Synthesis and Carbon-13 NMR Spectroscopy of Diterpenoids

Abstract

<p>With the view to functionalizing the C-20 angular methyl group of the ring C-aromatic diterpenoids, synthetic objectives centred on finding an easily accessible intermediate to which a 1,6-hydrogen transfer could be attempted by photolysis of the nitrite ester. Attempted functionalization through the 12β-hydroxy group in perhydropodocarpic acid 29b and through the C-7 alcohol group in methyl 7β-hydroxy-O-methylpodocarpate 37 and in 6β, 7β-dihydroxy-O-methyl-podocarp-19, 6β-lactone 48 were unsuccessful. Recourse to the normal 1, 5-hydrogen transfer through the nitrate ester of methyl 6β, 7β-dihydroxy-O-methyl-podocarpate 95, a compound readily obtained in good yield after six steps from 12-acetoxypodocarpic acid 38, was inconclusive.</p> <p>During the course of the synthesis, an improved procedure of benzylic oxidation of phenolic compounds, or easily hydroyzable derivatives, was developed.</p> <p>The carbon-13 nuclear magnetic resonance spectra of podocarpic acid 25, a ring C-aromatic diterpenoid, and a number of derivatives were recorded and analyzed. Substituent effects on the aliphatic carbons were determine for a variety of oxidation levels at carbon 19.</p> <p>These substituent effects, among other techniques, are applied to the analysis of some labdane intermediates, ozonolysis products of podocarpic acid, and abietic acid and derivatives.</p>