Chirogenic [3 + 2]-photocycloaddition reactions of 2-substituted naphthoquinones with cyclic alkenes.

The formal [3 + 2]-photocycloaddition of 2-hydroxy- (2a) and 2-amino-1,4-naphthoquinone (2b) to olefins was studied in various solvents aiming at a possible enantioselective reaction course. The reaction conditions were optimised for irradiation at low temperature in a nonpolar solvent employing external fluorescent lamps as irradiation sources. Best yields for the reaction of 2-hydroxy-1,4-naphthoquinone (2a) with 1-methyl-2-butene were obtained when using a large excess of the olefin (200 equiv.) in toluene as the solvent at an irradiation wavelength of λ = 419 nm. Under these conditions a variety of cyclic alkenes (cyclopentene, cyclohexene, dihydropyran, 1-methylcyclohex-1-ene) underwent the photocycloaddition in yields of 22-84%. Reactions with 2-hydroxy-1,4-naphthoquinone (2a) could be performed enantioselectively at -60 °C in toluene as the solvent employing a chiral hydrogen bonding template. The enantiomeric excess (≤11%) remained low, however. Possible reasons for this lack of selectivity are discussed.