Studies of chromenes. Part 5. Reaction of the Vilsmeier reagent with 7-methoxy-2,2-dimethylchroman-4-ones. 4-Chloro-7-methoxy-2,2-dimethyl-2H-chromenes and their nitration products.

Abstract

Action of the Vilsmeier reagent on 7-methoxy-2,2-dimethylchroman-4-one and its 6-bromo and 6-methoxy analogues gave low yields of 4-chlorochromene-3-carbaldehydes. 4-Chloro-2H-chromenes, which are not precursors of the carbaldehydes, were obtained in high yields. 4-Chloro-7-methoxy-2,2-dimethyl-6-nitrochroman was too labile to allow a dehydrogenation to the 6-nitrochlorochromene but the latter was obtained by regioselective nitration of 4-chloro-7-methoxy-2,2-dimethyl-2H-chromene. 6-Amino-7-methoxy-2,2-dimethylchroman-4-one, protected as the N-ethoxycarbonyl derivative, gave the 6-aminochlorochromene. The 6-amino-5-chloro analogue largely underwent N-formylation. The chlorochromenes have potential synthetic value since those without electron-withdrawing substituents are readily hydrolysed back to the chroman-4-one.