Investigating the Mechanism of 1,3,5-Cycloheptatriene Formation: Condensation of 1-Methylethyl 3-Oxo-4-(triphenylarsoranylidene)butanoate with 7-Methoxy-3,7-dimethyloctanal

Condensation of [3-(1-methylethoxy-carbonyl)-2-oxopropyl]triphenylarsonium bromide 8 with 7-methoxy-3,7-dimethyloctanal 10 in the presence of a base gave bis(1-methylethyl) 4,7-dihydroxy-2-(6-methoxy-2,6-dimethylheptyl)-3,5,7-cycloheptatriene-1,3- dicarboxylate 7b. The formation of 7b can be explained by a number of consecutive steps. The mechanism, strongly supported by liquid secondary ion mass spectrometry, suggests that an aldol condensation of the γ-arsonium ylide 2b and the aldehyde 10 takes place as the initiating step and is followed by a Michael addition of the aldol product and the γ-arsonium ylide 2b. Then an intramolecular nucleophilic substitution and elimination of triphenylarsine, followed by base-catalyzed elimination of a second molecule triphenylarsine takes place to produce 7b. © 1998 John Wiley & Sons, Inc.