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1.5 hr at ambient temperature, and extracted with three 100-mL portions of diethyl ether. The organic layer is washed with 50 mL of water, dried over anhydrous magnesium sulfate, and concentrated with a rotary evaporator. The crude oil it subjected to column chromatography using 100 g of silica gel (Note 17). After the first fraction, eluted with 800 mL of hexane, is removed, the second fraction, eluted with 500 mL of diethyl ether, is concentrated (Note 18). Recrystallization of the residue from diethyl ether-hexane gives 8.03 g (65%) of material, mp 79-80°C (Note 19). The mother liquor is concentrated and again subjected to column chromatography (Note
20) to give the same material, which, after recrystallization from diethyl ether-hexane, melts at 77-79°C (1.5 g, 12%). The total yield amounts to 77%.
15. Hexamethylphosphoric triamide is distilled from calcium hydride under reduced pressure of nitrogen. In place of hexamethylphosphoric triamide, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)pyrimidinone (DMPU), which is dried and purified similarly,7 can be used.
16. TASF was prepared according to the procedure of reference 3. Typically, diethylamino(trimethyl)silane (6.4 g, 8.3 mL, 44 mmol) is added drop by drop under a dry inert atmosphere to an ethereal solution (20 mL) of diethylaminosulfur trifluoride (DAST, purchased from Aldrich Chemical Company, Inc., and used directly) (3.2 g, 2.4 mL, 20 mmol) under cooling with a dry ice/acetone bath. The mixture is allowed to warm to room temperature and stirred for 72 hr at room temperature. The initial homogeneous solution separates into two layers. The upper layer is removed with the aid of a syringe. The lower layer is washed with dry ether (10 mL x 3) and dried under reduced pressure to afford TASF as a solid (6.0 g, 16.6 mmol, 83% yield). All the isolation operations should be carried out under an inert atmosphere such as nitrogen. The solid is dissolved in THF to give a 1-M solution (the volume of the solution is 16.6 mL) which is stored under a dry nitrogen atmosphere.
17. A glass column (35 mm x 20 cm) packed with Wakogel C-200 is used.
18. A 400-MHz 1H NMR analysis of the crude oil showed exclusive formation of the threo isomer of the material (>99%).
19. Spectral characteristics are as follows: 1H NMR (CDCI3) 8: 1.22 (d, J = 7.3 H), 1.1-1.7 (m, 6 H), 2.8-3.8 (m, 5 H), 4.7-4.8 (m, 2 H), 7.31 (s, 5 H); IR (KBr) cm1: 3380, 1606; MS (rel intensity) m/z 247 (M+; 6), 232 (16), 141 (100), 112 (23), 84 (39), 79 (15). Anal. Calcd for C15H2iN02: C, 72.84; H, 8.56; N, 5.66. Found: C, 72.70; H, 8-63; N, 5.65.
20. A glass column (20 mm x 25 cm) packed with 50 g of Wakogel C-200 is used. After Ihe first fraction, eluted with 300 mL of dichloromethane, was removed, the second fraction, eluted with 300 mL of dichloromethane-diethyl ether (1:4), was concentrated.
Aldols of the erythro configuration are prepared by aldol condensation of various metal enolates.4 An alternative approach is reduction of p-keto esters5 or amides6 with zinc borohydride. The hydrosilane-based reduction described here provides erythro aldols under high stereocontrol and is practical because of the mild conditions and easy handling of readily available hydrosilanes.7 The scope of this reduction is summarized in Table I. No epimerization at the chiral center is observed as shown in the last entry. The erythro-selective reduction with the PhMe2SiH/CF3COOH reagent is also applicable to the reduction of 2-oxy or 2-amino ketones.89
Preparation of threo aldols is sometimes a problem. For stereoselective synthesis by aldol condensation, propionate esters of mesitol must be employed.10 A general, alternative approach to threo aldols is threo-directed reduction of p-keto esters.11 Although the stereoselectivity of this reduction is usually low, reduction of fi-keto amides with potassium triethylborohydride (KBHEt3) is extremely selective.12 The hydrosilane/F' reduction of p-keto amides provides threo aldols of high diastereomeric purity when aroyl-substituted amides are employed.7 The scope of this reduction is summarized in Table II. High threo selectivity is observed only for reduction of 2-aroylpropanoates, whereas the reduction of 2-alkanoylpropanoates proceeds with poor selectivity and gives erythro isomers as the major product.13 The
tiydrosilane/F' reduction is also applicable to the threo-selective reduction of a-oxy and a-amino ketones.8
1. Sagami Chemical Research Center, 4-4-1 Nishiohnuma, Sagamihara, Kanagawa 229, Japan.
2. Marvel, C. S.; Lazier, W. A. Org. Synth., Coll. Vol. I, 2nded. 1941, 99.
3. Middleton, W. J. U.S. Patent 3 940 402, 1976; Chem. Abstr. 1976, 85, P638Sj. Also see; Middleton, W. J. Org. Synth. 1986, 64, 221.
4. (a) Heathcock, C. H. in "Asymmetric Synthesis;" Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, pp. 111-212; (b) Evans, D. A.; Nelson, J. V.; Taber, T. R. Top. Stereochem. 1982, 13, 1-115; (c) Bal, B.; Buse, C. T.; Smith, K.; Heathcock, C. H. Org. Synth. 1985, 63, 89.