Organocatalytic asymmetric tandem Michael addition-hemiacetalization: a route to chiral dihydrocoumarins, chromanes, and 4H-chromenes.

Asymmetric tandem Michael addition-hemiacetalization between aliphatic aldehydes and (E)-2-(2-nitrovinyl)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 enantioselectivities (up to 99% ee) but with moderate diastereoselectivities (2.8:1 to 10:1). Prolylprolinols are another type of efficient catalyst. Among them, l,l-prolylprolinol 5c is identified as the optimal species, showing high catalytic activity, good enantioselectivities (up to 89% ee), and excellent diasereoselectivities (up to 50:1 dr). Various aliphatic aldehydes and substituted (E)-2-(2-nitrovinyl)phenols were proven to be well tolerated in this tandem reaction. In addition, the chroman-2-ols 3 yielded in the above reactions could be conveniently transformed to synthetically and biologically significant chiral dihydrocoumarin, chroman, and 4H-chromene derivatives.