Stannane high-purity

A convenient route to highly enantiomerically enriched a-alkoxy tributylslannanes 17 involves the enanlioselective reduction of acyl stannanes 16 with chiral reducing agents10. Thus reaction of acyl stannanes with lithium aluminum hydride, chirally modified by (S)-l,l -bi-naphthalene-2,2 -diol, followed by protection of the hydroxy group, lead to the desired a-alkoxy stannanes 17 in optical purities as high as 98 % ee. 

Nucleophilic allysilanes or -stannanes react with aldehydes in the presence of Lewis acids (294). As illustrated in Scheme 122, the use of a chiral boron compound as catalyst affords the corresponding homoal-lylic alcohols of high enantiomeric purity (295). 

Nakai and coworkers90 have demonstrated that the carbanion cyclization of enantio-enriched a-(homoallyloxy)alkyllithiums, prepared from the corresponding stannanes 192, proceeds with complete retention of configuration at the lithium-atom-bearing carbon centre. The obtained tetrahydrofurans 193 are formed with high trans-selectivity and without losing the enantiomeric purity. This means that the lithium might coordinate to the... 

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. 

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