The conversion of simple precursors into optically active products is a continuing goal in chemistry to efficiently access complex architectures for use in total synthesis and medicinal chemistry. 1 An especially important substitution pattern is a carbon atom bonded to four other carbon substituents (i.e., quaternary), and the generation of these congested centers is a significant challenge. 2 The desymmetrization of achiral molecules is a powerful strategy for the synthesis of optically active materials and presents a distinct method for the construction of stereogenic quaternary carbon centers.3 In this Communication, we report the enantioselective synthesis of α,α-disubstituted cyclopentenes (2) by an intramolecular aldol reaction of achiral tricarbonyl compounds (1) catalyzed by chiral N-heterocyclic carbenes (eq 1).
The complex of AgF and (R)-DIFLUORPHOS has been shown to be an effective catalyst for the asymmetric Sakurai-Hosomi allylation of simple ketones. A significaant improvement of the reactivity was observed by using THF as the solvent. The catalyst turnover was increased by addition of 1 equiv of MeOH. AgF and (R)-DIFLUORPHOS predominantly formed a 1:1 complex that provided high enantioselectivity. This catalyst system can be applied to various simple ketones, and corresponding tertiary homoallylic alcohols were obtained with excellent enantioselectivities (up to 96% ee). Only 1,2-adducts were obtained from both acyclic and cyclic conjugate ketones. The regio-, diastereo-, and enantioselective crotylation has also been achieved. E- or Z-crotyltrimethoxysilane gave a similar diastereomer ratio with high enantioselectivities. This finding introduces the utility of racemic allylsilanes for the enantioselective Sakurai-Hosomi allylation reaction.
Metal‐less Michael: A highly diastereo‐ and enantioselective intramolecular Michael addition of α,β‐unsaturated aldehydes to enones catalyzed by an N‐heterocyclic carbene (NHC) has been developed. The reaction is tolerant of alkyl and aromatic substituents, as well as saturated and unsaturated tethers between the enal and conjugate acceptor (see scheme).
A catalytic amount of KF.18-crown-6 complex is effective as a soluble fluoride source to activate an asymmetric Sakurai-Hosomi allylation with BINAP and silver(I) triflate catalyst. The allylation of a variety of aromatic, alpha,beta-unsaturated and aliphatic aldehydes with allylic trimethoxysilane resulted in high yields and remarkable enantioselectivities. In addition, the asymmetric Mukaiyama-type aldol reaction is achieved by using trimethoxysilyl enol ethers in the presence of the same catalysts. High anti selectivity is obtained from E-silyl enol ether, while Z-silyl enol ether gives syn selectivity.
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