Crotyl-2,5-dimethylborolanes

A number of optically active allylic boron compounds have been used, including380 B-allylbis(2-isocaranyl)borane (28),381 E- and Z-crotyl-(/ ,/ )-2,5-dimethylborolanes (29),382... 

Excellent double diastereoselection has also been realized in the reactions of (151) and chiral crotyl-boron reagents (Table 7). Interestingly, the best selectivity for diastereomers (153) and (156) is obtained by using the tartrate crotylboronates (S,S)-(18) and () , )-(19), respectively (entries 2 and 3), while Masamune s 2,5-dimethylborolane reagents (HJl)-(221) and (5,5)-(222) provide the greatest selectivity for diastereomers (152) and (157 entries 7 and 10). Comparative data for the diastereoselectivity ol tained with the achiral crotylteronates (1) and (2) appear in the last two entries of Table 7. 

Masamune and cowotkers48 designed a set of chiral rrans-2,5-dimethylborolane-based reagents that show remarkable enantioselectivities in hydroboration,49 ketone reduction50 and crotyl addition to aldehydes.51 The chirality transfer also turns out to be highly efficient in the aldol reaction. Initial experiments were aimed at defining the structural parameters which affect the E(0) Z(0) ratios of the enolates prepared from various alkanethioates with 2,5-dimethylborolanyl triflate (90) under standard conditions... 

Garcia J, Kim B, Masamune S. Asymmetric addition of (Eland (Z)-crotyl-trans-2,5-dimethylborolanes to aldehydes. J. Org. Chem. 1987 52 4831 832. 

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