The Paterno-Buchi reaction of alpha-alkyl-substituted enecarbamates and benzaldehyde

The alpha -subsiituted enecarbamates and enamides 5a-d, 5f and 6 were prepa red in two steps from the corresponding ketone, N-benzylamine and an approp riate acylating agent (Boc(2)O, Ac2O). The [2+2] photocycloaddition reactio ns of benzaldehyde to the alkenes 5a-d which bear a primary or secondary al kyl substituent proceeded smoothly and gave the 3-aminooxetanes 8a-d in mod erate to good yields (46-71%). The alpha -phenyl-substituted enecarbamate 5 f did not produce a photocycloaddition product presumably due to rapid ener gy transfer (triplet sensitization) from the photoexcited aldehyde. For les s obvious reasons the tert-butyl-substituted enamide 6 did not react in the Paterno-Buchi 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/tr ans-8) in the direction of the thermodynamically more stable cis-product (2 9:71 for R = CH3 up to 57:43 for R = cyclohexyl). The separated oxetane dia stcreoisomers cis-8a and trans-8a (R = CH3) underwent a smooth ring opening /cyclization reaction upon treatment with trifluoroacetic acid. Oxetane tra ns-8a yielded the oxazolidinones 9 and trans-10 (92%), oxetane cis-8a gave exclusively the oxazolidinone cis-10 (54%).