Oxidative Dimerization of (E)- and (Z)-2-Propenylsesamol with O2 in the Presence and Absence of Laccases and Other Catalysts: Selective Formation of Carpanones and Benzopyrans under Different Reaction Conditions

Abstract

The oxidative dimerization of 2-propenylsesamol to carpanone with O(2) as the oxidant, which probably proceeds as a domino phenol oxidation/anti-β,β-radical coupling/intramolecular hetero Diels-Alder reaction, can be efficiently catalyzed by laccases. Experiments with laccases and other catalysts like a Co(salen) type catalyst and PdCl(2) clearly demonstrate that the diastereoselectivity of the carpanone formation does not depend on the catalyst but on the double-bond geometry of the substrate. With (E)-2-propenylsesamol as the substrate, carpanone and a so far unknown carpanone diastereoisomer are formed in a 9:1 ratio. When (Z)-2-propenylsesamol is used as starting material, carpanone is accompanied by two carpanone diastereoisomers unknown so far in a 5:1:4 ratio. All three carpanone diastereoisomers have been separated by HPLC, and their structures have been elucidated unambiguously by NMR spectroscopy, DFT calculations, and spin work analysis. When the oxidation of 2-propenylsesamol with O(2) is performed in the absence of any catalyst two diastereoisomeric benzopyrans are formed, probably as the result of a domino oxidation/intermolecular hetero Diels-Alder reaction. Under these conditions, carpanone is formed in trace amounts only.