Investigation of the Cyclopentenone Formation via the α-Alkynone Cyclisation: Synthesis of the Acorone Intermediate 8-Methylspirodeca-3,7-dien-2-one

Article abstract of DOI:10.1002/hlca.19850680208

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 3 starting 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 (22percent).The thermolysis of 2 at 550 deg provided the target molecule 3 (48percent) together with the product 9 (20percent) of a competing intramolecular ene reaction and its dimer 10 (4percent).At a higher thermolysis temperature (650 deg), the spiro ketone 3 was found to be unstable, affording the retro-Diels-Alder fragments 4-methylidene-2-cyclopentenone (12) (33percent) and isoprene (32percent).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 (5percent overall with Pd(II)-catalysed acylation) with that of its non-olefinic analogue 17-->21 (53percent overall with Friedel-Crafts acylation).