Lithium trisiamylborohydride, reduction ketones with

Diastereoselective reduction of acyclic a, -dialkoxy ketones.3 Reduction of the ketone 2 with this hydride results in the alcohol 3 with high syn-selectivity (97 3). The same syn-selectivity is observed with lithium trisiamylborohydride, but the yield is lower. The product was used for a synthesis of (+ )-4, a pheromone of a species of ants. 

Stereoselective reduction of ketones. This borohydride (1) is comparable to lithium tri-.ver-butylborohydride (4. 312-313) for stereoselective reduction of cyclic ketones to the less stable alcohols, but less stereoselective than lithium trisiamylborohydride (7, 216-217). The by-product formed in reductions with I can be removed as an insoluble ate complex formed by addition of water, simplifying isolation of the reduction product. 

The same two ketones featured again in a synthesis of ( )-lasubine II (910) by Pilli et al. (Scheme 119) (368,369). In this case, reaction of A -Boc-2-ethoxypiperidine (925) with enone 926 in the presence of trimethylsilyl triflate sparked off a remarkably efficient (90%) one-pot synthesis involving condensation (via an A -acyliminium ion to give the intermediate 927), deprotection, and intramolecular conjugate addition. The familiar products 921 and 922 were obtained as a 3 2 mixture. Base-induced epimerization of the mixture enriched the latter component, which was converted into ( )-910 by reduction with LS-Selectride (lithium trisiamylborohydride) according to the procedure developed by Comins (vide infra). 

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