SOLID-STATE PHOTOCHEMISTRY - DIASTEREOSELECTIVE SYNTHESIS OF THIETANE-FUSED BETA-LACTAMS FROM AN ACYCLIC MONOTHIOIMIDE WITH A CHIRAL GROUP

Irradiation of N-[(R)-1-phenylethyl]-N-tigloythiobenzamide in benzene solution resulted in both [2 + 2] thietane and thioketone formation reactions (via beta-hydrogen abstraction by the thiocarbonyl sulfur). Diastereoselectivity in the thietane formation was not observed, in that four beta-lactams were obtained in similar yields. In contrast to the solution photochemistry, irradiation of the crystals provided remarkably high diastereoselectivities. At 15 degrees C, de of the syn-beta-lactams was 61% and the ratio of syn/anti was to 8.7 (chemical yield: 78%). The solid-state photoreaction proceeded even at -78 degrees C, with higher diastereoselectivity. At low conversion (9%), only syn isomers (de = 93%) were obtained, and 71% de was observed even at 90% conversion. An X-ray crystal structure analysis of the monothioimide indicates that the crystal is orthorhombic, of space group P2(1)2(1)2(1), 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 Angstrom, respectively. The twist angle tau of the C(=O)-N bond is 40.8 degrees and that of the C(=S)-N bond is 19.9 degrees. On the other hand, the sulfur atom and abstractable hydrogen atom are in close proximity, and the value for d is 2.40 Angstrom, for theta is 127.3 degrees, for Delta is 67.9 degrees, and for omega is 6.2 degrees. From these values, it is apparent that this reactant is well-positioned for hydrogen transfer in the crystalline state.