Investigation of the Cyclopentenone Formation via the α‐Alkynone Cyclisation: Synthesis of the Acorone Intermediate 8‐Methylspiro[4.5]deca‐3,7‐dien‐2‐one

As a further application of the cyclopentenone formation A→C via the thermal α‐alkynone cyclisation B→C and in order to test the fate of an isolated C,C‐double bond within a molecule under these conditions, we investigated the synthesis of the acorone intermediate 3starting from the known carboxylic acid 1. The α‐alkynone 2 was obtained from 1 via the acyl chloride 6 and a Pd(II)‐catalysed route (22%). The thermolysis of 2 at 550° provided the target molecule 3 (48%) together with the product 9 (20%) of a competing intramolecular ene reaction and its dimer 10 (4%). At a higher thermolysis temperature (650°), the spiro ketone 3 was found to be unstable, affording the retro‐Diels‐Alder fragments 4‐methylidene‐2‐cyclopentenone (12) (33%) and isoprene (32%). A further example of the influence of an isolated double bond on the yield of the cyclopentenone‐formation sequence A→C was provided by the comparison of the annelation 14→20 (5% overall with Pd(II)‐catalysed acylation) with that of its non‐olefinic analogue 17→21 (53% overall with Friedel‐Crafts acylation).