Journal of the Chemical Society. Perkin transactions I p. 224 - 230 (1981)
Update date:2022-08-10
Topics:
Dean, Francis M.
Johnson, Robert S.
In general, chromones activated by electron-withdrawing groups at position 3 are alkylated (at position 2) by diazoalkanes in the same manner as the isomeric coumarins.For example, 6-methylchromone-3-carbonitrile (3a) is converted by diazoethane into 2-ethyl-6-methylchromone-3-carbonitrile (3c). 2-Diazopropane affords cyclopropane by-products as well, and a 3-formyl group usually suffers homologation to the appropriate ketone, as when 2-diazopropane converts 3-formyl-6-methylchromone into 2-isopropyl-6-methyl-3-(methylpropanoyl)chromone (8).In marked contrast to coumarin chemistry, there is no ring expansion into the 1-benzoxepin series.Chromones activated at position 2 show diverse reactions.Although nitriles are usually considered to be unreactive towards diazoalkanes, 6-methylchromone-2-carbonitrile slowly gives a triazole; subsequent protropy and further alkylation results in the isolation of what is provisionally considered to be the 2-(1,2,3-triazol-4-yl)chromone (12a). 2-Formylchromone (13a) is converted by diazomethane into a mixture of 2-acetylchromone and the 2-oxiranylchromone (14) but it does undergo ring expansion by diazoethane and 2-diazopropane giving derivatives of 1-benzoxepin, e.g. (15).Ethyl chromone-2-carboxylate (17a) reacts very slowly but affords 1-benzoxepin derivatives as well as 3-alkylated chromones.The ring-expansions are considered to occur by way of sigmatropic shifts in unstable pyrazolines as in diagram (20) for a 2-acylchromone adduct.There is no corresponding shift (and therefore no ring-expansion) for any 3-acylchromone adduct
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