Easy synthesis of 7-alkylbicyclo[3.3.1]non-6-en-3-ones by silica gel-promoted fragmentation of 3-alkyl-2-oxaadamant-1-yl mesylates

A synthesis of 7-alkylbicyclo[3.3.1]non-6-en-3-ones4b-f and 4j, k by reaction of the corresponding 3-alkyl-2-oxaadamant-1-yl mesylates3 with silica gel in methylene chloride at room temperature, is described. The method failed to give enones4a, g and the related compounds4l, m, what can be rationalized on mechanistic grounds. The synthesis of several bicyclic enones4 from the readily available diketones1a-c [X = CH2, (CH3O)2C, (CH3O)(CH3)C] via the corresponding oxaadamantanols2 and mesylates3, is described