Synthesis of C-glycosyl compounds. Part 2. Reactions of aldonic acid lactones with ethyl isocyanoacetate

Abstract

2,3 : 5,6-Di-O-isopropylidene-D-mannono-1,4-lactone reacted with ethyl isocyanoacetate and 1,5-diazabicyclo-[4.3.0]non-5-ene (DBN) to give ethyl 5-[(1S)-1,2 : 4,5-di-O-isopropylidene-D-arabinitol-1-yl]oxazole-4-carboxylate (1), in contrast with the products, (E)- and (Z)-ethyl 3,6-anhydro-2-deoxy-2-formylamino-4,5 : 7,8-di-O-isopropylidene-D-manno-oct-2-enonate, obtained when potassium hydride was used as the base. Similarly, 2,3 : 5,6-di-O-isopropylidene-D-allono-1,4-lactone gave the oxazole (9) and the oct-2-enonates [(15) and (16)], respectively, when DBN and potassium hydride were used as the base. With DBN as the base, 2,3-O-isopropylidene-4-O-methyl-L-rhamnono-1,5-lactone (20) gave the oxazole (21) in low yield, and with potassium hydride as the base both the oxazole (21) and an oct-2-enonate (23) were obtained in low yield. The reaction with 2,3,4,6-tetra-O-benzyl-D-glucono-1,5-lactone and with potassium hydride as the base gave the oxazole (27) as the major product but no oct-2-enonates. With DBN as the base, elimination occurred and only the unsaturated lactone (24) was identified. Similarly, 5,6-di-O-isopropylidene-2,3-di-O-methyl-L-erythro-hex-2-enono-1,4-lactone gave only elimination products, the lactones (33) and (34), when either of the two bases was used. The oxazoles (1), (9), (21), and (27) were converted into a series of derivatives by a sequence of acetylation, hydrolysis, and hydrogenolysis experiments. The reaction mechanism whereby the oxazoles and oct-2-enonates are produced is discussed briefly.