SummaW
The potential of using a chiral organosilane as a single antipode at silicon in the preparation and separation of diastereomers has been demonstrated.
Very little success has been encountered in the relatively few attempts to utilize organosilanes, as a single antipode at silicon, in asymmetric syntheses.3 The most successful diastereoselectivity in such a system is that reported by Brook and coworkers,4 who found that carboxylation of precipitated (l-naphthylphenylmethylsilyl)benzyllithium gave an 87:13 mixture of in separable diastereomeric a-silyl acids.
It occurred to us that it should be possible to prepare organosilicon-containing organometallic reagents, wherein the silicon moiety is an optically discrete l-naphthylphenylmethylsilyl group, and to react these reagents with prochiral carbonyl species to provide diastereomers. The separation of these diastereomerss followed by desilylation would leave an optically active, silicon-free product.
We wish to report herein on our initial attempts to test this concept employing reagent 4.6 Deprotonation ethylation of 17 (i. LDA/THF/-78 ii. Etl) provided 2 (mp 106-1080; [a]0 -4.97O) in 97 % yield. Thermolysis of 2 in DMF (12061 h) gave bromide 3 (99%; mp 64-66'; [a]D +4.130). Lithium reagent 4 could be prepared in low yield with n-butyllithium or in moderate yield with set-butyllithium, but was best prepared by treatment of 3 with 2 equivalents of tert-butyllithium in THF. (Scheme I)