Thermal acid-catalysed rearrangements of natural chrysanthemic acids

Abstract

Treatment of trans-chrysanthemic acid (1) with pyridine hydrochloride at 210 °C leads to (E)-2-isopropylidene-5-rnethylhex-3-enoic acid (3)(22%) and to a mixture of neutral products (55%) containing (E)-2,6-dimethylhepta-2,4-diene (4), three butanolides [(5), (6), and (8)], and a pentenolide (7). The conversions (1)→(3) and (1)→(4) proceed with cleavage of the cyclopropane 2,3-bond via a thermal homodienyl 1,5 H-shift, leading to a dienoic acid intermediate (11), followed by decarboxylation [to (4)] and isomerisation [to (3)]; the dienoic acid (3) is converted further into the butanolide (6). The butanolide (8) and the pentenolide (7) result from cleavage of the cyclopropane 1,2-bond. Separate treatment of the butanolide (8) with pyridine hydrochloride leads to a mixture of isomeric pentenolides (7) and (13). Cleavage of the cylcopropane 1,3-bond in (1) accounts for the formation of the butanolide (5).

The principal neutral products from the thermal decomposition of chrysanthemumdicarboxylic acid (2) are shown to be a cyclohexadienone (19), 2,6-dimethylphenol (21), a pentenolide (20), and a butanolide (22). The conversion (2)→(19) takes place via acid-catalysed cyclodehydration of an intermediate (Z)-dienoic acid (16), whereas the formation fo the phenol (21) is envisaged as proceeding via demethylation of the methyl ether product (25) of [1,3]-sigmatropic rearrangement of the cyclohexadienone (19).