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.