Syn-selective silicon Mukaiyama-type aldol reactions of (pentafluoro-λ6-sulfanyl)acetic acid esters with aldehydes

Aldol reactions belong to the most frequently used C-C bond forming transformations utilized particularly for the construction of complex structures. The selectivity of these reactions depends on the geometry of the intermediate enolates. Here, we have reacted octyl pentafluoro-λ6-sulfanylacetate with substituted benzaldehydes and acetaldehyde under the conditions of the silicon-mediated Mukaiyama aldol reaction. The transformations proceeded with high diastereoselectivity. In case of benzaldehydes with electron-withdrawing substituents in the para-position, syn-α-SF5-β-hydroxyalkanoic acid esters were produced. The reaction was also successful with meta-substituted benzaldehydes and o-fluorobenzaldehyde. In contrast, p-methyl-, p-methoxy-, and p-ethoxybenzaldehydes led selectively to aldol condensation products with (E)-configured double bonds in 30-40% yields. In preliminary experiments with an SF5-substituted acetic acid morpholide and p-nitrobenzaldehyde, a low amount of an aldol product was formed under similar conditions.