The Paternò-Büchi Reaction of α-Alkyl-Substituted Enecarbamates and Benzaldehyde

Abstract

The α-substituted enecarbamates and enamides 5a-d, 5f and 6 were prepared in two steps from the corresponding ketone, N-benzylamine and an appropriate acylating agent (Boc2O, Ac2O). The [2+2] photocycloaddition reactions of benzaldehyde to the alkenes 5a-d which bear a primary or secondary alkyl substituent proceeded smoothly and gave the 3-aminooxetanes 8a-d in moderate to good yields (46-71 %). The α-phenyl-substituted enecarbamate 5f did not produce a photocycloaddition product presumably due to rapid energy transfer (triplet sensitization) from the photoexcited aldehyde. For less obvious reasons the tert-butyl-substituted enamide 6 did not react in the Paternò-Büchi reaction either. The 3-alkyl-3-aminooxetanes 8 were obtained as a mixture of cis- and trans-diastereoisomers. An increase in the steric bulk of the alkyl substituent R shifted the diastereomeric ratio (cis-8/trans-8) in the direction of the thermodynamically more stable cis-product (29:71 for R = CH3 up to 57:43 for R = cyclohexyl). The separated oxetane diastereoisomers cis-8a and trans-8a (R = CH3) underwent a smooth ring opening/cyclization reaction upon treatment with trifluoroacetic acid. Oxetane trans-8a yielded the oxazolidinones 9 and trans-10 (92 %), oxetane cis-8a gave exclusively the oxazolidinone cis-10 (54 %).