The palladium-catalysed vinylic substitution of vinyl triflates with β-substituted-α,β-unsaturated carbonyl compounds. An application to the synthesis of cardenolides

Abstract

Vinyl triflates react with β-substituted-α,β-unsaturated aldehydes, ketones, and esters in the presence of catalytic amounts of Pd(OAc)2 and an exeess of KOAc, omitting phosphine ligands, to give vinylic substitution products in good to high yield with high regioselectivity. The added vinyl unit is preferentially linked to the β-carbon atom. As to the stereochemistry, vinylic substitution products contain the carbonyl group on the same side of the preexisting β-substituent. The use of KOAc has been proved to be superior both to tertiary amines and to carbonatc or bicarbonatc bases with or without the addition of salts such as LiCl and n-Bu4NCl. The application of the reaction to the synthesis of a cardenolide derivative is reported. Depending on the nature of β-substituted-α,β-unsaturated carbonyl compounds, the reaction can produce hydrovinylation (formal conjugated addition) products.