Tandem michael addition/aldol reaction of allenic ketones with alkyl vinyl ketones: Versatile synthesis of 2-alkynyl 1,5-diketones, 4-alkynyl-3- hydroxycyclohexanones and 4-alkynylcyclohexenones

摘要

The reactivity of alkynylenolate was investigated in the reactions of allenic ketones and vinyl ketones. We found that different products could be obtained under different conditions. The tetrabutylammonium fluoride (TBAF)-mediated Michael addition of allenic ketones to vinyl ketones gave 2-alkynyl 1,5-diketones as the product. Under stronger basic conditions, a tandem Michael addition/aldol reaction of the two starting materials occurred in one pot, and 4-alkynyl-3-hydroxycyclohexanone could be isolated at low temperature, whereas 4-alkynylcyclohexenones was obtained at room temperature. It should be noted that only one diastereoisomer of 4-alkynyl-3- hydroxycyclohexanone was isolated from the reaction even when ethyl vinyl ketone was used as the Michael acceptor, as determined by ~1H and ~(13)C NMR spectroscopy. The configurations of the diastereoisomers were confirmed by NOESY spectroscopy. Further investigations on the chemistry of alkynylenolate were performed and a tandem Michael addition/aldol reaction of allenic ketones with vinyl ketones was developed. Various products were obtained from the same starting materials under different conditions and only one diastereoisomer of hydroxycyclohexanone was isolated.