Organocatalytic Asymmetric Tandem Michael Addition-Hemiacetalization: A Route to Chiral Dihydrocoumarins, Chromanes, and 4H-Chromenes

Asymmetric tandem Michael addition -hemacetalization between aliphatic aldehydes and (L)-2-(2-nitiovinyl)phenols was investigated for constructing benzopyran backbones. Interestingly. the diastereo- and enantioselectivities changed markedly when the reaction was mediated by different types of secondary amine catalysts The diphenylprolinol silyl ether 7 promoted the reaction with excellent enantioselectivitics (up to 99% cc) hut with model ate diastereoselectivitics (2 8 1 to 10) Prolylprolinols are another type of efficient catalyst Among them. L,L-prolylprolimol 5c is identified as the optimal species. showing high catalytic activity. good enantioselectivities (up to 89% ee). and excellent diasereoselectivities (up to 50 1 dt) Various aliphatic aldehydes and substituted (L)-2-(2-nitrovinyl)phenols were proven to he well tolerated in this tandem reaction In addition the chroman-2-ols 3 yielded in the above reactions could he conveniently transformed to synthetically and biologically significant chiral dihydrocoumarin chroman. and 4H-chromene derivatives