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 (Boc2 O, Ac2 O). 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 %).
Keywords
cycloadditions - heterocycles - oxetanes - Paternò-Büchi reactions - photochemistry
