Crotylborane reagent

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. 

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-... 

The reaction proceeds with excellent enantioselectivity as shown, but in some cases the diastereoselectivity is modest, which is true when the allylborane reagent is substituted, as in crotylborane derivatives. In the Omura, Smith and co-workers synthesis of (+)-lactacystin, for example, reaction of the B-crotyl derivative shown with aldehyde 158 gave a 4 1 mixture of 159 and 160 in high yield. Each diastereomer was formed with high enantioselectivity. 

Cyclopropanated allylboration reagents have been used to homoallylate and homocrotylate aldehydes. A zinc-mediated a-prenylation of aldehydes and ketones has been reported conventional conditions have yielded mainly the y-adduct to date. " A racemic allene has been hydroborated in an enantioconvergent manner the enantioenriched crotylborane then reacts with aldehydes in high ee to give ( j-stannyl-homoallylic alcohols. A copper-catalysed y-selective and stereospecific allylic alkylation of ketene silyl acetals has been reported. ... 

Further exploration in this area has led to signiAcant advances in the area of higher-order allylboration. The entire range of pinane-based allylborane reagents snch as crotylborane, alkoxyallylborane,... 

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