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  • Title: Organocatalytic asymmetric tandem Michael addition-hemiacetalization: a route to chiral dihydrocoumarins, chromanes, and 4H-chromenes.
    Author: Lu D, Li Y, Gong Y.
    Journal: J Org Chem; 2010 Oct 15; 75(20):6900-7. PubMed ID: 20857918.
    Abstract:
    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.
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