Ring expansion of 3-acetylcoumarin by diazoethane: lactones derived from oxepin, oxocin, and oxonin

Diazoethane partly alkylates 3-acetylcoumarin giving 3-acetyl-4-ethylcoumarin, and partly expands the lactone ring giving, after addition of a second molecule of diazoethane, the benzoxepinopyrazoline (IX) as a racemate of assigned stereochemistry. This compound readily loses nitrogen and undergoes a second ring expansion to form the benzoxocin derivative (XII) as a single (racemic) stereoisomer. Treatment of (XII) with diazoethane induces immediate ring expansion giving the benzoxonin derivative (XIII), again as a single (racemic) stereoisomer. There is no further reaction with diazoalkanes, notwithstanding the presence of a highly activated double bond; the resistance is attributed to the internal compression that would result within a highly convoluted molecule if addition occurred.The structures and conformations of the foregoing products and of some by-products have been established by spectroscopic methods. Additionally, the benzoxocin derivative (XII) has been shown to yield meso-dimethyl-succinic acid upon exhaustive ozonolysis, and a convenient method is described for the preparation of small amounts of dimethylmaleic anhydride by the reaction of diazomethane with methylmaleic anhydride.A parallel is drawn between these reactions and certain of those found with quinone–diazoalkane adducts, and it is suggested that ring expansions are observed because the methyl group introduced with the diazoethane molecule forces the pyrazoline into the necessary conformation.