BF₃-Promoted Reactions of Cyclic 1,2-Diketones and Paraformaldehyde - New Structural Motifs by Multiple Hydroxymethylation and Acetal Formation

The BF₃-promoted reaction of paraformaldehyde with the cyclic 1,2-diketones 1, 3, 8 and 10 was studied. Under identical reaction conditions, the substrates yielded structurally different products depending on their ring size. 1,2-Cyclopentanedione (3a) was converted into the spiro compound 4 (49%) whereas its 3-methyl analog 3b gave a butterfly-like compound 5b (56%). A structurally related compound 2 was obtained (84%) by the reaction of 1,2-cyclohexanedione (1) with paraformaldehyde. 1,2-Cycloheptanedione (8) yielded the simple tricyclic acetal 9 (72%) whereas 1,2-cyclooctanedione (10) underwent a twofold hydroxymethylation in the α- and α′ -positions yielding the acetal 11 (53%). The structures of all new compounds were proven by NMR experiments and X-ray crystallography. The mechanism of product formation is discussed based on steric and electronic arguments.