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Enantioselective Synthesis of Masked Benzoquinones Using Designer Chiral Hypervalent Organoiodine(III) Catalysis Muhammet Uyanik, Niiha Sasakura, Masahiro Mizuno, and Kazuaki Ishihara ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03380 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016
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ACS Catalysis
Enantioselective Synthesis of Masked Benzoquinones Using Designer Chiral Hypervalent Organoiodine(III) Catalysis Muhammet Uyanik, Niiha Sasakura, Masahiro Mizuno and Kazuaki Ishihara* Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan. ABSTRACT: A highly enantioselective oxidative dearomatization of ortho- and para-hydroquinone derivatives to the corresponding masked ortho- and para-benzoquinones has been achieved by using chiral organoiodine(III) catalysts. Importantly, the remote steric effects of the hydrogen bonding designer organoiodine catalysts allowed us to achieve high chemo- and enantioselectivity for the oxidative dearomatization of para-hydroquinone derivatives through an associative iodine(III)–phenoxide intermediate. KEYWORDS: dearomatization, masked benzoquinones, hypervalent iodine, enantioselective catalysis, hydrogen bonds
Masked benzoquinones (MBs), cyclohexa-2,4- or 2,5dienone derivatives, are highly attractive synthons for the synthesis of various natural products and biologically active compounds, since they can be used for various transformations such as 1,2- and 1,4-additions, Diels–Alder reactions, sigmatropic rearrangement, etc.1 Conventionally, these compounds are synthesized by oxidation of the corresponding ortho- or para-alkoxyphenols using hypervalent iodine(III) reagents (Scheme 1a).1 However, most of the existing methods give racemic or achiral products. Recently, a substrate-controlled methodology has been developed for the synthesis of optically active masked ortho-benzoquinones (MOBs) through the hypervalent iodine(III)-mediated oxidative dearomatization of phenols O-tethered to a chiral ethanol2a,b or sugar unit2c,d (R2 in Scheme 1a) at the ortho-position. However, to the best of our knowledge, a reagent or catalyst-controlled strategies for the enantioselective synthesis of MOBs or masked parabenzoquinones (MPBs) using chiral hypervalent iodine(III) compounds have not yet been developed. This might be attributed to the fact that the racemic path would readily proceed via the dissociation of chiral organoiodine during the oxidation of phenols bearing strongly electron-donating ortho- or paraalkoxy substituents (Scheme 1b).1c,3 Moreover, the distance of the developing stereocenter from chiral environment created by O-bound iodine(III) makes the oxidative dearomatization of para-hydroquinone derivatives highly challenging (Scheme 1b).4 Recently, we developed chiral hypervalent iodine(III) catalysts, which are generated in situ from conformationally flexible iodines(I) with meta-chloroperbenzoic acid (m-CPBA), for the enantioselective oxidative dearomatization of phenols to give the corresponding spirolactones.5,6 Moreover, we achieved rational control of the desired associated pathway for iodine(III)-catalyzed oxidative dearomatization reactions using alcohol additives such as methanol.5c X-Ray and NMR analyses of in situ-generated organoiodines(III) show that a suitable chiral environment around the iodine(III) center is constructed via intramolecular hydrogen-bonding interactions between acidic amido protons and ligands of iodine(III)
(Scheme 1d).5c Based on these findings, we envisioned that our designer organoiodine(III) catalysis could be applied to the enantioselective oxidative dearomatization of phenols O- tethered to an acetic acid or ethanol unit at the ortho- or parapositions (Scheme 1c). This “tether strategy” would realize Scheme 1. Reaction Design for the Enantioselective Synthesis of MBs a) Previous methods OH
O
O
OR3
PhIL 2 OR2
R1
o- or p-hydroquinone derivatives
OR2
*
R1
R 3OH
natural products
R1 * R 3O OR2 MPB
MOB
Conventional: racemic or achiral (R 2 = R 3) Diastereoselective: enantioenriched substrates (R 2 is chiral)2 b) Challenges for enantioselective dearomatization to MBs O I(L)Ar*
O
Ar*(L)I–O
O OR2 or
R1
OR2
fast
R1 OR2
associative
far
or
R1
–ArI, –L
R1
R 3OH ?
R 3OH
Enantioenriched MBs
Racemic MBs
c) This work: First enantioselective oxidative dearomatization to MBs R2
OH
O iodine(I) precat.
O R1
O
OH
R1
mCPBA Ar*(L)I–O R1
H O O
R2
O O
R2 R2 ortho- or para"tether strategy"
OR2
dissociative
MOBs
O or
O R1 O O R2 2 R O MPBs
high chemo- and enantioselectivities
via intramolecular cyclization d) Conformationally flexible designer chiral hypervalent iodine catalysts L Y H I Y I H H N [O] L N O O N Y O O Y N in situ H extented iodine(I) precatalyst
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folded iodine(III) active species
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dissociation of the chiral iodine moiety. Here, we report the first enantioselective oxidative dearomatization of hydroquinone derivatives to give the corresponding MBs with high enantioselectivities using chiral organoiodine(III) catalysts. First, we examined the oxidative cyclization of 1,2naphthohydroquinone derivative 2a tethered to a carboxylic acid moiety (Scheme 2).7 To our delight, the reaction of 2a using 10 mol% of precatalyst 1a5c in the presence of 1.2 equivalents of m-CPBA as an oxidant in distilled ethanol-free chloroform at –20 °C gave the desired dioxolanone-type MOB 3a in 88% yield with 86% ee. In sharp contrast, the use of lactate-derived conformationally less-flexible diamide 1b5a,b,8 or diester 1c,9 which contained no acidic proton on its side chain, gave low reactivities and enantioselectivities. These preliminary results highlight the substantial scope of a conformationally flexible hydrogen-bonding organoiodine(III) catalyst for the challenging oxidative dearomatization of hydroquinone derivatives.
ysis.11 The absolute stereochemistries of other dioxolanones 3 were determined to be (S) through analogy. Table 1. Enantioselective Dearomatization to Dioxolanone-type MOBs 3 OH
O
OH O
OH
O O 3a
CHCl 3, –20 ºC, 12 h
2a
O
O
entry
2 (R)
1 2 3 4 5d
2b (3-Me-6-iPr) 2c (3,5-Me2) 2d (4,6-tBu2) 2e (4-Br-5-iPr) 2f (4-iPr-6-SiMe3)
Mes
I N H
O
O O
1a: 88% yield, 86% ee
N H
O Mes X
I O
3, yield,a ee (%, %) Method Ab Method Bc 82, 40 95, 91 90, 56 90, 84 99, 66 86, 96 78, 84 75, 95 92, 79 93, 92e
O
O 1a (10 mol%) m-CPBA (1.2 equiv)
OH
O O
O
CHCl 3, –20 ºC, 12 h
X
2g
Br
Encouraged by our initial success with 1-naphthols, we next examined the oxidation of much more challenging phenols (Table 1). The reaction of phenol 2b under conditions identical to those for 2a gave the desired 3b in good chemical yield, but with low enantioselectivity (Method A, 40% ee, entry 1). Fortunately, the enantioselectivity could be improved to 89% ee through the use of an excess amount of methanol (60 equiv) as an additive (Method B, entry 1). Similarly, the oxidation of various substituted phenols 2c–f gave the corresponding MOBs 3c–f with high enantioselectivities (up to 96% ee) in the presence of methanol (entries 2–5). As in our previous studies, methanol might improve the enantioselectivity as a ligand of I(III) by suppressing the dissociative pathway.5c,6e However, a prolonged reaction time was required for oxidations in the presence of methanol, since methanol suppressed the regeneration of I(III) species from I(I). Interestingly, the oxidation of 2f proceeded efficiently at 0 °C in the presence of a much lower amount of methanol (10 equiv) to give 3f with high enantioselectivity (entry 5). Notably, bulky trimethylsilyl as a protecting group was introduced at the 6-position of 2f to induce high enantioselectivity. These MOBs 3a–f are stable enough for isolation. However, 3g from the oxidation of 2g was unstable and could not be isolated (Scheme 3). Hence, after the oxidation of 2g was completed, methyl vinyl ketone was added to the same flask to trap unstable MOB 3g in situ.5c,10 As a result, the Diels–Alder adducts 4a and 4b were obtained in 67% combined yield with 97% ee for each diastereomer (Scheme 3). Interestingly, high enantioselectivity was observed for the oxidation of 2g even in the absence of methanol. However, the diastereoselectivity was low for the cycloaddition step. The relative and absolute stereochemistry of 4b was confirmed by X-ray structural anal-
3
Scheme 3. Oxidation of 2g to 3g and Tandem Cycloaddition OH
1b (X = NHMes): 22% yield, 61% ee 1c (X = OEt): 14% yield, 27% ee
O
O
Isolated yield. b w/o MeOH, 12 h. c w/ MeOH (60 equiv), 72 h. At 0 °C, 3 h (Method A), 4 h (Method B). e MeOH (10 equiv).
O O
O
R
CHCl 3, –20 ºC Method A or B
2
O O
O
1a (10 mol%) m-CPBA (1.2 equiv)
OH
2
R
d
O
1 (10 mol%) m-CPBA (1.2 equiv)
O
1
a
Scheme 2. Enantioselective Dearomatization of 2a
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Br
3g (unstable)
O
O
Ac
O O
O
Br
O O
+
O
RT, 12 h
Ac Br
O
4a, 97% ee* 4b, 97% ee* 67% combined yield (2 steps), 4a:4b = 1.4:1 * >99% ee after a single recrystallization
= (ORTEP)
As mentioned above, some of the dioxolanone-type MOBs were found to be epimerized and decomposed due to easy ring-opening of the dioxolanone moiety.12 Therefore, we sought to identify spiroketal-analogue 6 as a much more stable MOB, which would be obtained from the oxidation of phenols 5 derived from 1,2-diol (Scheme 4). In contrast to phenolic carboxylic acids,5,6 oxidative dearomatizative cyclization of alcohols using hypervalent iodine compounds has rarely been exploited.2,13 For the development of a highly enantioselective method to obtain spiroketals 6, various issues need to be addressed. First, undesired competitive intermolecular attack of excess methanol or meta-chlorobenzoic acid (m-CBA), which is generated in situ from m-CPBA during the reaction, might proceed to give undesired ortho-quinone acyclic ketal derivatives. Moreover, the enantioselective cyclodearomatization of phenolic alcohols has not been reported to date. In this case, the changes in catalyst-substrate interactions due to the lower acidity of a hydroxyl group compared to that of a carboxylic acid would influence the requisite chiral environment around the iodine(III) center. Scheme 4. Reaction Design for Spiroketals as a Stable MOB R2
O R1
chemo- and enantioselectivity? OH ArI O OH O R1 R1 [O] R2 R2 Spiroketal 6 (stable) 5 O
O O
O
Dioxolanone 3 (unstable)
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O
R2
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To identify whether our conformationally flexible chiral organoiodine(III) catalysis could be applied to the oxidative cyclization of alcohols 5, first, we investigated the oxidation of 1-naphthol derivatives (Table 2). The reaction of 4-bromo1-naphthol 5a in the presence of 1a and m-CPBA under the same conditions as in Scheme 2 gave the corresponding spiroketal 6a in 70% yield with 13% ee, along with m-CBA adducts and their decomposed products, ortho-quinones (entry 1). As expected, in contrast to the oxidative cyclization of carboxylic acids 2, the use of methanol as an additive was not appropriate for the oxidative cyclization of alcohol 5. Indeed, a complex mixture that included an unidentified methanol adduct was observed and 6a was isolated in only low yield with low enantioselectivity (entry 2). On the other hand, the rate of the intramolecular cyclization was accelerated with the introduction of two propyl groups (5b) instead of two methyl groups, and the corresponding 6b was obtained in higher yield after a shorter reaction time (entry 3). However, the enantioselectivity was not improved. Next, the reaction conditions for 5b as a model substrate were optimized using a non-halogenated solvent such as toluene (entries 4–8). Surprisingly, the oxidation of 5b with m-CPBA proceeded efficiently in the absence of catalyst (entry 5), which could explain the low enantioselectivities observed. The background reaction could be moderately suppressed in a toluene-water (or phosphate buffer, pH 7.0) biphasic system (entry 6). To our delight, the oxidation of 5b in a mixed solvent gave 6b in 79% yield with 86% ee (entry 7). Finally, the enantioselectivity was improved to 89% ee with pre-sonication of 5b and 1a in toluene/H2O before the addition of m-CPBA (entry 8). Table 2. Enantioselective Dearomatization of 5 R
OH O
OH
R R Br
O
1a (10 mol%) m-CPBA (1.2 equiv)
O
R
O
Solvent, 0 ºC
R = Me (5a) n-Pr (5b)
Br
6a or 6b
enantioselectivities (entries 1–5). On the other hand, the oxidation of low-reactive phenols 5h–j using Method D gave the corresponding MOBs 6h–j with high enantioselectivities (entries 6–8). Interestingly, the use of slightly modified new precatalyst 1d (Scheme 5) was found to be superior to the use of 1a for some cases. Especially, this remote electronic effect was found to enhance enantioselectivity for the oxidation of 5f–h in a toluene-buffer (pH 7.0) biphasic solvent (shown in parentheses for entries 4–6).14 Inspired by Quideau’s report,2b spiroketal 6h was successfully converted to bis(monoterpene) (–)-biscarvacrol, which is also known as a synthetic intermediate for chamaecypanone C, a novel microtubule inhibitor.15,16 Table 3. Enantioselective Dearomatization to Spiroketals 6 OH
O
OH O
R
n-Pr
OH
1a (10 mol%) m-CPBA (1.2 equiv)
6c–g
Method C or D, 0 ºC
OH
O
O R
n-Pr
n-Pr n-Pr O
R
n-Pr
5c–g
O
O
R
n-Pr
5h–j
n-Pr n-Pr
O
6h–j
entry
5 (R)
1 2 3 4 5 6 7 8
5c (H) 5d (4-Cl) 5e (4-Ph) 5f (4-Me) 5g (6-MeO) 5h (4-iPr-6-SiMe3) 5i (3,5,6-Me3-4-Cl) 5j [4-Br-5,6-(CH2)4]
method,a time (h) C, 12 C, 18 C, 12 C, 11 C, 11 D, 12 D, 24 D, 24
yield (%)b
ee (%)
60 (56)c 81 61 (60)c 66 (62)c 53 (50)c 54 (60)c 67 80
86 (88)c 93 89 (91)c 82 (88)c 84 (88)c 75 (78)c 91 90
a
entry 1 2b 3 4 5c 6c 7 8
5 5a 5a 5b 5b 5b 5b 5b 5b
solvent CHCl3 CHCl3 CHCl3 Toluene Toluene Toluene/H2Od Toluene/H2Od Toluene/H2Od,e
time (h) 12 12 4 10 5 5 10 10
yield (%) 70 44 87 88 78 45 79 82
a
ee (%) 13 12 17 18 – – 86 89
a
Isolated yield. b MeOH (60 equiv) was used. c In the absence of 1a. d Toluene/H2O (5/1, v/v). e The mixture of 5b and 1a in toluene/H2O was sonicated for 1 h at room temperature before the addition of m-CPBA.
To explore the generality and scope of the new enantioselective oxidative cyclization of alcohols to spiroketal-type MOBs 6, various 1-naphthol and phenol derivatives 5 were prepared and examined under optimized conditions (Table 3). Two solvent systems that are well mixed via pre-sonication were developed: Method C (Toluene/H2O 5:1 v/v) and Method D (CHCl3/H2O 5:1 v/v). For the oxidation of high-reactive 1naphthols 5c–g, which are prone to uncatalyzed reactions, Method C was found to be optimal and the corresponding MOBs 5c–g were obtained in good to high yield with high
Method C: Pre-sonicated toluene/H2O (5/1, v/v). Method D: Pre-sonicated CHCl3/H2O (5/1, v/v). b Isolated yield. c Precatalyst 1d was used instead of 1a in toluene/buffer (pH 7.0) (5/1, v/v), 24 h.
Scheme 5. Precatalyst 1d O
F 3C
I N H
O
O O
1d
N H
CF 3
Finally, our hydrogen bonding organoiodine catalysis could be successfully applied to the enantioselective oxidative dearomatization of para-hydroquinone derivatives that is developing stereocenter far from chiral environment created by phenoxide-bound iodine(III) (Table 4). Oxidation of 2-bromo1,4-hydroquinone derivative 7a tethered to a carboxylic acid moiety in the presence of precatalyst 1a gave the dioxolanonetype MPB 8a in 68% isolated yield, albeit with low enantioselectivity (entry 1). A number of unidentified side products were also observed. In contrast to that of MOBs, the use of lactate derived precatalyst 1b, which is conformationally less flexible, afforded 8a with higher enantioselectivity (entry 2 versus Scheme 2). Proximal and distal steric hindrances of C2symmetric organoiodine catalysts (Scheme 6) had opposite
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effects on the both chemo- and enantioselectivities. Although precatalyst 1e bearing isopropyl groups at adjacent stereocenters gave inferior results (entry 3), both chemical yield and enantioselectivity of 8a were increased by using precatalysts 1f–h bearing sterically congested terminal secondary amides derived from m-terphenylamines (entries 4–6). Thus, these remote steric effect of terminal groups of the iodine(III) catalyst might allow constructing a requisite deep chiral cavity for the challenging para-dearomatization4 through folding of iodine(III) catalyst via intramolecular hydrogen bonding interactions. Moreover, substituent at ortho-position of phenol was found to be critical to improve both chemo- and enantioselectivity (entries 7 and 8). To our delight, oxidation of 7c, bearing sterically hindered TBDPS group as a removable auxiliary, in the presence of 1h gave desired MPB 8c in 87% yield with 89% ee (entry 8).16 Notably, oxidation to spiroketal type MPBs by using para-analogues of 5 has been failed and a complex reaction mixture was obtained. Table 4. Oxidation of para-Hydroquinones to MPBs 8
O O
MeOH (10 equiv) CH2Cl2, 0 °C, 24 h
OH
O
entry 1 2 3 4 5 6 7b 8b
O O
7
8
7 (R) 7a (Br) 7a (Br) 7a (Br) 7a (Br) 7a (Br) 7a (Br) 7b (TBS) 7c (TBDPS)
precat. 1 1a 1b 1e 1f 1g 1h 1h 1h
yield (%)a 68 49 39 70 70 84 90 87
ee (%) 24 46 10 52 61 63 82 89
a
Isolated yield. b ClCH2CH2Cl and EtOH were used instead of CH2Cl2 and MeOH, respectively.
Scheme 6. Precatalysts 1e–h O
I O
N H
O O
N H
1e Ar
Ar
N H
O
Ar O
I O
O
N H
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedure, characterization data, copies of NMR spectra and HPLC traces (PDF).
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Kazuaki Ishihara: 0000-0003-4191-3845 Muhammet Uyanik: 0000-0002-9751-1952
R
1 (10 mol%) m-CPBA (1.2 equiv)
chemo- and enantioselectivity for the oxidative dearomatization of para-hydroquinone derivatives through an associative iodine(III)–phenoxide intermediate. A simple application of these masked benzoquinones to the enantioselective synthesis of a bis(monoterpene) natural product, such as (–)-biscarvacrol, highlights the substantial synthetic scope of the present enantioselective dearomatization with tether strategy.
ORCID ID
O
OH R
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Ar
Ar = Ph (1f), 3,5-Me2C6H 3 (1g) 4-[3,5-(CF3)2C6H 3]C6H 4 (1h)
In conclusion, we have achieved the first chiral organoiodine(III)-catalyzed highly enantioselective oxidative dearomatization of hydroquinone derivatives O-tethered to an acetic acid or ethanol unit at the ortho- or para-positions to give the corresponding MOBs or MPBs in moderate to high yields with high to excellent enantioselectivities. Importantly, the “tether strategy” would realize rapid intramolecular cyclization enantioselectively prior to dissociation of the chiral iodine moiety. Moreover, the remote steric effects of the hydrogen bonding designer organoiodine catalysts allowed us to achieve high
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT Financial support for this project was partially provided by CREST from JST, JSPS.KAKENHI (24245020, 15H05755, 15H05484), the Program for Leading Graduate Schools: IGER Program in Green Natural Sciences (MEXT), and JSPS Research Fellowships for Young Scientists (N.S.).
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Int. Ed. 2008, 47, 3787–3790. (b) Quideau, S.; Lyvinec, G.; Marguerit, M.; Bathany, K.; Ozanne-Beaudenon, A.; Buffeteau, T.; Cavagnat, D.; Chenede, A. Angew. Chem., Int. Ed. 2009, 48, 4605– 4609. (c) Boppisetti, J. K.; Birman, V. B. Org. Lett. 2009, 11, 1221– 1223. (d) Dohi, T.; Takenaga, N.; Nakae, T.; Toyoda, Y.; Yamasaki, M.; Shiro, M.; Fujioka, H.; Maruyama, A.; Kita, Y. J. Am. Chem. Soc. 2013, 135, 4558–4566. (e) Bosset, C.; Coffinier, R.; Peixoto, P. A.; El Assal, M.; Miqueu, K.; Sotiropoulos, J.-M.; Pouységu, L.; Quideau, S. Angew. Chem., Int. Ed. 2014, 53, 9860–9864. (f) Murray, S. J.; Ibrahim, H. Chem. Commun. 2015, 51, 2376–2379. (g) Zhang, D.-Y.; Xu, L.; Wu, H.; Gong, L.-Z. Chem. Eur. J. 2015, 21, 10314–10317. (h) Uyanik, M.; Sasakura, N.; Kaneko, E.; Ohori, K.; Ishihara, K. Chem. Lett. 2015, 44, 179–181. (i) Bekkaye M.; Masson, G. Synthesis 2016, 48, 302–312. (7) Berney, D.; Deslongchamps, P. Can. J. Chem. 1969, 47, 515– 519. (8) (a) Farid, U.; Wirth, T. Angew. Chem., Int. Ed. 2012, 51, 3462– 3465. (b) Farid, U.; Malmedy, F.; Claveau, R.; Albers, L.; Wirth, T. Angew. Chem., Int. Ed. 2013, 52, 7018–7022. (c) Mizar, P.; Wirth, T. Angew. Chem., Int. Ed. 2014, 53, 5993–5997. (d) Basdevant, B.; Legault, C. L. Org. Lett. 2015, 17, 4918–4921. (e) Haubenreisser, S.; Wöste, T. H.; Martínez, C.; Ishihara, K.; Muñiz, K. Angew. Chem., Int. Ed. 2016, 55, 413–417. (f) Monár, I. G.; Gilmour, R. J. Am. Chem. Soc. 2016, 138, 5004–5007. (9) (a) Fujita, M.; Yoshida, Y.; Miyata, K.; Wakisaka, A.; Sugimura, T. Angew. Chem., Int. Ed. 2010, 49, 7068–7071. (b) Fujita, M.; Wakita, M.; Sugimura, R. Chem. Commun. 2011, 47, 3983–3985. (c) Röben, C.; Souto, J. A.; Gonzáles, Y.; Lishchynskyi, A.; Muñiz, K. Angew. Chem., Int. Ed. 2011, 50, 9478–9482. (d) Fujita, M.; Mori, K.; Shimogaki, M. Sugimura, T. Org. Lett. 2012, 14, 1294–1297. (e)
W. Kong, P. Feige, T. de Haro, C. Nevado, Angew. Chem. Int. Ed. 2013, 52, 2469–2473. (f) Shimogaki, M.; Fujita, M.; Sugimura, T. Eur. J. Org. Chem. 2013, 7128–7138. (g) Banik, S. M.; Medley, J. W.; Jacobsen, E. N. Science 2016, 353, 51–54. (h) Woerly, E. M.; Banik, S. M.; Jacobsen, S. M. J. Am. Chem. Soc. 2016, 138, 13858– 13861. (10) Selected representative examples for related Diels–Alder reactions of MOBs: (a) Drutu, I.; Njardarson, J. T.; Wood, J. L. Org. Lett. 2002, 4, 493–496. (b) Gagnepain, J.; Méreau, R.; Dejugnac, D.; Léger, J.-M.; Castet, F.; Deffieux, D.; Pouységu, L.; Quideau, S. Tetrahedron 2007, 63, 6493–6505. (11) X-ray crystallographic data for compound 7b has been deposited with the Cambridge Crystallographic Data Centre database (http://www.ccdc.cam.ac.uk/) under code CCDC 1437470. (12) Unfortunately, attempts to realize the enantioselective synthesis of (+)-biscarvacrol from requisite dioxolanone-type MOB (S)-3f, which could be obtained with higher enantioselectivity than that of (R)-6h, gave irreproducible results due to the instability of the dioxolanone moiety under these conditions. (13) An example using iodine(III)-mediated cyclodearomatization of alcohol as a key step for the natural product synthesis: Cook, S. P.; Polara, A.; Danishefsky, S. J. J. Am. Chem. Soc. 2006, 128, 16440– 16441. (14) To improve the enantioselectivity of 6h, which is a requisite spiroketal for (–)-biscarvacrol, we investigated several precatalysts. Unfortunately, however, no further increase in ee was achieved. (15) Dong, S.; Hamel, E.; Bai, R.; Covell, D. G.; Beutler, J. A.; Porco, J. A., Jr. Angew. Chem., Int. Ed. 2009, 48, 1494–1497. (16) For details, see Supporting Information.
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ACS Catalysis
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