Geminal Acylation with Methyl-Substituted Analogues of 1,2-Bis[(trimethylsilyl)oxy]cyclobutene

BF3 . Et2O-catalyzed geminal acylation of ketones and acetals with 3-methyl-1,2-bis[(trimethylsilyl)oxy)]cyclobutene (3) provided methylcyclopentanediones in yields that ranged from 40 to 94%. The best substrates were unhindered cyclohexanones. With acetals, stereochemical preferences in the initial Mukaiyama-like aldol step giving cyclobutanones translated into the stereochemistry of the ultimate cyclopentanedione products. With ketones, equilibration of the initial cyclobutanone compounds resulted in cyclopentanedione products with a different stereochemical preference. The gem-dimethylcyclobutene reagent 4 reacted with ketones to give gem-dimethylcyclopentanediones in modest yield. The process was much more stereochemically efficient than the reaction with 3. Rearrangement from the initial cyclobutanone compound was partially diverted toward air-sensitive 3-furanone compounds and ring-opened 1,2-diones. Only furanones (e.g., 52 and 53) were isolated from reactions with the tetramethylcyclobutene 51.