2- Hydroxy-l,2,2-triphenylethyl acetate

Another application of this method is the stereoselective addition of (7 )-2-hydroxy-l,2,2-triphenylethyl acetate, via the lithium enolate, to propenal (acrolein) which affords mainly the ester 13 (d.r. 92 8). When the acid, obtained in the subsequent alkaline hydrolysis, is converted into the ammonium salt derived from (.S)-l -phenylethylaminc, and the salt recrystallized once, then the amine liberated (/f)-3-hydroxy-4-pentenoic acid is obtained in 41 % yield [relative to the (/ )-acetate] and >99.8% ee82. 

R)-(+)-2-Hydroxy-l, 2,2-triphenylethyl acetate (1). Preparation from methyl (R)-( —)-mandelate ... 

HMPA, HMPT HOAt HOBt HYTRA IBX Im hexamethylphosphoramide l-hydroxy-7-azabenzotriazole 1 -hydroxybenzotriazole (7 )-(+)-2-hydroxy-l,2,2-triphenylethyl acetate o-iodoxybenzoic acid Imidazolyl... 

Addition to 632 of chiral enolates generated from mandelic acid-derived (S) or (i )-2-hydroxy-l,2,2-triphenylethyl acetate (HYTRA) produces syn or /-dihydroxypentanoates of the type 647 or 648 [195]. In order to achieve the highest diastereoselectivity, the lithium enolates are transmetallated to magnesium in conjunction with carrying out the reaction at —125 to — 135 °C in THF/2-methylbutane cosolvent. 

An alternative to 138 and 122 is (R)- or (5)-2-hydroxy-l,2,2-triphenylethyl acetate ((/ )-or (5)-HYTRA) (14.3). a chiral acetate which does not need an auxiliary heteroatomic substituent. Compound 143 is readily available through acylation of 1,2,2-triphenyl-1,2-ethandiol with acetyl chloride or acetic anhydride. Its dianion, generated by double deprotonation with LDA and transmetallation with MgBr2 or Mgl2, reacts with aliphatic and aromatic aldehydes to give )8-hydroxy acid derivatives with diastereomeric ratios ranging from 92 8 to 98 2 . The crude adducts can be readily purified by crystallization. Removal of the auxiliary can be accomplished either hydrolytically (KOH, aq. MeOH) or transesterification (MeONa, MeOH/THF) to provide the respective j8-hydroxy acid/ester in enantiomerically pure form. 

It is highly desirable that chiral a-unsubstituted enolates should be available by simple methods from enantiomerically pure starting materials that are inexpensive and readily accessible in both enantiomeric forms. This postulate seems to be fulfilled to a reasonable extent by (R)- and (S)-2-hydroxy-l,l,2-triphenylethyl acetate 83 ( HYTRA ) [53, 128, 129]. It is readily prepared from methyl mandelate vhich is first converted into triphenylglycol 82 and subsequently converted into the acetic ester 83 by treatment vith acetyl chloride (Eq. (34)). Both enantiomers of the reagent are readily accessible, because both (R)- and (S)-mandelic acid are industrial products [130]. Diol 82 and acetate 83 are commercially available. 

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