Studies of chromens. Part 3. Routes to 2,2-dimethylchroman-3-ones and 4-ethyoxycarbonyl-2,2-dimethylchromens. Synthesis of a stable chromenopyrazolinone

Abstract

2,2-Dimethyl-3,4-epoxychroman (6a) was converted into 2,2-dimethylchroman-3-one which readily underwent ethoxycarbonylation to 4-ethoxycarbonyl-2,2-dimethylchroman-3-one (5c). The derived 4-ethoxycarbonyl-2,2-dimethylchromen (1b) reacted with hydrazine to give the pyrazolidinone (4a), oxidation of which gave the stable pyrazolinone (11). Lead tetra-acetate acetoxylation of 7-methoxy-2,2-dimethylchroman-4-one, followed by metal hydride reduction, afforded the 3,4-diol as a mixture of isomers. The relative proportions of cis and trans isomers could be varied from 1:1 to 5:1, respectively, by the choice of reducing conditions. Acid-catalysed dehydration of the diol mixture gave 7-methoxy-2,2-dimethylchroman-3-one (5b). This was unstable and could not be ethoxycarbonylated, but 4-ethoxycarbonyl-7-methoxy-2,2-dimethylchroman-3-one (5d)/(1j) was obtained by a standard synthesis.