Phenolation of (+)-Catechin with Mineral Acids. II. Identification of New Reaction Products

To investigate the reactions that occur in the flavanoid unit during the liquefaction of tannin in phenol, the phenolysis of (+)-catechin was studied using either H₂SO₄, HCl, or BF₃ 2H₂O as acid catalyst. In addition to 2-3-(3,4-dihydroxyphenyl)-2-hydroxy-3-(4-hydroxyphenyl)propyl]-1,3,5-benzenetriol (1) and 2-(3,4-dihydroxyphenyl)(4-hydroxyphenyl)methyl]-2,3-dihydro-4,6-benzofurandiol (3) that have been described previously, eight additional reaction products were isolated, four of which were compounds that have not been described previously. The novel compounds described here are: 2-3-(3,4-dihydroxyphenyl)-2-hydroxy-3-(2-hydroxyphenyl)propyl]-1,3,5-benzenetriol (2), 2-(3,4-dihydroxyphenyl)(2-hydroxyphenyl)methyl]-2,3-dihydro-4,6-benzofurandiol (4), 2-(3,4-dihydroxyphenyl)(4-hydroxyphenyl)methyl]-2,3-dihydro-7-(4-hydroxyphenyl)methyl-4,6-benzofurandiol (5), and 2-(1,3,5-trihydroxyphenyl)methyl-3-(3,4-dihydroxyphenyl)-6-(3,4-dihydroxyphenyl)(4-hydroxyphenyl)methyl]-2,3,5,6-tetrahydrobenzo1,2-b:5,4-b′]-difuran-4-ol (6). The structures of these and other previously described products are consistent with opening of the pyran ring of catechin and reaction at C-2 by either the para or the ortho position of phenol. Additional products resulting from reaction between pyran ring cleavage products and catechin, and from reaction of cleavage products were found. Similar reactions would be expected to take place during the phenolysis of condensed tannins.