Methyl 2,3,4-Tri-<i>O</i>-benzyl-α-<scp>d</scp>-glucopyranosyl-Derived γ-Silyloxy Allylic Stannanes as Reagents for S_E2‘ Additions to Aldehydes

Abstract

The methyl alpha-D-glucopyranoside allylic stannanes 3.1 and 3.2 were prepared from the 3-(5-pyranosyl)-2-propenal 2.1 and the cuprate Bu(3)Sn(Bu)CuCNLi(2), followed by trapping of the derived enolate with TBSCl. The major stannane, 3.1, underwent BF(3)-promoted addition to 2-nonynal to afford a single syn adduct 4.1 in 70-90% yield. The minor stannane, 3.2, gave rise to a 70:30 mixture of adduct 4.1 and the diastereomeric syn adduct 5.1 under these conditions. The stereochemistry of the adduct 4.1 was proven by degradation of the bis-TBS derivative 4.2 to aldehyde 4.3, prepared independently from the (R,Z)-gamma-OTBS crotyl stannane 4.4 and 2-nonynal along similar lines. Analogous degradation of the adducts 4.1 and 5.1 led to a ca. 65:35 mixture of aldehydes 4.3 and its enantiomer. Accordingly, it can be surmised that the addition of stannane 3.2 to 2-nonynal takes place mainly by a syn S(E)2' pathway. BF(3)-promoted additions to enal 6.1 proceeded as expected. Stannane 3.1 afforded the syn adduct 6.2 in 87-97% yield, and stannane 3.2 gave rise to a 9:1 mixture of syn and anti adducts 7.1 and 7.4 in 70-80% yield.