Dehydrobromination of methyl 11α-bromo-12-oxopodocarpan-19-oate

The configuration and stereochemistry of methyl 11α-bromo-12-oxopodocarpan-19-oate 1 have been determined by application of the nuclear Overhauser effect. The dehydrobromination of 1 with dimethylacetamide-calcium carbonate results in a predominant 1,4-elimination process which for short reaction periods yields the non-conjugated ketone 8 and for long reaction periods yields the conjugated ketone 2 . The 1,2-elimination of hydrogen bromide is a minor process and takes place to the extent of 20-22%. The ketone 8 has been oxidatively cleaved to the keto-acid 11 , a valuable intermediate for the synthesis of bicyclic diterpenoids.