X-ray crystallographic and NMR structural studies of trans-2,3-dichloro-5-ethyl-2,3-dihydrothieno[2 3-b]pyridine syn-1-oxide reactions of thiophene rings with hypochlorite reagents

X-ray analysis of a crystalline product obtained by treatment of 5-ethylthi eno[2,3-b]pyridine with excess acidified hypochlorite establishes its stere ochemistry as trans-2,3-dichloro -5-ethyl-2,3-dihydrothieno[2,3-b]pyridine syn-1-oxide (5), wherein the pyridine ring is planar and the dihydrothiophe ne ring is nonplanar with a C2-S-C7a angle of 86.6 degrees. The trans geome try is corroborated by a proton-proton coupling constant J(2,3) of 6.8 HZ C omparison of H-1 and C-13 nmr data for 5 with analogous crystalline 2,3-dic hloro-1-oxide addenda isolated in the isosteric benzo[b]thiophene and thien o[2,3-b]pyridine parent systems indicates that some proposed stereochemical assignments are questionable.