A Simple Procedure for the Synthesis of Carbohydrate Ortholactones

Abstract

3,4-Di-0-benzyl-2,6-dideoxy-~-arabino-hexono-l,5-lactone (1 b) was synthesized in three steps by starting from di-0-acetyl-Lrhamnal (6). Sequential treatment of 1 b or 3,4,6-tri-O-benzyl-2-deoxy-~-arabino-hexono-1,5-lactone (1 a) with triethyl-(5b) gave the spiro ortholactones 3a, b, and 4a, b. ethanolate has to our knowledge not successfully been applied to this problem. Moreover, it has been reportedr6] that 2,3,4,6-tetra-0-benzyl-D-gluconolactone does not at all react with triethyloxonium fluoroborate.We have now examined whether Meerwein's reaction can be applied to benzyl ether-protected 2-deoxy-aldono-l,5-lactones and whether spiro ortholactones can be synthesized by acid-catalyzed tran~orthoesterification['~ with simple diols (Scheme 1). We have chosen 3) PCC, CHzC12, ultrasound, 44% 3,4,6-tri-O-benzyl-2-deoxy-~-arabino-hexono-l,5-lactoneThe reaction of the lactones l a and b with triehtyloxonium tetrafluoroborate and sodium ethanolate furnishes the ortholactones 2a and b in yields of 51 and 62%, respectively. The lack of oxygen substituents at C-2 as compared to the gluconolactone mentioned above apparently enhances the reactivity of the lactones sufficiently.However, the reaction is accompanied by the formation of polar side products thus indicating that benzyl ether-protected aldonolactones are not completely stable under the reaction conditions. By treatment with catalytic amounts of hydrogen chloride, 2a or b and 1,2-ethanediol (Sa) or (1S,2S)-1,2-cyclohexanediol (5b) are converted into the spiro ortholactones 3a (83%), b (62%), 4a (8lY0),