TRANSANNULAR SULFUR NITROGEN INTERACTION IN 1,5-THIAZONINE DERIVATIVES - AN X-RAY STUDY

A sulfoxide-lactam (2) and sulfide-N-acetyllactam (3), each with a nin e-membered, boat-like 1,5-thiazonine ring, have been prepared and thei r molecular structures determined by X-ray diffraction. Only compound 2, with a trans-amide structure, exhibits sulfur-nitrogen close contac t with a sulfurane-like trigonal bipyramidal geometry about sulfur (r( S ... N) = 2.780(1)angstrom; theta(O=S ... N) = 171.0(1)-degrees). The interatomic distances r(S=O) = 1.501(1)angstrom and r(O=S ... N) = 4. 281(1) angstrom are in agreement with the usual S=O double bond and S ... N close contact rather than with O-S-N hypervalent bonds. Owing to the bulky N-acetyl group, the amide part in the ring of compound 3 ha s a cis structure with sulfur and nitrogen atoms lying 4.045(2) angstr om apart.