Chemoenzymatic Route to Oxyfunctionalized Cembranoids Facilitated by Substrate and Protein Engineering.

Cembranoids constitute a large family of 14-membered oxygenated macrocyclic diterpenoids with potential as therapeutic agents. Selective late-stage oxidations of cembranoid scaffolds remain a challenge for chemical catalysts but can be accomplished by enzymes. Here, a new chemoenzymatic route to oxyfunctionalized 14-membered macrocycles including cembranoids is described. This route combines a metal-catalyzed ring-closing metathesis with a subsequent P450 BM3-catalyzed hydroxylation and delivers cembranoid-like analogues. Systematic substrate probing with a set of synthetic 14-membered macrocycles revealed that the regioselectivity of a P450 BM3-based biocatalyst increased with increasing ring rigidity as well as size and polarity of the exocyclic substituents. Enzyme regioselectivity could further be improved by first-sphere active site mutagenesis. The V78A/F87A variant catalyzed hydroxylation of cembranoid-ol (9S/R)-3 d with 90 % regioselectivity for C5 position. Extensive NMR analysis of Mosher esters and single crystal X-ray structure determination revealed a remarkable diastereoselectivity of this P450 BM3 mutant depending on substrate stereochemistry, which led exclusively to the syn-cembranoid-diols (5S,9S)-4 and (5R,9R)-4.