Résumé
The effect of an electron attracting substituent in the Lewis acid catalyzed oligomerization of flavanols was investigated. The results showed that the presence of a COCF3, at the 8 position of a (+)-catechin unit strongly influenced the attack of the 6 free nucleophile flavanol position by the electrophile generated from a 4-O-alkyloxy protected catechin unit. This was observed either with TiCl4 or TMSOTf as Lewis acids in which the carbon-carbon bond formation was inhibited and the corresponding dimer was detected in small amount. On the contrary, the formation of a carbon-oxygen bond was observed and the corresponding C-4 -> O -> C-3 ether linked procyanidin dimer was isolated in a good yield. In order to avoid the participation of the C-3 hydroxyl group in the dimerization reaction, the two flavanol units were forced into C-4 -> C-8 coupling by use of an internal link. The structural elucidation of the isolated compounds was achieved through MS and NMR spectroscopy