Chemistry of Thienopyridines. XXIV. Two Transformations of Thieno[ 2,3-b] pyridine 7-Oxide

Refluxing thieno2,3-b ]pyridine 7-oxide ( 1 ) with aeetic anhydride plus subsequent hydrolysis produced a mixture of thieno2,3-b]pyrid-6(7H)one ( 2 ) (13%) and 5-hydroxythieno2,3-b]-pyridine ( 4 ) (4%). Refluxing 1 with phosphorus oxychloride yielded a mixture of 4- and 6-chloro-thieno2,3-b]pyridines, ( 5 ) (54%) and ( 6 ) (31%), respectively. Compound 6 was also obtained directly from 2 (31%). A mechanistic rationalization for the isomerization of 1 to form 2 and 4 is presented. Marked differences in the mass spectral fragmentations of these three isomeric compounds are noted.