Preparation of regioprotected morins by lipase catalysed transesterification

Different morin acetates have been prepared by alcoholysis of peracetate (6) in tetrahydrofuran with butanol, using Candida antarctica B or Rhizomucor miehei lipase as catalysts. The first enzyme recognised preferentially the acetate group located at position C-4 on B ring, giving 3,5,7,2-tetraacetylmorin (7). The second one, less selective, hydrolysed with comparable rates ester groups at position C-7 on A ring and C-4 on B ring, furnishing 7 and 3,5,2,4-tetraacetylmorin (8) respectively. Prolonging alcoholysis time, both lipases gave a morin ester having free OH groups at C-7 and C-4, 3,5,2-triacetylmorin (9). In any case lipases did not affect ester groups located at position C-3, C-5 and C-2. Morin derivative having free function OH in C-5, 3,2-diacetylmorin (11) was prepared in high yield using as substrate for alcoholysis the partial ester 3,7,2,4-tetraacetylmorin (10) easily prepared by chemical conventional method.