Activating groups for the ring expansion of coumarin by diazoethane: benzoyl, pivaloyl, arylsulphonyl, arylsulphinyl, and nitro

Abstract

The slow reaction between 3-ethoxycarbonylcoumarin and diazoethane affords the 4-ethyl derivative as the main product. 3-Benzoylcoumarin reacts faster but the main product is still the 4-ethyl derivative, though some ring expansion occurs, giving the oxepin derivative (VIII). 3-Pivaloylcoumarin suffers more ring expansion and gives an oxepin derivative (VIb) of a kind previously postulated in this work but never isolated. This oxepin further adds diazoethane giving a pyrazoline (IXa), comparable to (IXb) which is the first isolable product from 3-acetylcoumarin. Thermolysis of (IXa) gives largely the ethyloxepin (X), and the desired oxocin derivative (XIa) is formed to a minor extent only.

The unstable primary adduct from diazoethane and 3-(4-methylphenylsulphonyl)coumarin can be isolated but again collapses giving mainly the 4-ethyl derivative with a little of the oxepin derivative (XVI); no inverse addition was noted. The adduct from the corresponding sulphinylcoumarin eliminated the sulphenic acid immediately thus affording a simple synthesis of the [1]benzopyrano[3,4-c]pyrazole (XIXa) and its relatives. The use of 2-diazopropane gave the corresponding 3H-pyrazole derivative (XX) and of sodium azide the 1,2,3-triazole (XXII).

The reaction between 3-nitrocoumarin and diazoethane gave the 4-ethyl-3-nitrocoumarin in lesser amounts and considerable ring expansion resulted in the pyrazoline (XXIV), but small amounts of the cyclopropane derivatives (XXV) and (XXVI) were also noted as well as a trace of the cyclic nitrone (XXVII). The cyclopropane (XXVI) is the main product from thermolysis of the pyrazoline (XXIV), and no homologation was detected. The pyrazoline is itself further attacked by diazoethane (but not by diazomethane or 2-diazopropane) and suffers conversion of its lactonic carbonyl group into an oxiran system as in (XXX).

It is concluded that, in general, acetyl is the best activating group for inducing a series of ring homologations.