ASYMMETRIC INDUCTION IN INTRAMOLECULAR [2-PHOTOCYCLOADDITIONS OF 1,3-DISUBSTITUTED ALLENES WITH ENONES AND ENOATES(2])

Irradiation of optically active allenes (89-92% ee) appended to enones and enoates affords alkylidenecyclobutane photoadducts with high leve ls of asymmetric induction (83-100%) derived exclusively from the alle ne fragment. The substrates studied include allenes tethered to enones such as 1,3-cyclohexanedione and 1,3-cyclopentanedione, as well as al lenes tethered to functionalized coumarins. The enantiomer ratios of t he photoadducts were quantified by derivatization of the products as t he corresponding Mosher MTPA esters and analysis by H-1 NMR spectrosco py. The exo-methylenecyclobutanes obtained upon irradiation of allene- coumarins are isolated as single olefin diastereomers. Irradiation of a coumarin tethered at C(5) with an optically active allene affords an alkynyl-substituted oxepane with complete asymmetric induction.