Steric effects in the ring-expansion of coumarins by 2-diazopropane and t-butyldiazomethane

Abstract

3-Cyanocoumarin gives in high yields its 4-isopropyl derivative with 2-diazopropane and its 4-t-butyl derivative with t-butyldiazomethane showing that the 4-alkylation of 3-cyanocoumarin is general and unaffected by bulky substituents on the diazoalkane.With 2-diazopropane, 3-acetylcoumarin undergoes ring-expansion after which inverse cycloaddition of the diazoalkane leads to 3a-acetyl-3,3a,10,10a-tetrahydro-3,3,10,10-tetramethyl-4H-[1]benzoxepino[4,3-c]pyrazol-4-one (6a). In the similar reaction with 3-benzoylcoumarin the inverse cycloaddition is slow enough for the intermediate lactone (4b) to be isolated; a cyclopropane derivative is also obtained in low yield.t-Butyldiazomethane converts both 3-acetylcoumarin and 3-benzoylcoumarin into cyclopropane derivatives, e.g. 1a-benzoyl-1-t-butyl-1a,7b-dihydrocyclopropa[c][1]benzopyran-2(1H)-one (14b), and there is little or no ring-expansion.It is concluded that although some bulk is required in the diazoalkane to force the pyrazoline into the correct conformation for ring-expansion, too great a bulk is deleterious because collisions divert the reaction into other channels.A note is added on a ring-contraction (1-benzoxepin to benzofuran derivative) induced by silver nitrate-alumina columns used for chromatography.