Axially chiral dicarboxylic acid catalyzed asymmetric semipinacol rearrangement of cyclic β-hydroxy-α-diazo esters

Tetrahedron 68 (2012) 7630e7635

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Axially chiral dicarboxylic acid catalyzed asymmetric semipinacol rearrangement of cyclic b-hydroxy-a-diazo esters Takuya Hashimoto, Shingo Isobe, Cedric K.A. Callens, Keiji Maruoka * Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 March 2012 Received in revised form 31 May 2012 Accepted 8 June 2012 Available online 15 June 2012

The development of axially chiral dicarboxylic acid catalyzed desymmetrizing asymmetric semipinacol rearrangement of symmetrically substituted six-membered cyclic b-hydroxy-a-diazo esters is reported as a means to give chiral cycloheptanones with good enantioselectivities. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Chiral Brønsted acid Rearrangement Diazo compounds Desymmetrization Ring expansion

1. Introduction Rearrangement of b-diazonium alcohols constitutes a fundamental part of TiffeneaueDemjanov rearrangement (Fig. 1, (1)),1 as well as the acid catalyzed reactions of diazo compounds with aldehydes or ketones (2).2e4 For the generation of this intermediate, protonation of preformed b-diazo alcohols would be considered as an alternative pathway to generate the same intermediate (3). Although there are some studies investigating this reaction system with acid catalysis,5e7 its use in asymmetric catalysis has yet to be explored.

We have recently established acid catalyzed asymmetric reactions of 4-substituted cyclohexanones and diazo esters, wherein desymmetrization of the 4-substituent via the diazonium intermediate I gives cycloheptanones having a chiral stereogenic center (Fig. 2).3b,d,e In this context, we envisaged that it would also be possible to generate a similar intermediate II by the protonation of symmetrically substituted cyclic b-hydroxy-a-diazo esters (cis-2) in the presence of a chiral Brønsted acid. In conjunction with our effort in the development of axially chiral dicarboxylic acid catalyzed reactions,8 we became interested in the realization of this plan as a novel example of chiral Brønsted acid catalyzed asymmetric semipinacol rearrangements.9

Fig. 1. Generation of b-diazonium alcohols.

* Corresponding author. Tel./fax: þ81 (0) 75 753 4041; e-mail address: maruoka @kuchem.kyoto-u.ac.jp (K. Maruoka). 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.06.033

Fig. 2. Desymmetrization via diazonium intermediates.

T. Hashimoto et al. / Tetrahedron 68 (2012) 7630e7635

2. Results and discussion As a starting point of this study, we examined several axially chiral dicarboxylic acids (R)-1 using cis-2a as a representative substrate (Table 1). The structure of cis-2a projecting phenyl and diazo ester moieties in the equatorial orientation was determined unambiguously by X-ray crystallographic analysis (Fig. 3).10 To faTable 1 Optimization of the reaction conditionsa

Entry

Cat.

Solvent

Conditions [ C, time]

% Yieldb

% eec

1 2 3 4 5d 6e 7 8 9 10f 11g

1a 1b 1c 1d 1d 1d 1d 1e 1e 1e 1e

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 Toluene Toluene Toluene

rt, 6 h rt, 3 min rt, 30 min rt, 3 min rt, 3 min rt, 3 min 40, 24 h 40, 48 h 40, 48 h 40, 48 h 40, 48 h

79 67 76 72 78 73 64 63 64 62 61

26 12 4 34 25 30 58 68 74 80 71

a b c d e f g

Reactions were performed with (R)-1 (0.05 mmol), cis-2a (0.10 mmol). Isolated yield. Determined by chiral HPLC analysis. tert-Butyl ester was used instead of 2a. Benzyl ester was used instead of 2a. H2O (0.10 mmol) was added. MS 4  A (30 mg) was added.

cilitate the isolation and the determination of the enantioselectivity, the initial rearranged product was converted to cycloheptanone 3a via the Krapcho decarboxylation.11 This preliminary study revealed a remarkable difference in the reactivity depending on the 3,30 -substituents of the catalyst. Namely, the use of the sterically encumbered (R)-1a bearing the 4-tert-butyl-2,6dimethylphenyl group led to the full consumption of cis-2a after 6 h at room temperature (entry 1). On the other hand, the reaction catalyzed by (R)-1b bearing a 3,5-diphenylphenyl group led to the instantaneous consumption of the substrate (entry 2). The electronic property of the aryl group also affected the reactivity, and the apparent deceleration was observed with the electron-rich catalyst (R)-1c (entry 3). Although axially chiral dicarboxylic acids actually promoted the desired reaction, the enantioselectivity was low irrespective of the catalyst structure. The modest yield is due to the

Fig. 3. Ortep representation of cis-2a with ellipsoids shown at 50% probability level. Hydrogen atoms are omitted for clarity.

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retro-aldol process, affording ethyl diazoacetate and 4phenylcyclohexanone. In the next experiment, the catalyst (R)-1d having a 3,5-di(trifluoromethyl)phenyl group gave cycloheptanone 3a in 72% yield with 34% ee (entry 4). Replacement of the ester moiety with a tert-butyl or benzyl ester was detrimental to the enantioselectivity (entries 5 and 6). The enantioselectivity could be improved to 58% ee by lowering the reaction temperature to 40  C (entry 7). This result then prompted us to develop a catalyst bearing 3,5-dinitrophenyl groups at the 3,30 -position. To this end, it was found to be necessary to develop a new synthetic scheme, which utilizes the SuzukieMiyaura coupling of 3,30 -diboryl dicarboxylic acid ester and the corresponding aryl bromide (Scheme 1). This strategy is complementary to the previous procedure, which uses 3,30 -dibromo dicarboxylic acid ester and arylboronic acids as substrates,8a,b and allowed us to access 3,5-dinitrophenyl substituted catalyst (R)-1e in good overall yield. This catalyst was found to be promising as cycloheptanone 3a could be obtained with 68% ee (entry 7). The enantioselectivity could be further increased by using toluene as solvent (entry 8). Addition of 1 equiv of water was found to be beneficial to attain higher enantioselectivity in a reproducible fashion (entry 10). Water is assumed to be intervening in the key hydrogen-bonding of the substrate and the catalyst.12 On the other hand, addition of molecular sieves had an adverse effect on the enantioselectivity (entry 11).

Scheme 1. Synthesis of axially chiral dicarboxylic acid (R)-1e. (a) (i) Mg(TMP)2, THF; (ii) B(OMe)3, THF, 65%; (b) 3,5-(NO2)2eC6H3Br, Pd(OAc)2, PPh3, NaHCO3, DMF, 62%; (c) TBAF, THF, 82%.

With the optimized reaction conditions in hand, we turned our attention to the investigation of the substrate scope (Table 2). Cyclic b-hydroxy-a-diazo esters 2bef bearing a variety of aryl groups could be utilized to give the corresponding cycloheptanones 3bef without affecting the enantioselectivity (entries 2e6). In addition, other functional groups, such as alkyl and siloxy groups were also tolerated (entries 7 and 8). The absolute configuration of the Table 2 Asymmetric semipinacol rearrangement of cyclic b-hydroxy-a-diazo estersa

Entry

R

1 2 3 4 5 6 7 8

Ph 4-tolyl 3-Tolyl 4-MeOC6H4 4-ClC6H4 2-Naphthyl t-Bu TBSO

% Yieldb 2a 2b 2c 2d 2e 2f 2g 2h

60 68 65 57 72 60 73 68

% eec 3a 3b 3c 3d 3e 3f 3g 3h

80 79 80 78 81 77 82 73 (S)

a Reactions were performed with (R)-1e (0.05 mmol), cis-2 (0.10 mmol), and H2O (0.10 mmol) in toluene (1.0 mL). b Isolated yield. c Determined by chiral HPLC or GC analysis.

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product was determined by comparison of the optical rotation of 3h with the literature.13 The reaction could be extended to cyclic b-hydroxy-a-diazo ester 4 having two methyl groups at the 3- and 5-positions in a cis fashion to give 3,5-cis-dimethylcycloheptanone 5 with modest enantioselectivity (Scheme 2).

cis-2a Scheme 4. Diastereoselective alkylation of the ring-expanded product.

3. Conclusions

Scheme 2. Use of cyclic b-hydroxy-a-diazo ester with two methyl groups.

Furthermore, we implemented the reaction by using trans-2 placing the 4-substituent and the hydroxyl group in a trans-fashion (Scheme 3). This study revealed a unique discrepancy between trans-2a having a phenyl group and trans-2h having an OTBS group. Whereas the former was found to be completely inert under the reaction conditions, the latter gave the desired product (ent-3h) with 77% ee, having the opposite optical rotation with that of the reaction using the corresponding cis-2h.

Scheme 3. Use of trans-cyclic b-hydroxy-a-diazo esters.

DFT calculations of the relative energy differences (B3LYP/631G(d) for trans-2a, and B3LYP/6-31G(d,p) for trans-2h) indicated that trans-2a with the axial diazo ester moiety and trans-2h with the equatorial diazo ester moiety are the favored conformations (Fig. 4). This observation, in line with the X-ray analysis of the reactive substrate cis-2a and cis-2h,10 underlines the importance of the chair conformer with the axial hydroxyl group and the equatorial diazo ester to trigger the rearrangement. Finally, we decided to exploit the b-keto ester generated in the initial rearrangement (Scheme 4). After treatment of cis-2a with (R)-1e, the reaction was directly subjected to phase-transfer catalyzed benzylation in one pot to give cyclic b-keto ester 6 having an all-carbon quaternary center. The reaction proceeded smoothly at room temperature to give the desired compound in 55% yield with high diastereoselectivity.3b

Fig. 4. Computed relative energy differences of trans-2.

In conclusion, we succeeded in developing axially chiral dicarboxylic acid catalyzed asymmetric semipinacol rearrangement of symmetrically substituted cyclic b-hydroxy-a-diazo esters. This reaction provided an organocatalytic access to chiral heptanones with modest to good enantioselectivities. 4. Experimental section 4.1. General Infrared (IR) spectra were recorded on a Shimadzu IRPrestige-21 spectrometer. 1H NMR spectra were measured on a JEOL JNMFX400 (400 MHz) spectrometer. Data were reported as follows: chemical shifts in parts per million from tetramethylsilane as an internal standard in CDCl3 or CD2HOD (d¼3.30) in CD3OD, integration, multiplicity (s¼singlet, d¼doublet, t¼triplet, q¼quartet, dd¼double-doublet, m¼multiplet), coupling constants (hertz). 13C NMR spectra were measured on a JEOL JNM-FX400 (100 MHz) spectrometer with complete proton decoupling. Chemical shifts were reported in ppm from the residual solvent as an internal standard. High performance liquid chromatography (HPLC) was performed on Shimadzu 10A instruments at 210 nm or 254 nm using 4.6 mm25 cm Daicel Chiralpak AD-H column. Analytical gas-liquid phase chromatography (GLC) was performed on Shimadzu GC-14B instruments equipped with a flame ionization detector using a Chirasil-DEX CB (25 m0.25 mm). High-resolution mass spectra (HRMS) were performed on Bruker microTOF. Optical rotations were measured on a JASCO DIP-1000 digital polarimeter. For thin layer chromatography (TLC) analysis throughout this work, Merck precoated TLC plates (silica gel 60 GF254, 0.25 mm) were used. The products were purified by flash column chromatography silica gel 60 (Merck, 230e400 mesh) or Merck precoated preparative thin layer chromatography (PTLC) plate (silica gel 60 GF254, 0.5 mm). In experiments requiring dry solvent, tetrahydrofuran, and diethyl ether were purchased from Kanto Chemical Co. Inc. as ‘Dehydrated’. Dichloromethane and toluene were stored over 4  A molecular sieves. Other simple chemicals were purchased and used as such. 4.2. Representative procedure for the preparation of starting materials To a stirred solution of 4-phenylcyclohexanone (1.5 mmol, 261 mg) and ethyl diazoacetate (1.8 mmol, 205 mg) in THF (2.0 mL) was added lithium diisopropylamide [prepared by the addition of butyllithium in hexane (1.95 mmol, 1.25 mL) to a solution of diisopropylamine (1.95 mmol, 0.27 mL) in Et2O (2.0 mL) at 0  C] dropwise at 78  C. After stirring for 3 h at 78  C, saturated aqueous NH4Cl was added to the solution. The organic layer was extracted with ethyl acetate, dried over Na2SO4, and concentrated. The residue was purified by column chromatography on silica gel with hexane/ethyl acetate¼20:1 to give cis-2a [40% (173 mg)] and trans-2a [21% (90 mg)].

T. Hashimoto et al. / Tetrahedron 68 (2012) 7630e7635

4.2.1. Ethyl cis-2-diazo-2-(1-hydroxy-4-phenylcyclohexyl)acetate (cis2a). Yellow solid; mp 65e67  C; 1H NMR (400 MHz, CDCl3) d 7.32e7.17 (5H, m), 4.26 (2H, q, J¼7.1 Hz), 3.58 (1H, s), 2.54 (1H, m), 2.22 (2H, m), 2.03 (2H, m), 1.77 (2H, m), 1.66 (2H, m), 1.30 (3H, t, J¼7.1 Hz); 13C NMR (100 MHz, CDCl3) d 167.2, 146.6, 128.3, 126.8, 126.0, 68.7, 60.8, 43.5, 36.3, 32.5, 28.8. 14.4; IR (neat) 3473, 2930, 2087, 1670, 1448, 1369, 1301, 1099, 746, 700 cm1; HRMS (ESI) exact mass calcd for C16H20N2O3: m/z 311.1367 ([MþNa]þ), found: m/z 311.1368 ([MþNa]þ). 4.2.2. Ethyl trans-2-diazo-2-(1-hydroxy-4-phenylcyclohexyl)acetate (trans-2a). Yellow oil; 1H NMR (400 MHz, CDCl3) d 7.32e7.18 (5H, m), 4.26 (2H, q, J¼7.1 Hz), 3.56 (1H, s), 2.62 (1H, m), 2.38 (2H, m), 1.93 (2H, m), 1.79 (2H, m), 1.47 (2H, m), 1.31 (3H, t, J¼7.1 Hz); 13C NMR (100 MHz, CDCl3) d 166.9, 145.6, 128.3, 126.7, 126.2, 71.6, 60.8, 42.8, 36.7, 30.8, 14.3; IR (neat) 3452, 2931, 2088, 1682, 1294, 1085 cm1; HRMS (ESI) exact mass calcd for C16H20N2O3: m/z 311.1367 ([MþNa]þ), found: m/z 311.1360 ([MþNa]þ). 4.2.3. Ethyl cis-2-diazo-2-(1-hydroxy-4-(4-methylphenyl)cyclohexyl) acetate (cis-2b). Yellow solid; mp 68e70  C; 1H NMR (400 MHz, CDCl3) d 7.17e7.09 (4H, m), 4.26 (2H, q, J¼7.2 Hz), 3.57 (1H, s), 2.50 (1H, m), 2.32 (3H, s), 2.21 (2H, m), 1.99 (2H, m), 1.74 (2H, m), 1.66 (2H, m), 1.30 (3H, t, J¼7.1 Hz); 13C NMR (100 MHz, CDCl3) d 167.3, 143.6, 135.5, 129.0, 126.7, 68.7, 60.8, 43.1, 36.4, 28.9, 20.9. 14.4; IR (neat) 3483, 2926, 2086, 1672, 1300, 1111, 1041 cm1; HRMS (ESI) exact mass calcd for C17H22N2O3: m/z 325.1523 ([MþNa]þ), found: m/z 325.1512 ([MþNa]þ). 4.2.4. Ethyl cis-2-diazo-2-(1-hydroxy-4-(3-methylphenyl)cyclohexyl) acetate (cis-2c). Yellow solid; mp 62e64  C; 1H NMR (400 MHz, CDCl3) d 7.19 (1H, t, J¼7.5 Hz), 7.08e7.00 (3H, m), 4.26 (2H, q, J¼7.1 Hz), 3.60 (1H, s), 2.51 (1H, m), 2.34 (3H, s), 2.21 (2H, m), 2.02 (2H, m), 1.75 (2H, m), 1.65 (2H, m), 1.31 (3H, q, J¼7.1 Hz); 13C NMR (100 MHz, CDCl3) d 167.4, 146.6, 137.9, 128.3, 127.6, 126.9, 123.9, 68.8, 60.8, 43.6, 36.5, 28.8, 21.4, 14.4; IR (neat) 3480, 2926, 2089, 1676, 1301, 1101, 1041 cm1; HRMS (ESI) exact mass calcd for C17H22N2O3: m/z 325.1523 ([MþNa]þ), found: m/z 325.1515 ([MþNa]þ). 4.2.5. Ethyl cis-2-diazo-2-(1-hydroxy-4-(4-methoxyphenyl)cyclohexyl) acetate (cis-2d). Yellow solid; mp 58e60  C; 1H NMR (400 MHz, CDCl3) d 7.17 (2H, m), 6.84 (2H, m), 4.26 (2H, q, J¼7.1 Hz), 3.78 (3H, s), 3.58 (1H, s), 2.49 (1H, m), 2.20 (2H, m), 1.97 (2H, m), 1.73 (2H, m), 1.65 (2H, m), 1.30 (3H, t, J¼7.1 Hz); 13C NMR (100 MHz, CDCl3) d 167.4, 157.9, 138.8, 127.7, 113.8, 68.7, 60.8, 55.2, 42.7, 36.5, 29.0, 14.4; IR (neat) 3483, 2935, 2086, 1676, 1512, 1301, 1247, 1111, 1039 cm1; HRMS (ESI) exact mass calcd for C17H22N2O4: m/z 341.1472 ([MþNa]þ), found: m/ z 341.1476 ([MþNa]þ). 4.2.6. Ethyl cis-2-diazo-2-(4-(4-chlorophenyl)-1-hydroxycyclohexyl) acetate (cis-2e). Yellow solid; mp 68e70  C; 1H NMR (400 MHz, CDCl3) d 7.26 (2H, m), 7.17 (2H, m), 4.26 (2H, q, J¼7.1 Hz), 3.60 (1H, s), 2.52 (1H, m), 2.21 (2H, m), 1.97 (2H, m), 1.74 (2H, m), 1.65 (2H, m), 1.30 (3H, q, J¼7.1 Hz); 13C NMR (100 MHz, CDCl3) 167.3, 145.0, 131.7, 128.4, 128.2, 68.6, 60.9, 43.0, 36.3, 28.8, 14.4; IR (neat) 3475, 2931, 2088, 1674, 1301, 1093 cm1; HRMS (ESI) exact mass calcd for C16H19ClN2O3: m/z 345.0976 ([MþNa]þ), found: m/z 345.0974 ([MþNa]þ). 4.2.7. Ethyl cis-2-diazo-2-(1-hydroxy-4-(2-naphthyl)cyclohexyl)acetate (cis-2f). Yellow solid; mp 110e112  C; 1H NMR (400 MHz, CDCl3) d 7.81e7.77 (3H, m), 7.67 (1H, s), 7.46e7.39 (3H, m), 4.27 (2H, q, J¼7.1 Hz), 3.63 (1H, s), 2.71 (1H, m), 2.26 (2H, m), 2.12 (2H, m), 1.85 (2H, m), 1.72 (2H, m), 1.31 (3H, t, J¼7.1 Hz); 13C NMR (100 MHz, CDCl3) d 167.4, 144.1, 133.6, 132.2, 127.9, 127.6, 127.5, 126.0, 125.9,

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125.2, 124.7, 68.8, 60.9, 43.7, 36.5, 28.8, 14.4; IR (neat) 3468, 2928, 2856, 2087, 1445, 1369, 1298, 1098, 1042 cm1; HRMS (ESI) exact mass calcd for C20H22N2O3: m/z 361.1523 ([MþNa]þ), found: m/z 361.1505 ([MþNa]þ). 4.2.8. Ethyl cis-2-diazo-2-(4-tert-butyl-1-hydroxycyclohexyl)acetate (cis-2g). Yellow solid; mp 40e42  C; 1H NMR (400 MHz, CDCl3) d 4.24 (2H, q, J¼7.2 Hz), 3.44 (1H, s), 2.13 (2H, m), 1.63 (2H, m), 1.56e1.43 (4H, m), 1.28 (3H, t, J¼7.1 Hz), 1.01 (1H, m), 0.87 (9H, s); 13 C NMR (100 MHz, CDCl3) d 167.4, 69.0, 60.7, 47.5, 36.7, 32.4, 27.5, 22.2, 14.4; IR (neat) 3471, 2953, 2087, 1674, 1367, 1298, 1099, 1035 cm1; HRMS (ESI) exact mass calcd for C14H24N2O3: m/z 291.1679 ([MþNa]þ), found: m/z 291.1679 ([MþNa]þ). 4.2.9. Ethyl cis-2-diazo-2-(4-(tert-butyldimethylsilyloxy)-1hydroxycyclohexyl)acetate (cis-2h). Yellow oil; 1H NMR (400 MHz, CDCl3) d 4.24 (2H, q, J¼7.2 Hz), 3.92 (1H, m), 3.51 (1H, m), 2.00 (2H, m), 1.88 (2H, m), 1.78 (2H, m), 1.49 (2H, m), 1.29 (3H, t, J¼7.1 Hz), 0.88 (9H, s), 0.04 (6H, s); 13C NMR (100 MHz, CDCl3) d 167.3, 69.8, 66.5, 60.7, 31.2, 29.6, 25.8, 18.0, 14.4, 4.9; IR (neat) 3487, 2930, 2087, 1676, 1369, 1300, 1254, 1032 cm1; HRMS (ESI) exact mass calcd for C16H30N2O4Si: m/z 365.1867 ([MþNa]þ), found: m/z 365.1868 ([MþNa]þ). 4.2.10. Ethyl trans-2-diazo-2-(4-(tert-butyldimethylsilyloxy)-1hydroxycyclohexyl)acetate (trans-2h). Yellow oil; 1H NMR (400 MHz, CDCl3) d 4.24 (2H, q, J¼7.2 Hz), 3.68 (1H, s), 3.36 (1H, s), 2.08 (2H, m), 1.82e1.60 (6H, m), 1.28 (3H, t, J¼7.1 Hz), 0.89 (9H, s), 0.05 (6H, s); 13C NMR (100 MHz, CDCl3) d 167.1, 69.2, 69.0, 60.8, 33.6, 30.8, 25.8, 18.1, 14.4, 4.7; IR (neat) 3477, 2931, 2089, 1678, 1369, 1300, 1253, 1094 cm1; HRMS (ESI) exact mass calcd for C16H30N2O4Si: m/z 365.1867 ([MþNa]þ), found: m/z 365.1873 ([MþNa]þ). 4.2.11. Compound 4. Yellow solid; mp 55e57  C; 1H NMR (400 MHz, CDCl3) d 4.24 (2H, q, J¼7.2 Hz), 3.54 (1H, s), 2.04 (2H, m), 1.94 (2H, m), 1.70 (1H, m), 1.29 (3H, t, J¼7.1 Hz), 1.03 (2H, t, J¼12.6 Hz), 0.90 (6H, d, J¼6.8 Hz), 0.56 (1H, q, J¼12.2 Hz); 13C NMR (100 MHz, CDCl3) d 167.4, 70.4, 60.7, 44.2, 43.2, 27.4, 22.0, 14.4; IR (neat) 3480, 2951, 2086, 1674, 1303, 1128, 1093 cm1; HRMS (ESI) exact mass calcd for C12H20N2O3: m/z 263.1366 ([MþNa]þ), found: m/z 263.1371 ([MþNa]þ).

4.3. Preparation of catalyst (R)-1e 4.3.1. Bis(2-(trimethylsilyl)ethyl) (R)-3,30 -bis(dihydroxyborane)-1,10 binaphthyl-2,20 -dicarboxylate. To a 0.25 M THF solution of Mg(TMP)2 (25 mmol, 100 mL) prepared according to the literature14 was added bis(2-(trimethylsilyl)ethyl) (R)-1,10 -binaphthyl2,20 -dicarboxylate (5.0 mmol, 2.71 g)8a,b in THF (10 mL) dropwise at 78  C. The mixture was then allowed to warm to room temperature and stirred for 2 h. After being cooled to 78  C, B(OMe)3 (30 mmol, 3.34 mL) was added and the reaction was stirred for 12 h at room temperature. This mixture was then poured into cold 1 M HCl (30 mL), and extracted with EtOAc. The organic layer was washed with water and brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was washed with hexane to give the title compound as a pale yellow solid [65% (2.04 g)]. Mp decomp. >250  C; 1H NMR (400 MHz, CD3OD) d 7.93 (2H, s), 7.90 (2H, d, J¼8.2 Hz), 7.45 (2H, m), 7.17 (2H, m), 6.96 (2H, d, J¼8.7 Hz), 3.68 (4H, m), 0.07 (4H, m), 0.27 (18H, s); 13C NMR (100 MHz, CD3OD) d170.7, 140.7, 136.1, 134.5, 131.9, 131.3, 129.1, 129.1, 128.2, 128.0, 64.6, 17.6, 1.8; IR (neat) 3399, 1699, 1419, 1386, 1276, 1134, 1058 cm1; HRMS (ESI) exact mass calcd for C32H40B2O8Si2: m/z

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681.2666 ([M2H2Oþ2MeOHþNa]þ), found: m/z ([M2H2Oþ2MeOHþNa]þ); ½a24 D 21.7 (c 1.0, MeOH).

681.2661

4.3.2. Bis(2-(trimethylsilyl)ethyl) (R)-3,30 -bis(3,5-dinitrophenyl)-1,10 binaphthyl-2,20 -dicarboxylate. To a Schlenk tube equipped with a rubber septum and a magnetic stir bar were added Pd(OAc)2 (0.050 mmol, 10.2 mg), triphenylphosphine (0.20 mmol, 52.5 mg), 3,5-dinitrobromobenzene (1.5 mmol, 308 mg), NaHCO3 (1.5 mmol, 126 mg), and bis(2-(trimethylsilyl)ethyl) (R)-3,30 -bis(dihydroxyborane)-1,10 -binaphthyl-2,20 -dicarboxylate (0.50 mmol, 315 mg). The Schlenk tube was then evacuated and backfilled with argon. After adding THF (2.0 mL) and water (1.0 mL) to the Schlenk tube, the rubber septum was replaced by a glass stopper and the reaction was stirred for 12 h at 60  C. The mixture was then cooled to room temperature and poured into saturated aqueous NH4Cl. The organic layer was extracted with EtOAc, dried over Na2SO4, and concentrated. The residue was purified by column chromatography on silica gel with hexane/EtOAc¼20:1 to give the title compound as a colorless oil [62% (271 mg)]. 1H NMR (400 MHz, CDCl3) d 9.07 (2H, t, J¼1.9 Hz), 8.70 (4H, d, J¼1.9 Hz), 8.08 (2H, s), 8.04 (2H, d, J¼8.2 Hz), 7.67 (2H, m), 7.50 (2H, m), 7.35 (2H, d, J¼8.7 Hz), 3.65 (4H, m), 0.09 (4H, m), 0.18 (18H, s); 13C NMR (100 MHz, CDCl3) d 167.1, 148.4, 144.5, 136.2, 133.1, 132.9, 132.8, 131.0, 130.3, 128.7, 128.6, 128.6, 128.3, 127.5, 117.6, 63.7, 16.6, 1.9; IR (neat) 1720, 1541, 1342, 1274, 1136 cm1; HRMS (ESI) exact mass calcd for C44H42N4O12Si2: m/z 897.2230 ([MþNa]þ), found: m/z 897.2216 ([MþNa]þ); ½a24 D þ5.0 (c 1.0, CHCl3). 4.3.3. (R)-3,30 -Bis(3,5-dinitrophenyl)-1,10 -binaphthyl-2,20 -dicarboxylic acid (R)-(1e). To a stirred solution of the ester (0.31 mmol, 271 mg) in THF (1.0 mL) was added a 1 M solution of tetrabutylammonium fluoride in THF (3.1 mmol, 3.1 mL). After stirring for 8 h at room temperature, the mixture was treated with 1 M HCl. The organic layer was extracted with EtOAc, dried over Na2SO4, and concentrated. The residue was purified by column chromatography on silica gel with hexane/EtOAc/AcOH¼1:1:0.005 to give (R)-1e as a white solid [82% (222 mg)]. Mp decomp. >200  C; 1H NMR (400 MHz, CD3OD) d 8.94 (2H, t, J¼1.9 Hz), 8.73 (4H, d, J¼1.8 Hz), 8.18 (2H, s), 8.03 (2H, d, J¼8.2 Hz), 7.55 (2H, m), 7.35 (2H, m), 7.17 (2H, d, J¼8.7 Hz); 13C NMR (100 MHz, CD3OD) d 171.1,149.8,146.0,136.9,134.7,134.4,134.0,133.0, 131.7, 129.9, 129.7, 129.3, 129.2, 128.3, 118.4; IR (neat) 2922, 1737, 1537, 1342 cm1; HRMS (ESI) exact mass calcd for C34H18N4O12: m/z 673.0837 ([MþNa]þ), found: m/z 673.0823 ([MþNa]þ); ½a25 D þ8.9 (c 0.5, CH3OH). 4.4. General procedure for the asymmetric semipinacol rearrangement of cyclic b-hydroxy-a-diazo esters

mass calcd for C13H16O: m/z 211.1093 ([MþNa]þ), found: m/z 211.1087 ([MþNa]þ); HPLC analysis: Daicel Chiralpak AD-H, hexane/i-PrOH¼50:1, flow rate¼0.5 mL/min, retention time 19.7 min (minor) and 29.8 min (major); ½a32 D 104.2 (c 0.84, CHCl3 (80% ee)). 4.4.2. (S)-4-(4-Methylphenyl)cycloheptanone (3b). Colorless oil; 1H NMR (400 MHz, CDCl3) d 7.13e7.04 (4H, m), 2.68 (2H, m), 2.61e2.54 (3H, m), 2.31 (3H, s), 2.11 (1H, m), 2.00 (2H, m), 1.86 (1H, m), 1.76 (1H, m), 1.62 (1H, m); 13C NMR (100 MHz, CDCl3) d 214.7, 144.6, 135.7, 129.2, 126.3, 48.3, 43.8, 42.9, 38.4, 32.0, 23.8, 20.9; IR (neat) 2924, 1699, 1514 cm1; HRMS (ESI) exact mass calcd for C14H18O: m/z 225.1250 ([MþNa]þ), found: m/z 225.1249 ([MþNa]þ); HPLC analysis: Daicel Chiralpak AD-H, hexane/i-PrOH¼50:1, flow rate¼0.5 mL/min, retention time 25.3 min (minor) and 29.1 min (major); ½a32 D 98.3 (c 0.82, CHCl3 (79% ee)). 4.4.3. (S)-4-(3-Methylphenyl)cycloheptanone (3c). Colorless oil; 1H NMR (400 MHz, CDCl3) d 7.19 (1H, t, J¼7.5 Hz), 7.03e6.97 (3H, m), 2.73e2.56 (5H, m), 2.34 (3H, s), 2.12 (1H, m), 2.06e1.98 (2H, m), 1.87 (1H, m), 1.75 (1H, m), 1.63 (1H, m); 13C NMR (100 MHz, CDCl3) d 214.7, 147.6, 138.1, 128.4, 127.3, 127.0, 123.5, 48.7, 43.8, 42.9, 38.4, 31.9, 23.9, 21.4; IR (neat) 2926, 1699, 1452 cm1; HRMS (ESI) exact mass calcd for C14H18O: m/z 225.1250 ([MþNa]þ), found: m/z 225.1241 ([MþNa]þ); HPLC analysis: Daicel Chiralpak AD-H, hexane/i-PrOH¼50:1, flow rate¼0.5 mL/min, retention time 16.5 min (minor) and 19.5 min (major); ½a32 D 78.2 (c 0.64, CHCl3 (80% ee)). 4.4.4. (S)-4-(4-Methoxyphenyl)cycloheptanone (3d). Colorless oil; 1 H NMR (400 MHz, CDCl3) d 7.10 (2H, m), 6.84 (2H, m), 3.79 (3H, s), 2.73e2.52 (5H, m), 2.08 (1H, m), 2.05e1.96 (2H, m), 1.88e1.70 (2H, m), 1.60 (1H, m); 13C NMR (100 MHz, CDCl3) d 214.7, 157.9, 139.8, 127.3, 113.9, 55.2, 47.9, 43.8, 42.9, 38.5, 32.1, 23.8; IR (neat) 2924, 1697, 1510, 1456, 1244, 1178, 1033 cm1; HRMS (ESI) exact mass calcd for C14H18O2: m/z 241.1199 ([MþNa]þ), found: m/z 241.1199 ([MþNa]þ); HPLC analysis: Daicel Chiralpak AD-H, hexane/iPrOH¼50:1, flow rate¼0.5 mL/min, retention time 35.0 min (minor) and 42.2 min (major); ½a32 D 92.7 (c 0.78, CHCl3 (78% ee)). 4.4.5. (S)-4-(4-Chlorophenyl)cycloheptanone (3e). Colorless oil; 1H NMR (400 MHz, CDCl3) d 7.25 (2H, m), 7.10 (2H, m), 2.71e2.51 (5H, m), 2.11e1.94 (3H, m), 1.87e1.69 (2H, m), 1.58 (1H, m); 13C NMR (100 MHz, CDCl3) d 214.3, 145.9, 131.8, 128.6, 127.8, 48.1, 43.7, 42.7, 38.3, 31.8, 23.7; IR (neat) 2924, 1697, 1490, 1091, 1012 cm1; HRMS (ESI) exact mass calcd for C13H15ClO: m/z 245.0704 ([MþNa]þ), found: m/z 245.0709 ([MþNa]þ); HPLC analysis: Daicel Chiralpak AD-H, hexane/i-PrOH¼50:1 flow rate¼0.5 mL/min, retention time 28.5 (minor) min and 33.2 min (major); ½a32 D 92.1 (c 0.79, CHCl3 (81% ee)).

To a solution of (R)-1e (0.005 mmol, 3.3 mg) in toluene (1.0 mL) and H2O (0.10 mmol, 1.8 mL) was added cis-2 (0.10 mmol) at 40  C, and the reaction was stirred for 48 h at 40  C. The solvent was then removed under reduced pressure. The residue was purified by PTLC with hexane/EtOAc¼4:1 to give the corresponding cyclic bketo ester. To a solution of thus-obtained cyclic b-keto ester in DMSO (1.0 mL) was added lithium chloride (0.10 mmol, 4.3 mg) at room temperature. The mixture was stirred at 160  C for 3 h. The resulting solution was poured into brine and extracted with EtOAc. The combined organic layers were dried over Na2SO4 and removed under reduced pressure. The residue was purified by PTLC with hexane/EtOAc¼4:1 to give 3.

4.4.6. (S)-4-(2-Naphthyl)cycloheptanone (3f). Colorless oil; 1H NMR (400 MHz, CDCl3) d 7.81e7.77 (3H, m), 7.61 (1H, s), 7.48e7.40 (2H, m), 7.33 (1H, dd, J¼8.5, 1.7 Hz), 2.86 (1H, m), 2.75 (1H, m), 2.66e2.60 (3H, m), 2.20 (1H, m), 2.15e1.93 (3H, m), 1.88e1.69 (2H, m); 13C NMR (100 MHz, CDCl3) d 214.6, 144.9, 133.6, 132.2, 128.1, 127.6, 126.0, 125.5, 125.4, 124.4, 48.8, 43.8, 42.9, 38.3, 31.8, 23.9; IR (neat) 2922, 1697, 1452, 1342 cm1; HRMS (ESI) exact mass calcd for C17H18O: m/z 261.1250 ([MþNa]þ), found: m/z 261.1251 ([MþNa]þ); HPLC analysis: Daicel Chiralpak AD-H, hexane/i-PrOH¼100:1, flow rate¼0.5 mL/min, retention time 21.7 min (minor) and 25.5 min (major); ½a32 D 80.2 (c 0.95, CHCl3 (77% ee)).

4.4.1. (S)-4-Phenylcycloheptanone (3a).3e Colorless oil; 1H NMR (400 MHz CDCl3) d 7.32e7.17 (5H, m), 2.65 (5H, m), 2.11 (1H, m), 2.03 (2H, m), 1.89 (1H, m), 1.79 (1H, m), 1.65 (1H, m); 13C NMR (100 MHz, CDCl3) d 214.7, 147.6, 128.5, 126.5, 126.2, 48.8, 43.8, 42.9, 38.4, 31.8, 23.8; IR (neat) 2924, 1699, 1452 cm1; HRMS (ESI) exact

4.4.7. (S)-4-(tert-Butyl)cycloheptanone (3g). Colorless oil; 1H NMR (400 MHz, CDCl3) d 2.49e2.34 (4H, m), 2.02 (1H, m), 1.94e1.88 (2H, m), 1.50 (1H, m), 1.25 (1H, m), 1.10e0.97 (2H, m), 0.81 (9H, s); 13C NMR (100 MHz, CDCl3) d 215.6, 52.0, 43.6, 43.1, 33.6, 31.0, 27.5, 25.6, 24.2; IR (neat) 2951, 1703, 1367 cm1; HRMS (ESI) exact mass calcd

T. Hashimoto et al. / Tetrahedron 68 (2012) 7630e7635

for C11H20O: m/z 191.1406 ([MþNa]þ), found: m/z 191.1400 ([MþNa]þ); GLC analysis: Chirasil-DEX CB (25 m0.25 mm) column (100  C isotherm, N2: 80 kPa, He: 80 kPa), retention time 42.2 min (minor) and 43.0 min (major); ½a32 D 80.2 (c 0.95 CHCl3 (82% ee)). 4.4.8. (S)-4-(tert-Butyldimethylsilyloxy)-cycloheptanone (3h).13 Colorless oil; 1H NMR (400 MHz, CDCl3) d 4.02 (1H, m), 2.84 (1H, m), 2.50 (1H, m), 2.41 (1H, m), 2.28 (1H, m), 1.98 (1H, m), 1.88e1.75 (3H, m), 1.68 (1H, m), 1.55 (1H, m), 0.91 (9H, s), 0.08 (6H, s); 13C NMR (100 MHz, CDCl3) d 214.8, 70.0, 43.7, 38.4, 37.1, 32.1, 25.7, 18.0, 17.9, 4.9, 4.9; IR (neat) 2929, 1703, 1467, 1255, 1080 cm1; HRMS (ESI) exact mass calcd for C13H26O2Si: m/z 265.1594 ([MþNa]þ), found: m/z 265.1588 ([MþNa]þ); ½a24 D 4.70 (c 0.41, CHCl3 (73% ee)).

7635

saturated aqueous NH4Cl. The organic layer was extracted with EtOAc, dried over Na2SO4 and concentrated. The residue was purified by PTLC with hexane/EtOAc¼4:1 to give 6 as a colorless oil [55% (19.3 mg)]. 1H NMR (400 MHz, CDCl3) d 7.29e7.10 (10H, m), 4.23 (2H, m), 3.23 (2H, s), 2.64 (1H, m), 2.53 (1H, m), 2.33 (1H, m), 2.17 (1H, m), 1.98e1.17 (5H, m), 1.29 (3H, t, J¼7.1 Hz); 13C NMR (100 MHz, CDCl3) d 210.3, 172.4, 147.1, 136.1, 130.5, 128.5, 128.2, 126.9, 126.5, 126.3, 64.0, 61.2, 47.7, 43.0, 43.0, 33.3, 33.3, 31.2, 14.1; IR (neat) 2932, 1730, 1701, 1450, 1228, 1201 cm1; HRMS (ESI) exact mass calcd for C23H26O3: m/z 373.1774 ([MþNa]þ), found: m/z 373.1789 ([MþNa]þ); HPLC analysis: Daicel Chiralpak AD-H, hexane/i-PrOH¼99:1, flow rate¼0.5 mL/min, retention time 36.6 min (minor) and 44.5 min (major); ½a32 D 23.1 (c 1.02, CHCl3 (79% ee)). Acknowledgements

4.4.8.1. Determination of the enantioselectivity of 3h. To a solution of 3h (0.067 mmol, 16.2 mg) in MeOH (1.0 mL) was added NaBH4 (0.79 mmol, 30 mg) at 0  C. After stirring for 1 h at room temperature, the mixture was quenched with saturated aqueous NaHCO3 and the organic layer was extracted with Et2O. The combined organic layers were dried over Na2SO4 and concentrated. The residue was purified by column chromatography on silica gel with hexane/Et2O¼5:1 to give the cycloheptanol as a diastereo mixture. To a solution of thus-obtained cycloheptanol in THF (1.0 mL) was added pyridine (2.0 mmol, 16 mL) and N,N-dimethyl-4aminopyridine (0.033 mmol, 4.0 mg). The mixture was added BzCl (0.10 mmol, 12 mL) at 0  C and stirred for 2 h at room temperature. The resulting solution was quenched with saturated aqueous NH4Cl and the organic layer was extracted with EtOAc. The combined organic layers were dried over Na2SO4 and concentrated. The residue was purified by PTLC with hexane/CH2Cl2¼1:1 to isolate the upper fraction as a diastereo pure 4-((tert-butyldimethylsilyl)oxy)-cycloheptyl benzoate (relative configuration not assigned) [47% (11.0 mg)]. Colorless oil; 1H NMR (400 MHz, CDCl3) d 8.04 (2H, d, J¼8.3 Hz), 7.54 (1H, t, J¼7.4 Hz), 7.43 (2H, t, J¼7.6 Hz), 5.17 (1H, m), 3.91 (1H, m), 2.11e2.00 (2H, m), 1.90e1.60 (7H, m), 1.36 (1H, m), 0.90 (9H, s), 0.053 (3H, s), 0.049 (3H, s); 13C NMR (100 MHz, CDCl3) d 165.9, 132.7, 131.0, 129.5, 128.2, 74.9, 71.5, 38.1, 33.9, 31.8, 27.4, 25.9, 18.5, 18.1, 4.8, 4.8; IR (neat) 2953, 2854, 1716, 1273, 1111, 1068 cm1; HRMS (ESI) exact mass calcd for C20H32O3Si: m/z 371.2013 ([MþNa]þ), found: m/z 371.2016 ([MþNa]þ); HPLC analysis: Daicel Chiralpak AD-H, hexane/iPrOH¼400:1 flow rate¼0.3 mL/min, retention time 18.4 min (minor) and 19.3 min (major); ½a32 D 7.4 (c 0.75, CHCl3 (73% ee)). 4.4.9. (3R,5R)-cis-3,5-Dimethylcycloheptanone (5). Colorless oil; 1H NMR (400 MHz, CDCl3) d 2.53e2.44 (2H, m), 2.37 (2H, m), 1.88 (1H, m), 1.75 (2H, m), 1.60 (1H, m), 1.37 (1H, m), 1.05 (1H, m), 0.98 (3H, d, J¼6.8 Hz), 0.95 (3H, d, J¼6.8 Hz); 13C NMR (100 MHz, CDCl3) d 214.2, 52.1, 48.2, 43.0, 35.9, 32.5, 30.8, 24.4, 24.2; IR (neat) 2954, 1699, 1456 cm1; HRMS (ESI) exact mass calcd for C9H16O: m/z 163.1093 ([MþNa]þ), found: m/z 163.1092 ([MþNa]þ); GLC analysis: ChirasilDEX CB (25 m0.25 mm) column (100  C isotherm, N2: 80 kPa, He: 80 kPa), retention time 11.6 min (minor) and 12.3 min (major); ½a32 D 60.4 (c 1.07, CHCl3 (65% ee)). 4.5. Preparation of ethyl (1S,5S)-1-benzyl-2-oxo-5-phenylcycl oheptanecarboxylate (6) To a solution of (R)-1e (0.005 mmol, 3.3 mg) in toluene (1.0 mL) and H2O (0.10 mmol, 1.8 mL) was added cis-2a (0.10 mmol, 28.8 mg) at 40  C and stirred for 48 h at 40  C. The mixture was then allowed to warm to room temperature. To this solution were added benzyl bromide (0.15 mmol, 18 mL), Bu4NBr (0.02 mmol, 6.4 mg) and 1 M aqueous KOH (0.2 mmol, 200 mL). After stirring for 12 h at room temperature, the resulting solution was quenched with

This work was partially supported by a Grant-in-Aid for Scientific Research from the MEXT, Japan. T.H. thanks a Grant-in-Aid for Challenging Exploratory Research. Supplementary data Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.tet.2012.06.033. References and notes 1. Tiffeneau, M.; Weill, P.; Tchoubar, B. Compt. Rend. 1937, 205, 54e56. 2. For reviews, see: (a) Zhang, Y.; Wang, J. Chem. Commun. 2009, 5350e5361; (b) Johnston, J. N.; Muchalski, H.; Troyer, T. L. Angew. Chem., Int. Ed. 2010, 49, 2290e2298. 3. For our works on this subject, see: (a) Hashimoto, T.; Naganawa, Y.; Maruoka, K. J. Am. Chem. Soc. 2008, 130, 2434e2435; (b) Hashimoto, T.; Naganawa, Y.; Maruoka, K. J. Am. Chem. Soc. 2009, 131, 6614e6617; (c) Hashimoto, T.; Miyamoto, H.; Naganawa, Y.; Maruoka, K. J. Am. Chem. Soc. 2009, 131, 11280e11281; (d) Hashimoto, T.; Naganawa, Y.; Maruoka, K. Chem. Commun. 2010, 6810e6812; (e) Hashimoto, T.; Naganawa, Y.; Maruoka, K. J. Am. Chem. Soc. 2011, 133, 8834e8837. 4. For other recent works, see: (a) Moebius, D. C.; Kingsbury, J. S. J. Am. Chem. Soc. 2009, 131, 878e879; (b) Wommack, A. J.; Moebius, D. C.; Travis, A. L.; Kingsbury, J. S. Org. Lett. 2009, 11, 3202e3205; (c) Rendina, V. L.; Moebius, D. C.; Kingsbury, J. S. Org. Lett. 2011, 13, 2004e2007; (d) Li, W.; Wang, J.; Hu, X.; Shen, K.; Wang, W.; Chu, Y.; Lin, L.; Liu, X.; Feng, X. J. Am. Chem. Soc. 2010, 132, 8532e8533. € llkopf, U.; Ba nhidai, B.; Frasnelli, H.; Meyer, R.; Beckhaus, H. Liebigs Ann. 5. (a) Scho brault, D.; Uguen, D.; De Cian, A.; Fischer, J. Chem. 1974, 1767e1783; (b) He Tetrahedron Lett. 1998, 39, 6703e6706. 6. (a) Pellicciari, R.; Fringuelli, R.; Ceccherelli, P.; Sisani, E. J. Chem. Soc., Chem. Commun. 1979, 959e960; (b) Nagao, K.; Chiba, M.; Kim, S.-W. Synthesis 1983, 197e199; (c) Ye, T.; McKervey, M. A. Tetrahedron 1992, 48, 8007e8022. 7. (a) Pellicciari, R.; Natalini, B.; Sadeghpour, B. M.; Rosato, G. C.; Ursini, A. J. Chem. Soc., Chem. Commun. 1993, 1798e1800; (b) Pellicciari, R.; Natalini, B.; Sadeghpour, B. M.; Marinozzi, M.; Snyder, J. P.; Williamson, B. L.; Kuethe, J. T.; Padwa, A. J. Am. Chem. Soc. 1996, 118, 1e12; (c) Gioiello, A.; Venturoni, F.; Natalini, B.; Pellicciari, R. J. Org. Chem. 2009, 74, 3520e3523. 8. (a) Hashimoto, T.; Maruoka, K. J. Am. Chem. Soc. 2007, 129, 10054e10055; (b) Hashimoto, T.; Maruoka, K. Synthesis 2008, 3703e3706; (c) Hashimoto, T.; Hirose, M.; Maruoka, K. J. Am. Chem. Soc. 2008, 130, 7556e7557; (d) Hashimoto, T.; Uchiyama, N.; Maruoka, K. J. Am. Chem. Soc. 2008, 130, 14380e14381; (e) Hashimoto, T.; Kimura, H.; Maruoka, K. Tetrahedron: Asymmetry 2010, 21, 1187e1188; (f) Hashimoto, T.; Kimura, H.; Maruoka, K. Angew. Chem., Int. Ed. 2010, 49, 6844e6847; (g) Hashimoto, T.; Takagaki, T.; Kimura, H.; Maruoka, K. Chem. dAsian J. 2011, 6, 1936e1938; (h) Hashimoto, T.; Kimura, H.; Nakatsu, H.; Maruoka, K. J. Org. Chem. 2011, 76, 6030e6037; (i) Hashimoto, T.; Omote, M.; Maruoka, K. Angew. Chem., Int. Ed. 2011, 50, 3489e3492; (j) Hashimoto, T.; Omote, M.; Maruoka, K. Angew. Chem., Int. Ed. 2011, 50, 8952e8955; (k) Hashimoto, T.; Kimura, H.; Kawamata, Y.; Maruoka, K. Nat. Chem. 2011, 3, 642e646. 9. (a) Zhang, Q. W.; Fan, C. A.; Zhang, H. J.; Tu, Y. Q.; Zhao, Y. M.; Gu, P.; Chen, Z. M. Angew. Chem., Int. Ed. 2009, 48, 8572e8574; (b) Liang, T.; Zhang, Z.; Antilla, J. C. Angew. Chem., Int. Ed. 2010, 49, 9734e9736. 10. CCDC 868829 (cis-2a) and CCDC 868830 (cis-2h) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. 11. (a) Krapcho, A. P.; Glynn, G. A.; Grenon, B. J. Tetrahedron Lett. 1967, 8, 215e217; (b) Elsinger, F. In Organic Syntheses; Wiley: New York, 1973; Collect. Vol. 5, pp 76e81. 12. Ube, H.; Fukuchi, S.; Terada, M. Tetrahedron: Asymmetry 2010, 21, 1203e1205. 13. Hiroya, K.; Ogasawara, K. J. Chem. Soc., Chem. Commun. 1995, 2205e2206. 14. Ooi, T.; Uematsu, Y.; Maruoka, K. J. Org. Chem. 2003, 68, 4576e4578.