Hydroboration double-asymmetric synthesis

The facial selectivity of (43) was found to be 1.75 1 from the ratio of aldol products (48) and (49) obtained by the reaction with the achiral boron enolate (50). The latter is structurally similar to reagents (37) and (38). These experiments confirm again the validity of the rule of double asymmetric synthesis. The product (44) can be further converted through a sequence of reactions to provide (+)-Prelog-Djerassi lactonic acid (51 Scheme 27) (i) trimethylsilylation (ii) hydroboration with thexylborane (single asym-... 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Hydroboration provides a convenient and direct access to organoboranes from olefins via addition across the carbon-carbon double bond. A discussion of the hydroboration reaction and of the synthesis of enantiomerically pure organoborane intermediates by asymmetric hydroboration of proslereogenic olefins is given in Section D.2.5.2.1. The present section deals with subsequent stereoselective carbon-carbon bond formation from enantiomerically pure or enriched organoboranes which bear the boron atom at the stereogenic center1-8. 

In an amazing transformation, optically pure erythronolide template 927 is generated from 926 by a double dioxanone-dihydropyran [3,3] sigmatropic rearrangement [248] (Scheme 124). The template contains the fundamental 13-carbon framework of erythronolide A or B (600) with seven of its asymmetric centers established. The synthesis of 927 requires 18 steps to complete, and the genesis of all the stereochemistry can be traced to isobutyl D-lactate (913). It is speculated that the remaining six stereocenters of erythronolide B can be introduced by hydroboration of the three olefinic segments of the bis(dihydropyran) 926. 

Baker s yeast based asymmetric C=C bond hydrogenations have been exploited for the synthesis of several terpenes and sesquiterpenes, which are two important classes of naturalproducts.Thereductionof(R)-and(S)-23(Scheme3.5),easily prepared from the (R)- and (S)-enantioforms of Umonene 24, gives access to each diastereoisomer ofp-menth-l-en-9-ol 25 in >60% yields and vdth a very high optical purity [54]. This procedure compares well in terms of simplidty, stereoselectivity, and yield to other reported syntheses based on the hydroboration or hydroalumination of C(8)=C(9) double bond of limonene followed by fractional crystallization of nitrobenzoate... 

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