Spiroannelation via Intramolecular Ketocarbenoid Addition. Stereocontrolled Synthesis of (-)-Acorenone B and (+/-)-α-Chamigrene

Article abstract of DOI:10.1021/ja00397a040

The spirobicyclic structures of (+/-)-α-chamigrene (7) and (-)-acorenone B (6) were synthesized by means of a copper-catalyzed, intramolecular cycloaddition of diazo ketones 25 and 47, respectively.The former was prepared from 4-methyl-4-(p-methoxyphenyl)pentanoic acid (8) and the latter was obtained in optically active form from (R)-(+)-limonene (35).Reduction of 26 with lithium in ammonia gave 27, which was transformed to (+/-)-α-chamigrene (7) via olefin 30 and carbinol 32.The tricyclic ketone 53, from 47, was converted via olefin 55 to spiroketone 56.Interoduction of the conjugated olefin afforded (-)-acorenone B (6).Alternatively, 53 was reduced with lithium in ammonia to 54 and this, through a parallel sequence, was taken to (-)-4-epiacorenone B (67).