 100.0 (C, C-3), 69.0 (CH, C-7), 56.7 (CH2, C9), 42.0 (CH2, C-6), 28.3 (CH, C-11), 26.0 (3 × CH3, SiC(CH3)3), 23.9 (CH3, C-10), 21.0 (CH3, C-12), 20.9 (CH3, C-13), 18.5 (C, SiC(CH3)3), −5.3 (2 × CH3, SiCH3); HRESIMS m/z 385.2169 (calcd for C21H34O3SiNa, 385.2179). Preparation of 24 from 25. tert-Butyldimethylsilyl trifluoromethanesulfonate (33 μL, 144 μmol) was added to a stirred solution of 25 (12.9 mg, 35.6 μmol) and 2,6-lutidine (24 μL, 213 μmol) in CH2Cl2 (2 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 10 min. The reaction was then quenched by the addition of H2O, and the mixture was diluted with CHCl3 to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by preparative TLC (hexane−EtOAc, 40:1, v/v) to give 24 (16.9 mg, quant.). 7-(tert-Butyldimethylsilyloxymethyl)-6-(R)-[2-(tertbutyldimethylsilyloxy)propyl]-2-hydroxy-2-isopropylbenzofuran-3(2H)-one (26). OsO4 (10.6 mg, 41.7 μmol) was added to a stirred solution of 24 (15.7 mg, 32.9 μmol) in pyridine (2 mL) and Et2O (1 mL) at rt, and the resulting mixture was stirred under a nitrogen atmosphere at rt for 16 h. A solution of Na2SO3 in EtOH (2 mL) and H2O (1 mL) was added to the mixture. The resulting mixture was stirred under an aerobic atmosphere at 70 °C for 1.5 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. After the solution was filtrated through Celite, the organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 10:1 to 5:1, v/v) to give 26 (6.3 mg, 37%) as a 10.8:1 mixture of the ketal form (1.4:1 diasteromeric mixture at C-2) and diketone form as a pale yellow oil: [α]22D −80.8 (c 0.25, CHCl3); UV (CHCl3) λmax (log ε) 269 (4.28), 338 (3.89) nm; IR (neat) νmax 3446, 3419, 2956, 2929, 2889, 2856, 1724, 1612, 1468, 1431, 1385, 1365, 1333, 1255, 1190, 1126, 1076, 1002 cm−1; 1H NMR (CDCl3, 400 MHz, the signals derived from the ketal are indicated; signals marked with an asterisk are from the minor diastereomer at C-2) δ 7.48* (1H, d, J = 8.0 Hz, H-4), 7.46 (1H, d, J = 8.0 Hz, H-4), 6.96 (1H, d, J = 8.0 Hz, H-5), 6.94* (1H, d, J = 8.0 Hz, H-5), 4.89 (1H, d, J = 11.2 Hz, H-9), 4.81* (1H, d, J = 11.2 Hz, H-9), 4.79* (1H, d, J = 11.2 Hz, H-9), 4.69 (1H, d, J = 11.2 Hz), 4.11 (1H, m, H-7), 4.06* (1H, m, H-7), 3.26* (1H, brs, 11-OH), 3.25 (1H, brs, 11-OH), 2.99− 2.81 (2H, m, H-6), 2.99−2.81* (2H, m, H-6), 2.28 (1H, m, H-11), 2.28* (1H, m, H-11), 1.22 (6H, d, J = 6.8 Hz, H-12 and H-13), 1.22* (6H, d, J = 6.8 Hz, H-12 and H-13), 1.14 (3H, d, J = 6.8 Hz, H-10), 1.13* (3H, d, J = 6.8 Hz, H-10), 0.92 (9H, s, C(CH3)3), 0.91* (9H, s, C(CH3)3), 0.78 (9H, s, C(CH3)3), 0.78* (9H, s, C(CH3)3), 0.15 (3H, s, SiCH3), 0.14* (3H, s, SiCH3), 0.13 (3H, s, SiCH3), 0.11* (3H, s, SiCH3), −0.11* (3H, s, SiCH3), −0.17 (3H, s, SiCH3), −0.35* (3H, s, SiCH3), −0.41 (3H, s, SiCH3),; 13C NMR (CDCl3, 100 MHz) δ 199.6 (C, C-3), 199.6* (C, C-3), 169.9 (C, C-9b), 169.9* (C, C-9b), 152.5* (C, C-5a), 152.4 (C, C-5a), 126.6 (CH, C-5), 125.5* (CH, C-5), 124.5* (C, C-9a), 124.1 (C, C-9a), 123.3* (CH, C-4), 123.1 (CH, C4), 117.8 (C, C-3a), 117.5* (C, C-3a), 106.6 (C, C-2), 106.5* (C, C2), 69.9 (CH, C-7), 69.6* (CH, C-7), 55.4 (CH2, C-9), 55.4* (CH2, C-9), 43.5 (CH2, C-6), 43.0* (CH2, C-6), 34.1 (CH, C-11), 33.8* (CH, C-11), 25.9 (3 × CH3, SiC(CH3)3), 25.9* (3 × CH3, SiC(CH3)3), 25.8 (3 × CH3, SiC(CH3)3), 25.7* (3 × CH3, SiC(CH3)3), 24.6 (CH3, C-10), 24.6* (CH3, C-10), 18.4 (CH3, C-

13), 18.4* (CH3, C-13), 17.91 (C, SiC(CH3)3), 17.90* (C, SiC(CH3)3), 16.1* (C, SiC(CH3)3), 16.0 (C, SiC(CH3)3), 15.2 (CH3, C-12), 15.2* (CH3, C-12), −4.9* (CH3, SiCH3), −5.07* (CH3, SiCH3), −5.21 (CH3, SiCH3), −5.21 (CH3, SiCH3), −5.23* (CH3, SiCH3), −5.3* (CH3, SiCH3), −5.36 (CH3, SiCH3), −5.38 (CH3, SiCH3); HRESIMS m/z 531.2916 (calcd for C27H48O5Si2Na, 531.2933). (R)-7-(tert-Butyldimethylsiloxymethyl)-6-[2-(tertbutyldimethylsiloxy)propyl]-2-(propan-2-ylidene)benzofuran-3(2H)one (27). Burgess reagent (18.0 mg, 94.6 μmol) was added to a stirred solution of 26 (15.1 mg, 29.6 μmol) in toluene (3.0 mL) at rt, and the resulting mixture was stirred at 80 °C for 1 h. Burgess reagent (18.9 mg, 99.4 μmol) was added to the mixture at rt, and the resulting mixture was stirred at 80 °C for 30 min. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 20:1, v/v) to give 27 (11.8 mg, 81%) as a colorless oil: [α]22D −67.7 (c 1.18, CHCl3); UV (CHCl3) λmax (log ε) 291 (4.18), 320 (3.10) nm; IR (neat) νmax 3018, 2956, 2931, 2891, 1695, 1651, 1606, 1467, 1431, 1379, 1333, 1290, 1254, 1217, 1192, 1122, 1076, 1005 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.56 (1H, d, J = 7.6 Hz, H-4), 7.01 (1H, d, J = 7.6 Hz, H-5), 4.92 (1H, d, J = 11.2 Hz, H-9), 4.84 (1H, d, J = 11.2 Hz, H-9), 4.10 (1H, m, H7), 2.92 (2H, m, H-6), 2.37 (3H, s, H-13), 2.12 (3H, s, H-12), 1.23 (3H, d, J = 6.0 Hz, H-10), 0.91 (9H, s, C(CH3)3), 0.80 (9H, s, C(CH3)3), 0.12 (3H, s, SiCH3), 0.09 (3H, s, SiCH3), −0.13 (3H, s, SiCH3), −0.35 (3H, s, SiCH3); 13C NMR (CDCl3, 100 MHz) δ 184.0 (C, C-3), 163.2 (C, C-9b), 149.4 (C, C-5a), 145.4 (C, C-2), 130.9 (C, C-11), 126.0 (CH, C-5), 123.7 (C, C-9a), 122.8 (CH, C-4), 121.5 (C, C-3a), 69.8 (CH, C-7), 55.5 (CH2, C-9), 43.1 (CH2, C-6), 25.9 (3 × CH3, SiC(CH3)3), 25.8 (3 × CH3, SiC(CH3)3), 24.5 (CH3, C-10), 20.1 (CH3, C-12), 18.4 (C, SiC(CH3)3), 17.9 (C, SiC(CH3)3), 17.4 (CH3, C-13), −5.0 (CH3, SiCH3), −5.19 (CH3, SiCH3), −5.24 (CH3, SiCH3), −5.4 (CH3, SiCH3); HRESIMS m/z 513.2824 (calcd for C22H46O4Si2Na, 518.2827). Ustusorane A (1). p-TsOH·H2O (12.1 mg, 63.6 μmol) was added to a stirred solution of 27 (13.9 mg, 28.3 μmol) in MeOH (3.5 mL), and the resulting mixture was stirred for 1 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 20:1, v/v) to give 1 (6.8 mg, 92%) as white solids: mp 144−145 °C; [α]22D −4.3 (c 0.10, MeOH); UV (MeOH) λmax (log ε) 224 (4.05), 288 (4.27), 355 (3.72) nm; IR (KBr) νmax 3323, 2968, 2931, 2883, 1689, 1645, 1606, 1469, 1427, 1373, 1336, 1311, 1294, 1267, 1228, 1184, 1116, 1072, 1026 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.58 (1H, d, J = 8.0 Hz, H-4), 6.96 (1H, d, J = 8.0 Hz, H-5), 4.93 (1H, d, J = 12.0 Hz, H-9), 4.74 (1H, d, J = 12.0 Hz, H-9), 4.08 (1H, m, H-7), 3.81 (1H, brs, 7-OH), 2.96 (1H, dd, J = 13.6 Hz, 8.8 Hz, H-6), 2.90 (1H, dd, J = 13.6 Hz, 3.6 Hz, H-6), 2.57 (1H, brs, 9-OH), 2.36 (3H, s, H-13), 2.12 (3H, s, H-12), 1.38 (3H, d, J = 6.0 Hz, H-10); 13C NMR (CDCl3, 100 MHz) δ 183.7 (C, C-3), 163.3 (C, C-9b), 147.7 (C, C-5a), 145.3 (C, C-2), 132.3 (C, C-11), 124.7 (CH, C-5), 124.5 (C, C-9a), 123.6 (CH, C-4), 121.9 (C, C-3a), 69.0 (CH, C-7), 54.3 (CH2, C-9), 41.8 (CH2, C-6), 24.2 (CH3, C-10), 20.3 (CH3, C-12), 17.5 (CH3, C-13); HRESIMS m/z 285.1111 (calcd for C15H18O4Na, 285.1097). (R)-7-(tert-Butyldimethylsiloxymethyl)-6-(2-hydroxypropyl)-2(propan-2-ylidene)benzofuran-3(2H)-one (28). tert-Butyldimethylsilyl chloride (8.5 mg, 49.8 μmol) was added to a stirred solution of (R)1 (6.8 mg, 25.9 μmol), imidazole (8.8 mg, 129 μmol), and DMAP (0.3 mg, 2.5 μmol) in CH2Cl2 (4.5 mL) at rt, and the resulting mixture was stirred at rt for 2 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with CHCl3 to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 3:1, v/ G

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v) to give 28 (9.3 mg, 96%) as a colorless oil: [α]22D −2.9 (c 0.75, CHCl3); UV (CHCl3) λmax (log ε) 289 (3.33), 356 (2.81) nm; IR (neat) νmax 3503, 2956, 2929, 2889, 2856, 1697, 1653, 1608, 1466, 1431, 1375, 1333, 1290, 1254, 1227, 1184, 1120, 1072, 1004 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.63 (1H, d, J = 8.0 Hz, H-4), 7.03 (1H, d, J = 8.0 Hz, H-5), 4.97 (1H, d, J = 11.6 Hz, H-9), 4.82 (1H, d, J = 11.6 Hz, H-9), 4.07 (1H, m, H-7), 2.99 (1H, dd, J = 13.6 Hz, 4.0 Hz, H-6), 2.92 (1H, dd, J = 13.6 Hz, 8.8 Hz, H-6), 2.37 (3H, s, H-12), 2.12 (3H, s, H-13), 1.32 (3H, d, J = 6.0 Hz, H-10), 0.94 (9H, s, C(CH3)3), 0.18 (3H, s, SiCH3), 0.15 (3H, s, SiCH3); 13C NMR (CDCl3, 100 MHz) δ 183.7 (C, C-3), 163.0 (C, C-9b), 148.7 (C, C-5a), 145.4 (C, C-2), 131.4 (C, C-11), 124.9 (CH, C-5), 123.7 (C, C-9a), 123.3 (CH, C-4), 121.8 (C, C-3a), 68.5 (CH, C-7), 55.3 (CH2, C-9), 42.6 (CH2, C-6), 25.9 (3 × CH3, SiC(CH3)3), 24.2 (CH3, C-10), 20.1 (CH3, C-12), 18.5 (C, SiC(CH3)3), 17.5 (CH3, C-13), −5.3 (2 × CH3, Si(CH3)2); HRESIMS m/z 399.1970 (calcd for C21H32O4SiNa, 399.1962). 7-(tert-Butyldimethylsiloxymethyl)-6-(2-oxopropyl)-2-(propan-2ylidene)benzofuran-3(2H)-one (29). Dess-Martin periodinane (35.0 mg, 82.5 μmol) was added to a stirred solution of 28 (14.1 mg, 37.4 μmol) in CH2Cl2 (4.5 mL) at rt, and the resulting mixture was stirred at rt for 2 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with CHCl3 to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 5:1, v/v) to give 29 (13.5 mg, 96%) as white, amorphous solids: UV (CHCl3) λmax (log ε) 288 (3.38), 358 (2.91) nm; IR (KBr) νmax 3064, 3010, 2952, 2927, 2854, 1695, 1651, 1608, 1466, 1437, 1383, 1356, 1331, 1292, 1254, 1234, 1165, 1124, 1065, 1005 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.63 (1H, d, J = 8.0 Hz, H-4), 6.94 (1H, d, J = 8.0 Hz, H5), 4.84 (2H, s, H-9), 3.98 (2H, s, H-6), 2.37 (3H, s, H-13), 2.20 (3H, s, H-10), 2.12 (3H, s, H-12), 0.90 (9H, s, C(CH3)3), 0.08 (6H, s, Si(CH3)2); 13C NMR (CDCl3, 100 MHz) δ 204.9 (C, C-7), 183.6 (C, C-3), 162.6 (C, C-9b), 145.3 (C, C-5a), 143.5 (C, C-2), 131.6 (C, C11), 125.3 (CH, C-5), 123.9 (C, C-9a), 123.6 (CH, C-4), 122.5 (C, C3a), 55.7 (CH2, C-9), 48.1 (CH2, C-6), 29.8 (CH3, C-10), 25.9 (3 × CH3, SiC(CH3)3), 20.1 (CH3, C-12), 18.4 (C, SiC(CH3)3), 17.5 (CH3, C-13), −5.4 (2 × CH3, Si(CH3)2); HRESIMS m/z 397.1813 (calcd for C21H30O4SiNa, 397.1806). Pergillin (4). p-TsOH·H2O (3.8 mg, 20.0 μmol) was added to a stirred solution of 29 (6.8 mg, 18.2 μmol) in THF and H2O (4:1, v/v, 2.5 mL), and the resulting mixture was stirred for 48 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 3:1 to 2:1, v/v) to give 4 (4.0 mg, 85%) as white solids: mp 170−171 °C (lit.3 171−172 °C); UV (MeOH) λmax (log ε) 224 (4.32), 284 (4.42), 350 (3.92) nm; IR (KBr) vmax 3346, 2981, 2937, 2898, 2854, 1684, 1645, 1606, 1496, 1437, 1367, 1331, 1288, 1259, 1228, 1173, 1117, 1076, 1039 cm−1; 1H NMR (a 1:1 mixture of CDCl3 and DMSO-d6, 400 MHz) δ 7.44 (1H, d, J = 8.0 Hz, H-4), 6.88 (1H, d, J = 8.0 Hz, H-5), 6.00 (1H, s, OH), 4.90 (1H, d, J = 15.6 Hz, H-9), 4.80 (1H, d, J = 15.6 Hz, H-9), 2.93 (1H, d, J = 17.2 Hz, H-6), 2.84 (1H, d, J = 17.2 Hz, H-6), 2.32 (3H, s, H-13), 2.10 (3H, s, H-12), 1.50 (3H, s, H-10); 1H NMR (DMSO-d6, 400 MHz) δ 7.46 (1H, d, J = 8.0 Hz, H-4), 6.93 (1H, d, J = 8.0 Hz, H5), 6.09 (1H, s, OH), 4.84 (1H, d, J = 15.6 Hz, H-9), 4.78 (1H, d, J = 15.6 Hz, H-9), 2.91 (1H, d, J = 17.2 Hz, H-6), 2.81 (1H, d, J = 17.2 Hz, H-6), 2.29 (3H, s, H-13), 2.06 (3H, s, H-12), 1.45 (3H, s, H-10); 13 C NMR (a 1:1 mixture of CDCl3 and DMSO-d6, 100 MHz) δ 182.4 (C, C-3), 159.8 (C, C-9b), 144.5 (C, C-2), 142.1 (C, C-5a), 130.9 (C, C-11), 123.3 (CH, C-5), 120.9 (CH, C-4), 120.3 (C, C-3a), 118.7 (C, C-9a), 93.6 (C, C-7), 56.4 (CH2, C-9), 39.2 (CH2, C-6), 28.5 (CH3, C-10), 19.8 (CH3, C-13), 16.9 (CH3, C-12); 13C NMR (DMSO-d6, 100 MHz) δ 182.5 (C, C-3), 159.8 (C, C-9b), 144.5 (C, C-2), 142.7 (C, C-5a), 132.0 (C, C-11), 123.9 (CH, C-5), 121.3 (CH, C-4), 120.3 (C, C-3a), 118.9 (C, C-9a), 93.8 (C, C-7), 56.3 (CH2, C-9), 39.2 (CH2, C-6), 28.7 (CH3, C-10), 20.0 (CH3, C-13), 17.0 (CH3, C-12); HRESIMS m/z 283.0948 (calcd for C15H16O4Na, 283.0941).

Dihydropergillin (5) and Its 2-Epimer (5′). A solution of 4 (4.7 mg, 18.1 μmol) and 10% Pd/C (2.2 mg) in THF (3 mL) was stirred at rt under a H2 atmosphere for 8.5 h. The reaction mixture was then filtered through Celite and washed with EtOAc. The filtrate was concentrated, and the resulting residue was purified by flash column chromatography (hexane−EtOAc, 6:1, v/v) to give a 1:1 inseparable mixture of 5 and 5′ (3.8 mg, 80%) as white solids: mp 116−117 °C (lit.4 122−126 °C); UV (MeOH) λmax (log ε) 216 (3.46), 261 (3.15) nm; IR (KBr) νmax 3369, 2964, 2933, 2902, 2854, 1695, 1616, 1601, 1502, 1462, 1437, 1373, 1331, 1294, 1257, 1213, 1161, 1103, 1086, 1039, 1005 cm−1; 1H NMR (CDCl3, 400 MHz, the signals marked with an asterisk are from the diastereomer at C-2) δ 7.46 (1H, d, J = 8.0 Hz, H-4), 7.46* (1H, d, J = 8.0 Hz, H-4), 6.80 (1H, d, J = 8.0 Hz, H-5), 6.80* (1H, d, J = 8.0 Hz, H-5), 4.98 (1H, d, J = 16.0 Hz, H-9), 4.98* (1H, d, J = 16.0 Hz, H-9), 4.91 (1H, d, J = 16.0 Hz, H-9), 4.91* (1H, d, J = 16.0 Hz, H-9), 4.43 (1H, d, J = 3.6 Hz, H-2), 4.42* (1H, d, J = 3.6 Hz, H-2), 3.02 (1H, d, J = 17.2 Hz, H-6), 3.02* (1H, d, J = 17.2 Hz, H-6), 2.94 (1H, d, J = 17.2 Hz, H-6), 2.94* (1H, d, J = 17.2 Hz, H-6), 2.35 (1H, m, H-11), 2.35* (1H, m, H-11), 1.63 (3H, s, H-10), 1.63* (3H, s, H-10), 1.159 (3H, d, J = 6.8 Hz, H-12), 1.155* (3H, d, J = 6.8 Hz, H-12), 0.87 (3H, d, J = 6.8 Hz, H-13), 0.87* (3H, d, J = 6.8 Hz, H-13); 13C NMR (CDCl3, 100 MHz) δ 201.1 (C, C-3), 201.1* (C, C-3), 169.23 (C, C-9b), 169.18* (C, C-9b), 142.14 (C, C-5a), 142.09* (C, C-5a), 122.7 (CH, C-4), 122.7* (CH, C-4), 121.7 (CH, C-5), 121.7* (CH, C-5), 119.58 (C, C-3a), 119.55* (C, C-3a), 118.92 (C, C-9a), 118.89 (C, C-9a), 94.77 (C, C-7), 94.75* (C, C-7), 90.21 (C, C-2), 90.15* (C, C-2), 52.7 (CH2, C-9), 52.7* (CH2, C-9), 38.6 (CH2, C-6), 38.6* (CH2, C-6), 31.0 (CH, C-11), 30.9* (CH, C-11), 29.4 (CH3, C-10), 29.3* (CH3, C-10), 18.9 (CH3, C-13), 18.8* (CH3, C-13), 15.8 (CH3, C-12), 15.6* (CH3, C-12); HRESIMS m/z 285.1111 (calcd for C15H18O4Na, 285.1097). (±)-Penicicochrman A (6) and Ustusorane B (2). p-TsOH·H2O (3.6 mg, 18.9 μmol) was added to a stirred solution of 29 (6.2 mg, 16.6 μmol) in MeOH (3 mL), and the resulting mixture was stirred for 3 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by preparative TLC (hexane−EtOAc, 6:1, v/v) to give (±)-6 (4.0 mg, 65%) as white solids and 2 (0.5 mg, 12%) as yellow, amorphous solids. (±)-6: mp 148−149 °C; UV (MeOH) λmax (log ε) 224 (2.91), 285 (3.00), 351 (2.48) nm; IR (KBr) νmax 2991, 2937, 2914, 2854, 1699, 1655, 1603, 1498, 1437, 1375, 1356, 1329, 1286, 1261, 1227, 1182, 1144, 1117, 1074, 1053 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.53 (1H, d, J = 7.6 Hz, H-4), 6.84 (1H, d, J = 7.6 Hz, H-5), 4.93 (1H, d, J = 17.2 Hz, H-9), 4.73 (1H, d, J = 17.2 Hz, H-9), 3.35 (3H, s, OCH3) 3.00 (1H, d, J = 17.2 Hz, H-6), 2.92 (1H, d, J = 17.2 Hz, H-6), 2.36 (3H, s, H-13), 2.09 (3H, s, H-12), 1.53 (3H, s, H-10); 1H NMR (CD3OD, 400 MHz) δ 7.48 (1H, d, J = 8.0 Hz, H-4), 6.92 (1H, d, J = 8.0 Hz, H-5), 4.89 (1H, d, J = 15.6 Hz, H-9), 4.69 (1H, d, J = 15.6 Hz, H-9), 3.32 (3H, s, OCH3), 3.00 (1H, d, J = 17.6 Hz, H-6), 2.91 (1H, d, J = 17.6 Hz, H-6), 2.35 (3H, s, H-13), 2.12 (3H, s, H-12), 1.50 (3H, s, H-10); 13C NMR (CDCl3, 100 MHz) δ 183.4 (C, C-3), 160.4 (C, C9b), 145.2 (C, C-2), 140.7 (C, C-5a), 131.6 (C, C-11), 123.0 (CH, C5), 121.9 (CH, C-4), 121.2 (C, C-3a), 118.2 (C, C-9a), 97.4 (C, C-7), 57.7 (CH2, C-9), 49.0 (CH3, OCH3), 39.3 (CH2, C-6), 23.0 (CH3, C10), 20.2 (CH3, C-12), 17.4 (CH3, C-13); 13C NMR (CD3OD, 100 MHz) δ 184.9 (C, C-3), 161.7 (C, C-9b), 146.5 (C, C-2), 143.2 (C, C5a), 134.3 (C, C-11), 124.6 (CH, C-5), 122.5 (CH, C-4), 122.0 (C, C3a), 120.0 (C, C-9a), 98.8 (C, C-7), 58.4 (CH2, C-9), 49.3 (CH3, OCH3), 40.1 (CH2, C-6), 23.2 (CH3, C-10), 20.1 (CH3, C-12), 17.5 (CH3, C-13); HRESIMS m/z 297.1117 (calcd for C16H18O4Na, 297.1097). 2: UV (MeOH) λmax (log ε) 206 (3.12), 228 (3.18), 279 (2.82), 368 (3.16) nm; IR (KBr) νmax 2924, 2854, 1697, 1654, 1614, 1579, 1491, 1435, 1396, 1373, 1336, 1288, 1259, 1234, 1155, 1117, 1080, 1047, 1016 cm−1; 1H NMR (acetone-d6, 400 MHz) δ 7.47 (1H, d, J = 7.6 Hz, H-4), 6.77 (1H, d, J = 7.6 Hz, H-5), 5.85 (1H, s, H-6), 5.28 (2H, s, H-9), 2.30 (3H, s, H-13), 2.07 (3H, s, H-12), 1.95 (3H, s, H-10); 13C NMR (CDCl3, 100 MHz) δ 182.7 (C, C-3), 160.4 (C, C9b), 160.2 (C, C-7), 145.9 (C, C-2), 141.4 (C, C-5a), 130.6 (C, C-11), H

DOI: 10.1021/np5010483 J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products

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C-12), 24.4 (CH3, C-13), 20.7 (CH3, C-10), 2.3 (3 × CH3, Si(CH3)3); HRESIMS m/z 357.1501 (calcd for C18H26O4SiNa, 357.1493). 32: [α]22D −178.0 (c 0.15, CHCl3); UV (CHCl3) λmax (log ε) 322 (3.45) nm; IR (neat) νmax 2974, 2927, 2854, 1726, 1614, 1469, 1446, 1383, 1348, 1254, 1180, 1160, 1120, 1059 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.23 (1H, d, J = 7.2 Hz, H-4), 6.62 (1H, d, J = 7.2 Hz, H-5), 4.76 (1H, dd, J = 9.6 Hz, 6.0 Hz, H-2), 4.56 (1H, m, H-7), 3.22 (1H, dd, J = 16.0 Hz, 6.0 Hz, H-3), 3.14 (1H, dd, J = 16.0 Hz, 10.4 Hz, H3), 2.87 (1H, dd, J = 16.0 Hz, 10.4 Hz, H-6), 2.81 (1H, dd, J = 16.0 Hz, 3.6 Hz, H-6), 1.48 (3H, d, J = 6.4 Hz, H-10), 1.32 (3H, s, H-12), 1.27 (3H, s, H-13), 0.01 (9H, s, Si(CH3)3); 13C NMR (CDCl3, 100 MHz) δ 163.1 (C, C-9), 162.0 (C, C-9b), 138.5 (C, C-5a), 129.2 (C, C-9a), 129.0 (CH, C-4), 118.2 (CH, C-5), 107.6 (C, C-3a), 90.7 (CH, C-2), 75.0 (C, C-11), 74.8 (CH, C-7), 35.5 (CH2, C-6), 29.4 (CH2, C3), 27.4 (CH3, C-12), 24.5 (CH3, C-13), 20.8 (CH3, C-10), 2.3 (3 × CH3, Si(CH3)3); HRESIMS m/z 357.1449 (calcd for C18H26O4SiNa, 357.1493). Ustusorane E (3). LiBHEt3 (70 μL of 1.0 M solution in THF, 70 μmol) was added to a stirred solution of 31 (9.7 mg, 29 μmol) in THF (3 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 1.5 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue. A 1.0 M solution of TBAF (30 μL, 30 μmol) was added to a stirred solution of the residue (10.7 mg) in THF (3 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 30 min. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (EtOAc− MeOH, 20:1, v/v) to give 3 (5.5 mg, 75% in 2 steps) as a colorless oil: [α]22D −6.7 (c 0.10, MeOH); UV (MeOH) λmax (log ε) 206 (4.27), 288 (3.43) nm; IR (neat) νmax 3360, 2972, 2925, 2856, 1623, 1593, 1482, 1442, 1378, 1321, 1269, 1186, 1118, 1074, 1007 cm−1; 1H NMR (DMSO-d6, 400 MHz) δ 6.98 (1H, d, J = 7.6 Hz, H-4), 6.61 (1H, d, J = 7.6 Hz, H-5), 4.77 (1H, d, J = 4.4 Hz, 7-OH), 4.71 (1H, dd, J = 6.4 Hz, 4.4 Hz, 9-OH), 4.51 (1H, dd, J = 11.6 Hz, 6.4 Hz, H-9), 4.50 (1H, m, H-2), 4.48 (1H, s, 11-OH), 4.42 (1H, dd, J = 11.6 Hz, 4.4 Hz, H-9), 3.78 (1H, m, H-7), 3.11 (1H, dd, J = 15.6 Hz, 7.2 Hz, H-3), 3.05 (1H, dd, J = 15.6 Hz, 9.6 Hz, H-3), 2.74 (1H, dd, J = 13.6 Hz, 7.2 Hz, H-6), 2.60 (1H, dd, J = 13.6 Hz, 5.6 Hz), 1.12 (3H, s, H-12), 1.12 (3H, s, H13), 1.07 (3H, d, J = 6.0 Hz, H-10); 13C NMR (DMSO-d6, 100 MHz) δ 158.6 (C, C-9b), 138.5 (C, C-5a), 124.7 (C, C-3a), 123.5 (CH, C-4), 122.0 (CH, C-5), 121.5 (C, C-9a), 88.7 (CH, C-2), 70.5 (C, C-11), 67.8 (CH, C-7), 54.6 (CH2, C-9), 41.6 (CH2, C-6), 30.3 (CH2, C-3), 26.0 (CH3, C-12), 25.0 (CH3, C-13), 23.8 (CH3, C-10); HRESIMS m/z 289.1420 (calcd for C15H22O4Na, 289.1410). 2-epi-Ustusorane E (2-epi-3). LiBHEt3 (41 μL of a 1.0 M solution in THF, 41 μmol) was added to a stirred solution of 32 (4.0 mg, 12 μmol) in THF (3 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 30 min. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue. A 1.0 M solution of TBAF (20 μL, 20 μmol) was added to a stirred solution of the residue (6.6 mg) in THF (3 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 1 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (EtOAc− MeOH, 20:1, v/v) to give 2-epi-3 (2.5 mg, 79% in 2 steps) as white, amorphous solids: [α]22D +3.4 (c 0.10, MeOH); UV (MeOH) λmax (log ε) 207 (3.98), 288 (3.12) nm; IR (KBr) νmax 3345, 2970, 2925, 2856, 1622, 1593, 1483, 1442, 1379, 1319, 1271, 1255, 1184, 1120, 1072 cm−1; 1H NMR (DMSO-d6, 400 MHz) δ 6.98 (1H, d, J = 7.6 Hz, H-4), 6.61 (1H, d, J = 7.6 Hz, H-5), 4.72 (1H, d, J = 4.4 Hz, 7-OH), 4.69 (1H, t, J = 5.2 Hz, 9-OH), 4.50 (1H, m, H-2), 4.48 (1H, dd, J = 11.2 Hz, 5.2 Hz, H-9), 4.48 (1H, dd, J = 11.2 Hz, 5.2 Hz, H-9), 4.48

124.4 (CH, C-4), 122.3 (CH, C-3a), 118.3 (C, C-5), 109.6 (C, C-9a), 102.4 (CH, C-6), 63.0 (CH2, C-9), 19.8 (CH3, C-10), 19.8 (CH3, C12), 16.9 (CH3, C-13); HRESIMS m/z 265.0847 (calcd for C15H14O3Na, 265.0835). Preparation of Ustusorane B (2) from (±)-Penicicochrman A (6). (±)-10-Camphorsulfonic acid (3.5 mg, 15.1 μmol) was added to a stirred solution of (±)-6 (4.0 mg, 14.6 μmol) in toluene (3 mL), and the resulting mixture was stirred at 50 °C for 15 min. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by preparative TLC (hexane−EtOAc, 6:1, v/v) to give 2 (3.8 mg, quant.). (R)-2-(2-Hydroxypropan-2-yl)-7-methyl-6,7-dihydro-9H-furo[3,2h]isochromen-9-one (30). NaBH4 (26.6 mg, 703 μmol) was added to a stirred solution of 10 (30.3 mg, 117 μmol) and CeCl3·7H2O (262 mg, 703 μmol) in THF (3 mL) and MeOH (1 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 1 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue. A 0.2 M aqueous solution of H2SO4 (0.4 mL) was added to a solution of the residue in 1,2-dimethoxyethane (4 mL) at rt, and the resulting mixture was stirred at rt for 1 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane−EtOAc, 1:1, v/v) to give 30 (29.7 mg, 97%) as colorless, amorphous solids: [α]22D −107.7 (c 0.47, CHCl3); UV (MeOH) λmax (log ε) 227 (4.26), 268 (3.75), 3.13 (3.56) nm; IR (KBr) νmax 3435, 2981, 2935, 1707, 1618, 1585, 1429, 1383, 1356, 1290, 1259, 1211, 1159, 1126, 1081, 1055, 1030 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.67 (1H, d, J = 8.0 Hz, H-4), 7.07 (1H, d, J = 8.0 Hz, H-5), 6.59 (1H, s, H-3), 4.72 (1H, m, H-7), 3.41 (1H, brs, OH), 3.02 (2H, m, H-6), 1.72 (6H, s, H-12 and H-13), 1.54 (3H, d, J = 6.0 Hz, H-10); 13C NMR (CDCl3, 100 MHz) δ 165.1 (C, C-9), 162.7 (C, C-2), 153.6 (C, C-9b), 136.0 (C, C-5a), 129.4 (C, C-9a), 126.5 (CH, C-5), 121.8 (CH, C-4), 109.9 (C, C-3a), 98.8 (CH, C-3), 75.3 (CH, C-7), 69.0 (C, C-11), 35.1 (CH2, C-6), 28.7 (CH3, C-12), 28.6 (CH3, C-13), 20.8 (CH3, C-10); HRESIMS m/z 283.0951 (calcd for C15H16O4Na, 283.0941). (2R,7R)-7-Methyl-2-(2-trimethylsilyloxypropan-2-yl)-2,3,6,7-tetrahydro-9H-furo[3,2-h]isochromen-9-one (31) and (2S,7R)-7-Methyl2-(2-trimethylsilyloxypropan-2-yl)-2,3,6,7-tetrahydro-9H-furo[3,2h]isochromen-9-one (32). A solution of 30 (8.4 mg, 32 μmol) and 10% Pd/C (1.0 mg) in acetone (3 mL) was stirred at rt under a H2 atmosphere for 9.5 h. The reaction mixture was then filtered through Celite and washed with EtOAc. The filtrate was concentrated. Trimethylsilyl chloride (24 μL, 0.19 mmol) was added to a stirred solution of the resulting residue (9.7 mg) and imidazole (26 mg, 0.38 mmol) in CH2Cl2 (3 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 20 min. The reaction was then quenched by the addition of H2O, and the mixture was diluted with CHCl3 to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by preparative TLC (toluene−EtOAc, 20:1, v/v) to give 31 (7.7 mg, 72%) as a colorless oil and 32 (1.7 mg, 16%) as a colorless oil. 31: [α]22D −6.0 (c 0.39, CHCl3); UV (MeOH) λmax (log ε) 215 (4.13), 241 (3.67), 322 (3.61) nm; IR (neat) vmax 2976, 2954, 2902, 1730, 1614, 1471, 1446, 1383, 1348, 1254, 1180, 1160, 1119, 1059 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.23 (1H, d, J = 7.2 Hz, H-4), 6.62 (1H, d, J = 7.2 Hz, H-5), 4.71 (1H, dd, J = 10.0 Hz, 6.4 Hz, H-2), 4.60 (1H, m, H-7), 3.24 (1H, dd, J = 16.0 Hz, 6.4 Hz, H-3), 3.11 (1H, dd, J = 16.0 Hz, 10.0 Hz, H-3), 2.84 (2H, m, H6), 1.45 (3H, d, J = 6.0 Hz, H-10), 1.35 (3H, s, H-12), 1.31 (3H, s, H13), 0.05 (9H, s, Si(CH3)3); 13C NMR (CDCl3, 100 MHz) δ 162.7 (C, C-9), 161.9 (C, C-9b), 138.3 (C, C-5a), 129.1 (C, C-9a), 129.0 (CH, C-4), 118.3 (CH, C-5), 107.8 (C, C-3a), 90.8 (CH, C-2), 75.0 (C, C11), 74.5 (CH, C-7), 35.2 (CH2, C-6), 29.4 (CH2, C-3), 27.6 (CH3, I

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400 MHz) δ 7.00 (1H, d, J = 7.6 Hz, H-4), 6.68 (1H, d, J = 7.6 Hz, H5), 4.71 (2H, s, H-8), 4.62 (1H, dd, J = 8.8 Hz, 8.8 Hz, H-2), 3.16 (1H, dd, J = 15.6 Hz, 9.2 Hz, H-3), 3.09 (1H, dd, J = 15.6 Hz, 9.6 Hz, H-3), 2.62 (2H, m, H-9), 2.40 (1H, brs, OH), 1.68 (1H, brs, OH), 1.58 (2H, sextet, J = 7.6 Hz, H-10), 1.34 (3H, s, H-13), 1.20 (3H, s, H-14), 0.96 (3H, t, J = 7.6 Hz, H-11); 13C NMR (CDCl3, 100 MHz) δ 158.5 (C, C-7a), 140.9 (C, C-6), 124.6 (C, C-3a), 124.1 (CH, C-4), 121.9 (CH, C-5), 120.3 (C, C-7), 89.8 (CH, C-2), 71.7 (C, C-12), 57.3 (CH2, C8), 34.6 (CH2, C-9), 30.5 (CH2, C-3), 26.2 (CH3, C-13), 25.1 (CH2, C-10), 24.1 (CH3, C-14), 14.1 (CH3, C-11); HRESIMS m/z 273.1458 (calcd for C15H22O3Na, 273.1461). Computational Details. Conformational analysis of 5 and 5′ was performed using the conformational search algorithm implemented in version 1.4.2 of the BARISTA software (Conflex Corp., Tokyo, Japan).24 The lower energy conformers of each compound, which differed from the most stable conformer by less than 10 kcal/mol, were optimized using DFT calculations, at the B3LYP/6-31G(d,p) level, that were implemented in the Gaussian 09 program package.25 The lowest energy conformations of 5 and 5′ were determined by comparing the sum of the electronic and zero-point energies of each conformer.

(1H, s, 11-OH), 4.43 (1H, dd, J = 11.6 Hz, 5.2 Hz, H-9), 3.80 (1H, m, H-7), 3.09 (1H, dd, J = 15.6 Hz, 7.6 Hz, H-3), 3.05 (1H, dd, J = 15.6 Hz, 9.2 Hz, H-3), 2.75 (1H, dd, J = 13.2 Hz, 6.8 z, H-6), 2.60 (1H, dd, J = 13.2 Hz, 5.6 Hz), 1.13 (3H, s, H-12), 1.13 (3H, s, H-13), 1.05 (3H, s, H-10); 13C NMR (DMSO-d6, 100 MHz) δ 158.6 (C, C-9b), 138.4 (C, C-5a), 124.7 (C, C-3a), 123.4 (CH, C-4), 122.1 (CH, C-5), 121.5 (C, C-9a), 88.7 (CH, C-2), 70.4 (C, C-11), 67.6 (CH, C-7), 54.6 (CH2, C-9), 41.6 (CH2, C-6), 30.3 (CH2, C-3), 26.1 (CH3, C-12), 25.1 (CH3, C-13), 23.7 (CH3, C-10); HRESIMS m/z 289.1416 (calcd for C15H22O4Na, 289.1410). (R)-1-{[(R)-7-tert-Butyldimethylsilyloxy]methyl-2-(2-trimethylsilyloxypropan-2-yl)-2,3-dihydrobenzofuran-6-yl}propan-2-ol (33). LiBHEt3 (250 μL of a 1.0 M solution in THF, 250 μmol) was added to a stirred solution of 31 (27.1 mg, 81 μmol) in THF (3 mL) at 0 °C, and the resulting mixture was stirred at 0 °C for 1 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue (28.0 mg). tert-Butyldimethylsilyl chloride (15.6 mg, 104 μmol) was added to a stirred solution of the residue, N,N-diiopropylethylamine (30 μL, 172 μmol), and DMAP (1.2 mg, 10 μmol) in CH2Cl2 (5 mL) at rt, and the resulting mixture was stirred at rt for 13 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (hexane− EtOAc, 10:1, v/v) to give 33 (20.3 mg, 54%) as colorless oil and the corresponding diol (10.7 mg, 38%) as a colorless oil. 33: [α]21D −4.6 (c 1.23, CHCl3); UV (CHCl3) λmax (log ε) 290 (3.66) nm; IR (neat) νmax 3437, 2958, 2931, 2857, 1595, 1469, 1442, 1383, 1365, 1254, 1219, 1176, 1068, 1043, 1005 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.04 (1H, d, J = 7.6 Hz, H-4), 6.68 (1H, d, J = 7.6 Hz, H-5), 4.80 (1H, d, J = 11.2 Hz, H-9), 4.65 (1H, dd, J = 11.2 Hz, H-9), 4.51 (1H, dd, J = 9.6 Hz, 8.0 Hz, H-2), 3.97 (1H, m, H-7), 3.17 (1H, dd, J = 15.6 Hz, 7.6 Hz, H-3), 3.16 (1H, br s, 9-OH), 3.08 (1H, dd, J = 15.6 Hz, 9.2 Hz, H3), 2.86 (1H, dd, J = 13.6 Hz, 3.6 z, H-6), 2.72 (1H, dd, J = 13.6 Hz, 8.8 Hz), 1.31 (3H, s, H-12), 1.28 (3H, d, J = 6.4 Hz, H-10), 1.21 (3H, s, H-13), 0.92 (9H, s, C(CH3)3), 0.15 (3H, s, SiCH3), 0.13 (3H, s, SiCH3), 0.09 (9H, s, Si(CH3)3); 13C NMR (CDCl3, 100 MHz) δ 158.8 (C, C-9b), 138.6 (C, C-5a), 125.3 (C, C-3a), 124.3 (CH, C-4), 122.1 (CH, C-5), 120.2 (C-9a), 89.3 (CH, C-2), 74.7 (C, C-11), 68.9 (CH, C-7), 56.7 (CH2, C-9), 42.0 (CH2, C-6), 30.8 (CH2, C-3), 27.4 (CH3, C-12), 26.0 (3 × CH3, SiC(CH3)3), 24.5 (CH3, C-13), 24.0 (CH3, C10), 18.5 (C, SiC(CH3)3), 2.5 (3 × CH3, Si(CH3)3), −5.3 (2 × CH3, Si(CH3)2); HRESIMS m/z 475.2671 (calcd for C24H44O4Si2Na, 475.2670). (−)-Brassicadiol (7). Martin sulfurane (46.0 mg, 68.4 μmol) was added to a stirred solution of 33 (20.3 mg, 45.8 μmol) at rt, and the resulting mixture was stirred at 80 °C for 1.5 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with hexane to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (EtOAc−hexane, 10:1, v/v) to give a crude alkene. A solution of the alkene (20.0 mg) and 10% Pd/C (4.0 mg) in THF (2 mL) was stirred at rt under a H2 atmosphere for 4 h. The reaction mixture was then filtered through Celite and washed with EtOAc. The filtrate was concentrated. A 1.0 M solution of TBAF (30 μL, 30 μmol) was added to a stirred solution of the residue (21.3 mg) in THF (3 mL) at 0 °C, and the resulting mixture was stirred at rt for 1.5 h. The reaction was then quenched by the addition of H2O, and the mixture was diluted with EtOAc to give a biphasic solution. The organic layer was then collected and washed with brine before being dried over Na2SO4 and concentrated to a residue, which was purified by flash column chromatography (EtOAc−hexane, 1:1, v/v) to give (−)-7 (10.2 mg, 91% in three steps) as a colorless oil: [α]24D −17.5 (c 0.35, CHCl3); UV (MeOH) λmax (log ε) 207 (4.63), 288 (3.80) nm; IR (neat) νmax 3340, 2962, 2931, 2871, 1593, 1485, 1442, 1381, 1338, 1269, 1217, 1195, 1155, 1087, 1051, 1005 cm−1; 1H NMR (CDCl3,

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ASSOCIATED CONTENT

S Supporting Information *

Comparison of NMR spectroscopic data between natural products and synthetic compounds (Tables S1−S6) and the 1H and 13C NMR spectra of all of the new compounds. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81-75-703-5603. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Young Scientists (B) (25850083). This study was carried out using the mass spectrometers in the Joint Usage/Research Center (JURC) at the Institute for Chemical Research, Kyoto University. We are grateful to Prof. M. Nakamura, Prof. H. Takaya, and Prof. K. Isozaki of Kyoto University for assistance with HRMS measurements.

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REFERENCES

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Journal of Natural Products

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(b) Sato, Y.; Kuramochi, K.; Suzuki, T.; Nakazaki, A.; Kobayashi, S. Tetrahedron Lett. 2011, 52, 626−629. (10) Kuramochi, K.; Saito, F.; Nakazaki, A.; Takeuchi, T.; Tsubaki, K.; Sugawara, F.; Kobayashi, S. Biosci. Biotechnol. Biochem. 2010, 74, 1635−1640. (11) Kuramochi, K.; Tsubaki, K.; Kuriyama, I.; Mizushina, Y.; Yoshida, H.; Takeuchi, T.; Kamisuki, S.; Sugawara, F.; Kobayashi, S. J. Nat. Prod. 2013, 76, 1737−1745. (12) Li, W.; Brauers, G.; Ebel, R.; Wray, V.; Berg, A.; Sudarsono; Proksch, P. J. Nat. Prod. 2003, 66, 57−61. (13) Simonetti, S.; Larghi, E. L.; Bracca, A. B. J.; Kaufman, T. S. Org. Biomol. Chem. 2012, 10, 4124−4134. (14) (a) Kohno, J.; Hiramatsu, H.; Nishio, M.; Sakurai, M.; Okuda, T.; Komatsubara, S. Tetrahedron 1999, 55, 11247−11252. (b) Kohno, J.; Sakurai, M.; Kameda, N.; Nishio, M.; Kawano, K.; Kishi, N.; Okuda, T.; Komatsubara, S. J. Antibiot. 1999, 52, 913−916. (15) Ishii, H.; Ishikawa, T.; Takeda, S.; Ueki, S.; Suzuki, M. Chem. Pharm. Bull. 1992, 40, 1148−1153. (16) Atkins, G. M.; Burgess, E. M. J. Am. Chem. Soc. 1968, 90, 4744− 4745. (17) Oxidation of (R)-1 with MnO2 in CH2Cl2 gave (R)-13 in 45% yield. The specific rotation of the resultant (R)-13 was [α]24D −155.4 (c 0.10, CHCl3). On the other hand, the specific rotation of (R)-13, which was used as the starting material in this study, was [α]24D −158.8 (c 0.50, CHCl3). The enantiomeric excess of the starting (R)13 was determined to be 99% by conversion to pseudodeflectusin and the chiral HPLC analysis reported by our group.9a (18) The structure of natural pergillin (5) was determined by X-ray crystallography following its crystallization from a mixture of acetonitrile and water.4 Unfortunately, no crystals of 5 were obtained from the mixture of 5 and 5′. (19) The ratio of 5 and 5′ did not change after treatment of 5 and 5′ with TsOH·H2O (1.0 equiv) in THF−H2O or NaH (1.0 equiv) in THF. (20) Luche, J. L. J. Am. Chem. Soc. 1978, 100, 2226−2227. (21) Gabriele, B.; Mancuso, R.; Salerno, G. J. Org. Chem. 2008, 73, 7336−7341. (22) Treatment of synthetic 3 with an excess amount of (S)-αmethoxy-α-(trifluoromethyl)phenylacetyl chloride (MTPA-Cl) and Et3N gave the corresponding bis-(R)-MTPA ester in quantitative yield as a single diastereomer. Since the signals derived from other diastereomers were observed in the 1H and 13C NMR spectra, the high optical purity (>95%) of synthetic 3 was verified (Supporting Information). (23) Arhart, R. J.; Martin, J. C. J. Am. Chem. Soc. 1972, 94, 5003− 5010. (24) (a) Goto, H.; Osawa, E. J. Am. Chem. Soc. 1989, 111, 8950− 8951. (b) Goto, H.; Osawa, E. Tetrahedron Lett. 1992, 33, 1343−1346. (c) Goto, H.; Osawa, E. J. Chem. Soc., Perkin Trans. 2 1993, 187−198. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Gaussian, Inc.: Wallingford, CT, 2009.

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DOI: 10.1021/np5010483 J. Nat. Prod. XXXX, XXX, XXX−XXX