Reaction of 2,2,4,4-tetramethylcyclobutane-1,3-dione (2) under phase-transfer-catalysis (PTC) conditions (CHCl3/aqueous NaOH) yielded a complex mixture of unexpected products (Scheme 2). From the organic phase, three ring-enlarged products 7-9 with a cyclopentane-1,3-dione (cf. 7 and 9) or a cyclopentenone skeleton (cf. 8) were isolated in low yield. After acidification of the aqueous phase, the oily residue was treated with CH2N2 , and methyl 3-oxopentanoate 12 and dimethyl 2-hydroxybutanedioate 13 were obtained in almost equal amounts. The structures of 8 and 9 were established by X-ray crystal-structure analysis (Fig.). Mechanisms for the formation of the products, initiated by nucleophilic attack of trichloromethanide ion and opening of the cyclobutane ring, are proposed in Schemes 3 and 4.