Furopyridines. IX. Syntheses and properties of 3‐ethoxy derivatives of furo[2,3‐b]‐, furo[3,2‐b]‐, furo[2,3‐c]‐ and furo[3,2‐c]pyridine

Abstract Reaction of ethyl 3‐ethoxycarbonylmethoxyfuropyridine‐2‐carboxylates 2a‐2d with sodium ethoxide afforded 3‐ethoxy derivatives 3a‐3d which converted to 3‐ethoxyfuropyridines 5a‐5d by hydrolysis and decarboxylation of the ester group. Vilsmeier reaction of 5a and 5b gave 2‐formyl‐3‐ethoxy derivatives 6a and 6b and 2‐formyl‐3‐chloro derivatives 7a and 7b , while 5c and 5d did not give any formyl compound. Bromination of 3‐ethoxyfuropyridines with 1 equivalent mole of bromine gave 2‐bromo‐3‐ethoxyfuropyridines 9a‐9d , whereas reaction with 3 equivalents of bromine yielded 2,2‐dibromo‐3,3‐diethoxy‐2,3‐dihydrofuropyridines ( 10a and 10b ) and/or 2‐bromo‐3,3‐diethoxy‐2,3‐dihydrofuropyridines 11b , 11c and 11d . Treatment of compounds 5a‐5d with n ‐butyllithium in hexane‐tetrahydrofuran at −70° and subsequent addition of N,N ‐dimethylformamide yielded 2‐formyl derivatives 6a‐6d .