Using the readily accessible chiral auxiliaries 1-3 the sulfonamide-shielded 0 -silylated esters 5 underwent n-face-selective a-acetoxylation on successive treatment with Pb(OAc), and NEt, HF to give after recrystallization a-acetoxy ester 6 in S5-67% yields and in 95-100% d.e. Starting from conjugated enoates addition of RCu and subsequent acetoxylation 1 0 + l l + l 2 yielded aJ-bifunctionalized esters 12 with > 95% configurational control at both C, and Cg. Nondestructive removal of the auxiliary(6-r7,6+8 and 12-13) gaveeither a-hydroxycarboxylic acids or terminal cc-glycols in high enantiomeric purity. The prepared glycols 8c and 13a are key intermediates for previously reported syntheses of the natural products 16 and 17, respectively.Recently, we have shown that the readily available, crystalline alcohols 1-3 are efficient and practical chiral auxiliaries for asymmetric Diels-Alder reactions [ 11, organocopper additions 121 and enolate alkylations [2]. Thus, CI -and p-functionalizations of esters I were selectively accomplished from the least hindered n-face.
-N S o ! dWe report here the first extension of this principle to enantioselective preparations of CI -hydroxy-carboxylic acids and terminal M -glycols which may serve as chiral building blocks in organic synthesis2).Kinetically controlled 0 -silylation [7] of saturated esters 43) [i) LDA (I. 1 mol-equiv.), THF, -78", ii) Me,SiCl (1.75 mol-equiv.)] furnished ketene acetals 5,) which on successive treatment with dry Pb(OAc), (1.1 mol-equiv., C H Q , -15", 15 min, then +20°, 30 min) and NEt,.HF (3.0 mol-equiv., +2W, 4 h), according to the method described by Rubottom et al. [9], afforded, with 88 to 96 % diastereoface selection, a-acetoxyesters 64). All new compounds were characterized by 'H-NMR, IR and MS.
4,