Abstract
γ-Substituted butenolides are useful building blocks for organic synthesis and various methods have been investigated for their asymmetric synthesis. Among them, Lewis acid-catalyzed addition reactions of 2-trimethylsilyloxyfuran (TMSOF) and various electrophiles have recently emerged as the most straightforward ones. In this study, second-generation chiral (salen)chromium complex was demonstrated to be an effective catalyst for asymmetric Mukaiyama aldol-type addition reaction of TMSOF and aldehydes. Remarkably, addition of water (10 equivalent to the catalyst) was essential to realized high stereoselectivity and reproducibility in this reaction. Thus, in the presence of (salen)chromium complex (2.5 mol%) in wet dichlomethane at -20 °C, various aliphatic aldehydes efficiently underwent the reaction with TMSOF in high enantioselectivities up to 97 % ee and 92 % ee for anti- and syn-products, respectively, though the diastereoselectivities were moderate. Since Mukaiyama aldol addition reaction is principally reversible, it should be a critical point for high stereoselectivity and reproducibility in the presence system to suppress the reversereaction. Thus, water is considered to promote desilylation and protonation of the primary zwitterionic adduct, which should accelerate the conversion of this intermediate to the product and consequently, implede the undersired retro-addition.
