Crotylborane, chiral

Diisopropyl crotylboronates, 1. These chiral crotylboranates are prepared in 98-99% ee from diisopropyl tartrate. 

The synthesis of 108 began with homoallylic alcohol 114. Protection of the alcohol was followed by ozonolysis to give aldehyde 115. Reaction of the aldehyde with Z-crotyldiisopinocampheylborane, a chiral crotylborane, gave 116 in high yield with excellent diastereo- and enantioselectivity (see Calcimycin-13... 

Chiral Dialkylboranes. Several allylic boranes have been developed as chiral auxiliary reagents (Fig. 5). The introduction of terpene-based reagents such as 12 and 64-68 has been pioneered by H.C. Brown, and the most popular class remains the bis(isopinocampheyl) derivatives (structures 12, 64-66). A wide variety of substituted analogs have been reported, including the popular crotylboranes but also a number of other reagents bearing heteroatom-... 

Kinetic resolution can be accomplished by addition of allyl boronates to aldehyde groups adjacent to the tricarbonyliron fragment [59]. For the synthesis of ikaruga-mycin, Roush and Wada developed an impressive asymmetric crotylboration of a prochiral meso complex using a chiral diisopropyl tartrate-derived crotylborane (Scheme 1.25) [60]. In the course of this synthesis, the stereo-directing effect of the tricarbonyliron fragment has been exploited twice to introduce stereospedfically a crotyl and a vinyl fragment. 

From the range of synthetic applications of Brown s pinene-derived allyl- and crotylborane reagents illustrated in these selected examples, it is clear that they are excellent reagents for the synthesis of chiral homoallylic alcohols from chiral and achiral aldehydes alike. That so many researchers have applied these reagents in their research also attests to the great utility of these reagents in organic synthesis. 

Brown HC, Bhat KS. Chiral synthesis via organoboranes. 7. Diastereoselective and enantioselective synthesis of erythro- and threo-p-methylhomoallyl alcohols via enantiomeric (Z)- and (E)-crotylboranes. J. Am. Chem. Soc. 1986 108 5919-5923. 

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