Diols, Diphenols and Related Compounds

A few secondary/tertiary diols have been used as chiral auxiliaries. The monoacetates of (R)- and (SJ-1.32 (R = Ph) [204] are interesting precursors for asymmetric aldol reactions [205-210], (S)-1 -Phenyl-3,3-bis(trifluoromethji)pro- 

The most popular lands of the diols for asymmetric synthesis are bis-secondary diols that have a C2 axis of symmetry [212]. The presence of the symmetry axis avoids the formation of diastereoisomeric esters or acetals [213], (1R, 27 )-Cyclohexanediol 1.34 (n = 1) has been used as an auxiliary in asymmetric cyclopropanation [214] and (IS, 2S)-cycloheptanediol 1.34 (n = 2) in 1,4-addition of cuprates[157], Dioxolane derivatives of 1.34 have been used for asymmetric P-ketoester alkylations [215] and cuprate 1,4-additions [216]. Linear 1,2-diols 1.35 (R = Me, i-Pr, c-CgH j, Ph) and functionalized 1,2-diols 1.36 (Y = COOalkyl, CONR 2, CH2OR ) are readily available from optically active tartaric acids 1.36 (Y = COOH). Acetals derived from these diols are valuable reagents m asymmetric synthesis [173, 213, 217], as the related 1,3-diols 1.37. Acetals of 1,3-butanediol 137 (R = Me, R = H) have also been used. When these acetals are formed from aldehydes under thermodynamic conditions, one 1,3-di-oxane stereoisomer often predominates. In this favored isomer, the substituent from the aldehyde and the methyl group from 1.37 are both in equatorial orientar 

Many workers have cleaved chiral acetals with nucleophilic reagents such as metal hydrides, alkyl- or arylmetals, allyl- or alkynylsilanes or enoxysilanes in the presence of Lewis acids [213, 220, 221, 222]. Functionalized hydroxyethers are 

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