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Asymmetric Synthesis of the Four Stereoisomers of 4-Methyl-3-heptanol via Boronic Esters: Sequential Double Stereodifferentiation Leads to Very High Purity
17 September 2002 (online)
(S,S)-Diisopropylethanediol (S) 6, (S,S)-DIPED] boronic esters 1, (S)-3, (S)-9, 11 with (dichloromethyl)lithium yield the chain extended (αR)-(α-chloroalkyl)boronic esters (S)-2, (1R,2R)-24, (1R, 2S)-24, (R)-27, respectively in ≥ 98% diastereoisomeric excess. The major (αR) isomers with Grignard reagents yield the products of displacement of chloride by alkyl (S)-3, (1R,2R)-25, (1R, 2S)-25, (S)-9. The enantiomers have been made from (R,R)-DIPED. Contrakinetic (S,S)-DIPED (αS)-(α-chloroalkyl)boronate (S)-7 has been prepared indirectly and found to undergo a different major reaction with methylmagnesium bromide which leads to B-methyl-1,4,2-dioxaborinane 10. Thus, in the two-step formation of a new chiral center, the minor isomer is twice discriminated against. The utility of this new chemistry has been demonstrated with the asymmetric synthesis of each of the four stereoisomers of 4-methyl-3-heptanol (26) in very high purity. Diastereoselectivities measured at 350: 1 to ≥ 1000: 1 per chiral center have been achieved.