Abstract Treatment of thieno [2, 3-b] pyridine (1a) with chlorine gas in chloroform (plus water) gave a mixture of two 2, 3-dichloro-2, 3-dihydrothieno [2, 3-b] pyridine 1-oxides [trans-syn (IIa), and cis-anti (IIb))[and 2, 3, 3-trichloro-2, 3-dihydrothieno [2, 3-b] pyridine syn-1-oxide (IVa), as well as a non-isolated fourth product (prohably the anti isomer of IVa) and sometimes a small amount of thieno [2, 3-b] pyridine 1, 1-dioxide (III). Treatment of Ia in a ...