Unprecedented 8,9′-Neolignans: Enantioselective Synthesis of

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Unprecedented 8,9′-Neolignans: Enantioselective Synthesis of Possible Stereoisomers for Structural Determination Masato Takahashi,†,∥ Noriyuki Suzuki,† Tsutomu Ishikawa,*,† Hung-Yi Huang,‡ Hsun-Shuo Chang,‡,§ and Ih-Sheng Chen*,‡,§ †

Graduate School of Pharmaceutical Sciences, Chiba University, 1-8-1 Inohana, Chuo, Chiba 260-8675, Japan Graduate Institute of Natural Products, College of Pharmacy, and §School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, Kaohsiung 807, Taiwan, Republic of China

‡

S Supporting Information *

ABSTRACT: (+)-Wutaienin (3) and its C-7 methyl ether (4), isolated from Zanthoxylum wutaiense, were found to be unprecedented 8,9′-neolignans containing an (S)-2-(1,1-dimethyl-1-hydroxymethyl)-7-methoxydihydrobenzofuran skeleton. Wutaienin (3) was present in the plant as an inseparable 1:1 mixture of the (7,8)-syn-diastereoisomers. The diastereoisomeric mixture was characterized by comparison with four possible diastereoisomers, which were enantioselectively synthesized from (S)-5-bromo-(1,1-dimethyl-1hydroxymethyl)-7-methoxydihydrobenzofuran using Evans’ oxazolidinoneassisted asymmetric aldol condensation to install the chiral centers at the C-7 and C-8 positions.

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3a−3d. The diastereoisomers were enantioselectively synthesized from (S)-5-bromo(1,1-dimethyl-1-hydroxymethyl)-7methoxydihydrobenzofuran (15) using Evans’ oxazolidinoneassisted asymmetric aldol condensation to install the chiral centers at the C-7 and C-8 positions. (+)-Wutaienin (3) was previously isolated from the root wood of Z. wutaiense as a pale yellow, viscous oil in a 1.1 × 10−2% yield.11 The molecular formula was deduced to be C30H40O8 from the EIMS (m/z 528), which corresponded to a wutaiensol (1) (C15H20O4) dimer. The IR spectrum showed a strong hydroxy absorption at 3590 cm−1. The 1H NMR spectrum (270 MHz; Figure 1S, Supporting Information) suggested the presence of two 5-substituted 2-(1,1-dimethyl-1hydroxymethyl)-7-methoxydihydrobenzofuran units [δ 1.21, 1.22, 1.36, and 1.38 (3H each, s), 3.00−3.28 (4H, m), 3.87 and 3.88 (3H each, s), 4.66 and 4.67 (1H each, t, J = 9.2 Hz), 6.68 (1H, s), 6.76 (2H, s), and 6.78 and 6.80 (1H total, each s, ArH)], the same prenylated phenyl unit as wutaiensol (1). In addition, the moiety between the two propyl units was assigned as (Ar)CH(O)CH(C)CH2(O) [δ 1.90−2.22 (1H, m), 3.75 (2H, dd, J = 11.0, 6.0 Hz), and 4.65 (1H, d, J = 7.8 Hz)], and the remaining unit as (E)-(Ar)CHCHCH2(C) [δ 1.90−2.22 (2H, m), 5.92 (1H, dt, J = 16.2, 7.5 Hz), and 6.25 (1H, d, J = 16.2 Hz)]. Decoupling experiments suggested that the connection is between the center methine carbon (C-8) in the (Ar)CH(O)CH(C)CH2(O) unit and the methylene carbon (C-9′) in the (E)-(Ar)CHCHCH2(C) unit. The total of 28 carbon signals (6 × CH3, 4 × CH2, 3 × CH, and 1 × C in the aliphatic and 8 × CH and 6 × C in the aromatic regions) in the 13 C NMR spectrum (67.5 MHz; Figure 2S, Supporting

imeric phenylpropanoids are common natural products found in plants.1 Lignan moieties, which consist of propylbenzene units, feature a bond between the C-8 carbons of phenylpropanoid monomers. Examples include the 2aryltetralin lignan, (−)-podophyllotoxin,2 which has antitumor activity, and the 2,6-diarylperhydrofurofuran, (+)-sesamin,3 which is an active compound found in sesame seeds (Sesamum indicum). In neolignans,4 the monomeric phenylpropanoid units contain bonds between each phenyl ring and the propyl groups. For example, honokiol5 is a 3,3′-neolignan (phenyl− phenyl bond) with central nervous system depressant activity, and kadsurenone6 is a 3,8′-neolignan (phenyl−propyl bond) with activity as a platelet-activating factor antagonist. Over 30 years ago, our group reported the isolation of new monomeric prenylated phenylpropanoids and the related compounds from the root wood of Zanthoxylum wutaiense I.S. Chen,7 a rutaceous plant endemic to Taiwan.8 Some of the compounds showed piscicidal7 and antitubercular activities.9,10 Typical examples of the isolates are (+)-wutaiensol (1) and (+)-wutaialdehyde (2), which contain an (S)-2-(1,1-dimethyl1-hydroxymethyl)-7-methoxydihydrobenzofuran skeleton as the prenylated phenyl unit.7 A dimerized wutaiensol product was also discovered, named (+)-wutaienin (3), in the more polar fraction, and this product was tentatively identified as an unprecedented 8,9′-neolignan containing a new type of propyl−propyl bond connection.11 However, the structure was not fully determined, even after chemical modifications, because of spectroscopic discrepancies. Recently, (+)-wutaienin (3) was reisolated from the stem wood of Z. wutaiense, together with the methylated homologue, (+)-wutaienin methyl ether (4). In this paper, the elucidation of wutaienin as an inseparable mixture of the syn-isomers (7S,8S)-3a and (7R,8R)-3b is described by comparison with four possible diastereoisomers, © XXXX American Chemical Society and American Society of Pharmacognosy

Received: August 11, 2014

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Figure 1. Structures of (+)-wutaiensol (1), (+)-wutaialdehyde (2), (+)-wutaienin (3), and (+)-wutaienin methyl ether (4).

Scheme 1. Chemical Modifications of Wutaienin (3)

Information) was consistent with the 1H NMR spectroscopic assignments when signal overlap was taken into account. The configuration of the two dihydrobenzofuran units was inferred as being the same as the S-configuration in the corresponding monomeric (+)-wutaiensol (1) used in biosynthesis. Thus, diastereoisomers 3a−3d were selected as candidate compounds for wutaienin (Figure 1). To confirm this assignment, wutaienin (3) was chemically modified (Scheme 1). The diacetate 5 and its dihydro derivative 6 were obtained in addition to the oxidatively cleaved aldehydes, (+)-wutaialdehyde (2) and 7. Aldehyde 7 showed positive Cotton effects in its CD spectrum, similar to (+)-wutaialdehyde (2), suggesting the dihydrobenzofuran unit has an S-configuration despite the presence of additional C-7 and C-8 chiral centers (CD [θ]max (MeOH): +4763 (301 nm) for 2; +7897 (240 nm) for 7). Furthermore, the 1,3-glycol functionality in wutaienin (3) was confirmed by the formation of cyclic acetal 8. The large coupling constant (J = 10.2 Hz) between the diaxial ring methine protons in the 1H NMR spectrum (Figure 18S, Supporting Information) indicated synstereochemistry, similar to acetal 8a or 8b. This suggested that the wutaienin candidate is either 3a, where all the chiral centers

were S, or the diastereomeric (7R,8R,2″S,2‴S)-isomer 3b. However, unassignable signals were found in the 1H NMR spectrum [for 3, δ 6.78 and 6.80 (1H total, each s, ArH) (Figure 1S, Supporting Information); for 5, 5.69 (1H dd, J = 8.4, 2.7 Hz, H-7), 6.20 and 6.22 (1H total, each d, J = 15.7 Hz, H-7′) (Figure 11S, Supporting Information); for 6, 5.64 and 5.65 (1H total, each d, J = 8.5 Hz, H-7′) (Figure 13S, Supporting Information); for 8, 6.83 and 6.85 (1H total, each s, ArH) (Figure 18S, Supporting Information)] and inexplicable carbon distributions in the aromatic region of the 13C NMR spectra of wutaienin (3) and its chemical derivatives [for 3, 8 × CH and 6 × C (Figure 2S, Supporting Information); for 8, 9 × CH and 7 × C (Figure 19S, Supporting Information)]. These discrepancies suggested that wutaienin may be a mixture of the syn-isomers (7S,8S)-3a and (7R,8R)-3b. Recent access to the chemical constituents of the stem wood of the same plant led to the reisolation of (+)-wutaienin (3) (1.2 × 10−4% yield) and the isolation of the (+)-methylated homologue 4 (4.4 × 10−5% yield), which was assigned as the C7 methyl ether of 3 based on the NMR spectra (Figures 7S− 10S, Supporting Information).12 The previous structural assignment for 3 based on 270 MHz NMR spectra (Figures B

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Scheme 2. Preparation of (S)-Dihydrobenzofuran 15 from Bromophenol 9 via Shi’s Epoxidation

Scheme 3. Preparation of (7S,8S,2″S,2‴S)-syn (3a) and (7R,8S,2″S,2‴S)-anti (3c) Isomers from (S)-Dihydrobenzofuran 15 through Evans’ Asymmetric Aldol Condensation Using the (R)-Oxazolidinone Auxiliary as a Key Step

constructed by Evans’ asymmetric aldol condensation16 of the 5-(benzofuran-5-yl)-4-pentenoic acid derivative (16) containing a chiral oxazolidinone with (+)-wutaialdehyde (2). These reaction partners can be derived from a common key intermediate, (S)-5-bromo-2-(1,1-dimethyl-1-hydroxymethyl)7-methoxydihydrobenzofuran (15), by a Heck reaction or by formylation, respectively. 4-Bromo-2-methoxyphenol17 (9) was propargylated, reduced with Lindlar’s catalyst, and subjected to a Claisen rearrangement to provide prenylated phenol 12. The phenol in 12 was protected with a tert-butyldimethylsilyl (TBS) group, and

1S−2S, Supporting Information) was supported by the higher resolution NMR (600 MHz) spectra (Figures 3S−6S, Supporting Information); however, no definitive information was obtained for determining the final structure. Therefore, four candidates, 3a−3d, were synthesized to help determine the structure of wutaienin. An (S)-2-(1,1-dimethyl-1-hydroxymethyl)-7methoxydihydrobenzofuran fragment 15 would be prepared by a Shi asymmetric epoxidation13−15 of the corresponding prenylated phenol derivative (13) (see Scheme 2). The C-7 and C-8 stereogenic centers of wutaienin (3) would be C

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Scheme 4. Possible Biosynthesis of a Unprecedented 8,9′-Neolignan, (+)-Wutaienin (3)

configuration of wutaienin (3). In addition, the discrepancy in the NMR spectra suggested that natural wutaienin is a mixture of both syn-diastereoisomers, (7S,8S,2″S,2‴S)-3a and (7R,8R,2″S,2‴S)-3b. Overlaying their 1H NMR spectra produced a spectrum identical to that of the natural product (Figures 56S and 57S, Supporting Information). Accordingly, it has been concluded that wutaienin was obtained an inseparable 1:1 mixture of the (7,8)-syn-isomers 3a and 3b, identified as an unprecedented 8,9′-neolignan. It should be noted that natural wutaienin has never been separated into its components, even by chiral HPLC. Epimerization was observed at the C-7 position resulting in a 1:1 mixture of syn-3a and anti-3c when a solution of syn-3a in chloroform was sonicated at 50 °C for 3 h in the presence of silica gel. These facts strongly suggest that natural wutaienin is composed of noninterconvertible synisomers. The 1H NMR spectrum of (+)-wutaienin methyl ether (4) resembled the signal pattern of wutaienin (3), particularly in the olefinic proton region at 5.5−7.0 ppm, although some signals appeared at different chemical shifts due to the presence of an additional methoxy group (Figure 7S, Supporting Information). This suggested that the wutaienin methyl ether (4) was also the same 1:1 syn mixture of (7S,8S, 2″S,2‴S)-4a and (7R,8R, 2″S,2‴S)-4b. Wutaienin (3) and its C-7 methyl ether (4), isolated from Z. wutaiense, were found to be 8,9′-neolignans containing an unprecedented (S)-2-(1,1-dimethyl-1- hydroxymethyl)-7methoxydihydrobenzofuran skeleton in a 1:1 inseparable mixture of (7S,8S,2″S,2‴S)- and (7R,8R,2″S,2‴S)-isomers. These 8,9′-neolignans may be biosynthesized by the following reaction sequences (Scheme 4): (1) dimerization of coniferyl alcohols (19) through bond formation between conjugated anion species 20, generated by the deprotonation of a phenolic hydrogen, and vinylogous quinone methide 21, produced by dehydration at the C-8 and the C-9 positions; (2) syn-selective hydration of the C−C bonded dimer 22; (3) prenylation; and (4) stereospecific cyclization to form the (S)-2-(1,1-dimethyl-1hydroxymethyl)dihydrobenzofuran skeleton. The possible presence of structurally related dimeric phenylpropanoids, such as demethoxy-congeners of 3, has been identified in this plant source, although they are not yet fully characterized. Therefore, 8,9′-neolignans may be found in plants other than Z. wutaiense.

asymmetric epoxidation using the Shi catalyst {(5S,8R,9R)-2,2dimethyl-10-oxo- 1,3,6-trioxaspiro[4.5]decane-8,9-diyl diacetate} afforded the (R)-epoxide 14 in 81% yield with 91% ee (Figure 58S, Supporting Information). Epoxide 14 underwent spontaneous cyclization with inversion of the chiral center14 to produce a quantitative yield of the desired (S)-(+)-dihydrobenzofuran 15 during TBS-deprotection with a fluoride reagent (Scheme 2). First, the synthesis of the (7S,8S)-syn-isomer 3a and the corresponding (7R,8S)-anti-one 3c were examined (Scheme 3). The aldol reaction partners (+)-wutaialdehyde (2) and (S)(benzofuranyl)pentenoyl-(R)-oxazolidinone ((S,R)-16) were prepared from 15. Compound 2 was prepared by lithiating 15 and by treatment with dimethylformamide. In turn, (S,R)-16 was prepared by treating 15 with (R)-N-(pent-4-enoyl)oxazolidin-2-one in the presence of palladium acetate. Although the aldol reactions for preparing the syn-isomer 3a under various conditions (TMSCl/MgCl/Et3N,18 Cy2BOTf/Et3N,19 Bu2BOTf/DIPEA/Et2AlCl20) failed, the reaction catalyzed by Bu2BOTf16 in the presence of Hünig’s base afforded an alternative anti condensation product 17c as a single isomer in 81% yield. Reduction with sodium borohydride yielded the (7R,8S)-anti-isomer 3c. Inversion at the C-7 position of anti17c was achieved by the Mitsunobu reaction with 4nitrobenzoic acid to provide the (7S,8S)-syn-isomer 3a after a hydride reduction. Isomers 3a and 3c showed the same TLC Rf value, and their 1H NMR spectra looked similar (Figures 46S and 50S, Supporting Information). However, the signals assigned to H-7 [δ 4.65−4.69 (m)], H-7′ [δ 6.25 (d, J = 15.0 Hz)], and H-8′ [δ 5.93 (ddd, J = 15.0, 7.8, 7.2 Hz)] in syn3a appeared at slightly higher chemical shifts than those in anti3c [δ 4.94 (br s, H-7), 6.33 (d, J = 15.6 Hz, H-7′), and 6.00 (ddd, J = 15.6, 7.8, 7.2 Hz, H-8′)] (Figures 54S and 55S and Table 1S, Supporting Information). On comparing these 1H NMR spectra with that of natural wutaienin, it was found that neither 3a nor 3c was identical to natural wutaienin, although the syn-isomer 3a showed a similar signal pattern. Therefore, the (7R,8R,2″S,2‴S)-syn-isomer 3b was prepared along with its C-7 epimer 3d. Using pentenoyl-(S)oxazolidinone (S,S)-16 in the key Evans’ asymmetric aldol condensation produced the intended products; however, no complete structural identification could be made. The large coupling constant (J = 10.2 Hz) between the ring methine protons in cyclic acetal derivative 8 indicated the (7,8)-syn D

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, NMR spectra for compounds 2−8 and 10−18 including the overlaying spectra of synthetic and natural 3, and NMR data as tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(T. Ishikawa) Tel: (+81)-(0)436-756611. Fax: (+81)-(0)436756611. E-mail: [email protected]. *(I. S. Chen) Tel: (+886)-(0)7-3121101, ext. 2191. Fax: (+886)-(0)7-3210683. E-mail: [email protected]. Present Address ∥

Faculty of Pharmacy, Chiba Institute of Science, 15-8 Shiomicho, Choshi, Chiba 288-0025, Japan. Notes

The authors declare no competing financial interest.

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REFERENCES

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