Perhydroazulenes. 6. 4-Keto Derivatives with Bridgehead Methyl Substituents₁

The two 9-methyl stereoisomers 3 and 4 and the two 10-methyl stereoisomers 8 and 9 of 4-ketoperhydroazulene have been prepared and fully characterized by means of spectra, analyses, and crystal structures. Several routes, including the selective methylation of the enone 2 to form the unsaturated ketone 12, were used (Schemes II and III) to form the 10-methyl compounds 8 and 9. Methylation of the lithium enolate (7) of 4-ketoperhydroazulene yielded a mixture of monoalkylated products containing 97% of the cis isomer 8 and 3% of the trans isomer 9. Probable conformations for the enolates and alkylated products obtained in this study are discussed.