Solid-State Photochemistry: Diastereoselective Synthesis of Thietane-Fused β-Lactams from an Acyclic Monothioimide with a Chiral Group

Article abstract of DOI:10.1021/jo00090a034

Irradiation of N-<(R)-1-phenylethyl>-N-tigloylthiobenzamide in benzene solution resulted in both <2 + 2> thietane and thioketone formation reactions (via β-hydrogen abstraction by the thiocarbonyl sulfur).Diastereoselectivity in the thietane formation was not observed, in that four β-lactams were obtained in similar yields.In contrast to the solution photochemistry, irradiation of the crystals provided remarkably high diastereoselectivities.At 15 deg C, de of the syn-β-lactams was 61percent and the ratio of syn/anti was to 8.7 (chemical yield: 78percent).The solid-state photoreaction proceeded even at -78 deg C, with higher diastereoselectivity.At low conversion (9percent), only syn isomers (de = 93percent) were obtained, and 71percent de was observed even at 90percent conversion.An X-ray crystal structure analysis of the monothioimide indicates that the crystal is orthorhombic, of space group P2₁2₁2₁, and the distances between the sulfur atom and the alkenyl carbon, and from the thiocarbonyl carbon to the alkenyl carbon, are 4.29 and 3.00 Angstroem, respectively.The twist anglle τ of the C(=O)-N bond is 40.8 deg and that of the C(=S)-N bond is 19.9 deg.On the other hand, the sulfur atom and abstractable hydrogen atom are in close proximity, and the value for d is 2.40 Angstroem, for Θ is 127.3 deg, for Δ is 67.9 deg, and for ω is 6.2 deg.From these values, it is apparent that this reactant is well-positioned for hydrogen transfer in the crystalline state.