Tobacco Chemistry. 43. Sensitized Photo-oxygenation of (12Z)-Abienol. Biomimetic Synthesis of Tobacco Labdanoids.

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Abstract
The reaction of (12Z)-abienol with singlet oxygen was found to occur with competition between 4 processes. The predominant route (68%) is an ene-process involving attack at C-13 and yielding, after reduction, the (11E,13S)- and (11E,13R)-11,14-labdadiene-8,13-diols. Less favored are an ene process (3%) involving attack at C-12 and leading to the formation of the (12S)- and (12R)-13(16),14-labdadiene-8,12-diols and a Diels-Alder process (5%) furnishing (12R)-12,15-epidioxy-13-labden-8-ol. The remaining process, accounting for some 12% of the overall yield, is proposed to involve an attack on the 12,13 double bond and to proceed through intermediate peroxiranes to give the (12R,13R)- and (12S,13S)-8,12-epoxy-14-labden-13-ols as well as the (12S,13S)- and (12R,13R)-8,13-epoxy-14-labden-12-ols. Exhaustive photo-oxygenation affected the concentrations of all these compounds and resulted in the generation of some additional products, i.e., 15,16-epoxy-8-hydroxy-13(16),14-labdadien-12-one, the (12S)- and (12R)-15,16-epoxy-13(16),14-labdadiene-8,12-diols and norambreinolide. Since several of the photo-oxygenation products obtained have previously been identified as tobacco constituents, the present results provide experimental support for the view that (12Z)-abienol may be a precursor of the labdanic compounds found in processed tobacco.