BF 3 ‐Promoted Reactions of Cyclic 1,2‐Diketones and Paraformaldehyde − New Structural Motifs by Multiple Hydroxymethylation and Acetal Formation

The BF 3 ‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)