546
F. Teraoka et al.
LETTER
1009; American Chemical Society: Washington DC, 2009,
31–56.
same for both cases in terms of the enantioselectivity
of the formation of the axially chiral enolates [i.e. the ab-
solute configuration (axial chirality) of C is same as that
of D].9
(6) Basel, Y.; Hassner, A. J. Org. Chem. 2000, 65, 6368.
(7) Brunner, M.; Saarenketo, P.; Straub, T.; Rissanen, K.;
Koskinen, A. M. P. Eur. J. Org. Chem. 2004, 3879.
(8) The most stable conformer B was generated by a molecular
modeling search (MCMM 50,000 steps) with OPLS 2005
force field and GB/SA solvation model for chloroform using
MacroModel (V. 9.0); see Supporting Information.
(9) The possibility that the present asymmetric migration
proceeds without the intervention of an axially chiral enolate
cannot be excluded. Alternative route may involve a
concerted SEi process. This route was excluded by the
experimental results in the case of asymmetric cyclization
shown in Scheme 5 (refs. 2a and 2c). By analogy, we assume
that the present asymmetric carbonyl migration would
proceed through an axially chiral enolate intermediate.
(10) (a) Mermerian, A. H.; Fu, G. C. J. Am. Chem. Soc. 2003,
125, 4050. (b) Shaw, S. A.; Aleman, P.; Vedejs, E. J. Am.
Chem. Soc. 2003, 125, 13368. (c) Shaw, S. A.; Aleman, P.;
Christy, J.; Kampf, J. W.; Va, P.; Vedejs, E. J. Am. Chem.
Soc. 2006, 128, 925.
In conclusion, we have developed asymmetric carbonyl
migration of the N-tert-butoxycarbonylcarbamates of a-
amino acid derivatives. This process provides a-amino
acid derivatives with an additional ester group at the new-
ly formed tetrasubstituted stereogenic center in a highly
enantioselective manner. Compounds with similar struc-
tural characteristics have been reported to be prepared by
catalytic C-acylation of lactone enolates10 or asymmetric
Sommelet–Hauser rearrangement.11 The present method
provides a unique and new entry to this class of amino
acid derivatives from readily available usual amino
acids.12
Supporting Information for this article is available online at
(11) (a) Takayama, E.; Nanbara, S.; Nakai, T. Chem. Lett. 2006,
35, 478. (b) Takayama, E.; Kimura, H. Angew. Chem. Int.
Ed. 2007, 46, 8869.
References and Notes
(12) One-Pot Procedure for 2a (Table 2): A solution of Boc2O
(105 mg, 0.48 mmol) in DMF (1.0 mL) was added to a
solution of 3 (R = Bn; 120 mg, 0.40 mmol) and DMAP (5.0
mg, 0.04 mmol) in DMF (4.1 mL) at r.t. After being stirred
for 30 min, the mixture was cooled to –60 °C, KHMDS (0.47
M in THF solution, 1.3 mL, 0.60 mmol) was added dropwise
to the mixture. The reaction mixture was stirred at –60 °C for
3 h and then poured into sat. aq NH4Cl and extracted with
EtOAc. The combined organic layers were washed with sat.
aq NaHCO3 and brine, dried over Na2SO4, filtered and
evaporated in vacuo. The residue was purified by preparative
TLC (SiO2, hexane–EtOAc = 9:1) to give (R)-2a (82 mg,
53%, 98% ee) as a colorless oil.
(1) For asymmetric intermolecular alkylation via memory of
chirality, see: (a) Kawabata, T.; Yahiro, K.; Fuji, K. J. Am.
Chem. Soc. 1991, 113, 9694. (b) Kawabata, T.; Wirth, T.;
Yahiro, K.; Suzuki, H.; Fuji, K. J. Am. Chem. Soc. 1994,
116, 10809. (c) Kawabata, T.; Suzuki, H.; Nagae, Y.; Fuji,
K. Angew. Chem. Int. Ed. 2000, 39, 2155.
(2) For asymmetric intramolecular alkylation via memory of
chirality, see: (a) Kawabata, T.; Kawakami, S.; Majumdar,
S. J. Am. Chem. Soc. 2003, 125, 13012. (b) Kawabata, T.;
Matsuda, S.; Kawakami, S.; Monguchi, D.; Moriyama, K.
J. Am. Chem. Soc. 2006, 128, 15394. (c) Kawabata, T.;
Moriyama, K.; Kawakami, S.; Tsubaki, K. J. Am. Chem.
Soc. 2008, 130, 4153.
(3) For asymmetric intramolecular conjugate addition via
memory of chirality, see: Kawabata, T.; Majuumdar, S.;
Tsubaki, K.; Monguchi, D. Org. Biomol. Chem. 2005, 3,
1609.
(4) For Dieckmann condensation via memory of chirality, see:
Watanabe, T.; Kawabata, T. Heterocycles 2008, 76, 1593.
(5) For recent reviews on asymmetric synthesis via memory of
chirality see: (a) Kawabata, T.; Fuji, K. Top. Stereochem.
2003, 53, 175. (b) Zhao, H.; Hsu, D.; Carlier, P. R. Synthesis
2005, 1. (c) Kawabata, T. Asymmetric Synthesis and
Application of a-Amino Acids, ACS Symposium Series
HPLC conditions: Daicel Chiralpak OJ-H; hexane–i-PrOH,
9:1; flow 0.5 mL/min; tR = 8.4 (R), tR = 9.9 (S); [a]D25 +2.6
(c = 2.1, CDCl3). 1H NMR (600 MHz, CDCl3): d = 7.33–7.38
(m, 5 H), 7.17–7.24 (m, 5 H), 5.91 (ddt, J = 15.1, 9.6, 5.5 Hz,
1 H), 5.20 (d, J = 15.1 Hz, 1 H), 5.16 (ABq, JAB = 12.3 Hz,
Dn = 10.6 Hz, 2 H), 5.08 (d, J = 9.6 Hz, 1 H), 3.22–3.29 (m,
1 H), 3.26 (ABq, JAB = 14.4 Hz, Dn = 18.4 Hz, 2 H), 3.19 (dd,
J = 13.1, 5.5 Hz, 1 H), 1.29 (s, 9 H). 13C NMR (150 MHz,
CDCl3): d = 170.1, 168.5, 135.9, 135.7, 135.2, 130.3, 128.9,
128.53, 128.49, 127.9, 126.8, 116.1, 82.6, 70.8, 67.1, 45.9,
36.8, 27.7. IR (CDCl3): 1728, 1456, 1369, 1190, 1151 cm–1.
ESI–MS (+): m/z = 418 [M + Na], 340, 278, 204. HRMS:
m/z calcd for C24H29NO4Na: 418.1994; found: 418.1953.
Synlett 2011, No. 4, 543–546 © Thieme Stuttgart · New York