4,5-diisopropyl-B-[(E)-[(3'-menthofuryl)dimethylsilyl]allyl]-1,3,2-dioxaborolane, an improved chiral reagent for the anti alpha-hydroxyallylation of aldehydes: Application to the enantioselective synthesis of (-)-swainsonine

Diisopropyl tartrate-modified (E)-[gamma-(furyldimethylsilyl)allyl]boronates 10 and 11 were designed for the enantioselective synthesis of substituted anti-3,4-dihydroxy-1-butenes 9 via the anti-alpha-hydroxyallylation of aldehydes. The reactions of aldehydes with 10 and 11 provided furyldimethylsilyl-substituted anti-1,2-silanols 12 and 13 with good enantioselectivity (74-82% ee), and the silanols were oxidized to the corresponding anti-1,2-diols 9 by a modified Fleming-Tamao oxidation protocol. The high reactivity of the (menthofuryl)silane unit toward electrophilic substitution allowed complete protodesilylation of menthofuryl-substituted silanols 13 and subsequent oxidation to diols 9 in a one-pot operation without competing protodesilylation of the allylsilane unit. However, a two-step protocol was required for the protodesilylation-oxidation of the less reactive 2-(5-methyl)-asymmetric reactions with three chiral aldehydes 23-35 (80 to >92% de, with the exception of the mismatched reactions with aldehyde 25, which were essentially unselective). However, [[[2-(5-methylfuryl)]dimethylsilyl]ally]boronate 10 could only be synthesized in less than or equal to 15% yield, and oxidations of the 2-(5-methylfuryl)-substituted silanols 12 were lower-yielding than oxidations of the corresponding menthofuryl-substituted silanols 13. Thus diisopropyl tartrate-modified (E)-[gamma-[(menthofuryl)dimethylsilyl]ally]boronate 11 proved to be the more useful of the two reagents. As a demonstration of the utility of 11 in the synthesis of polyhydroxylated natural products, this new reagent was used in a brief and highly stereoselective synthesis of the naturally occurring alkaloid (-)-swainsonine (42).