Chemistry of the Podocarpaceae. XXXII. Stereochemistry of 6-Bromo-7-oxo derivatives of ring-c aromatic diterpenoids

The stereochemistry of 6-bromo-7-oxo derivatives of diterpenoids possessing an aromatic ring c is discussed. Nan-stereospecific bromination of 7-oxo diterpenoids occurs for compounds with an unactivated aromatic ring provided that no methoxycarbonyl substituent is at C 4. It also occurs for a C 19 acetate providing an activating substituent is present at C 12, but not for compounds with a C 4 methoxycarbonyl group even if they possess a C 12 activating substituent. The assignment of configuration of 6-bromo substituents from nuclear magnetic resonance and nuclear Overhauser effect data is examined and limitations to the use of optical rotatory dispersion curves are discussed.