40 SYNTHETIC STUDIES ON CELLSURFACE GLYCANS
Details
Regioselective transformation of carbohydrates through trialkylstannylation of hydroxyl groups and the application of the procedure to the synthesis of cell surface glycan chains are described. For example, tributylstannylation of 30 and subsequent benzoylation with benzoyl chloride gave dibenzoate 32 in 90% yield via 31 as the activated intermediate. Similar regioselective benzoylation of mono- and di-saccharides are achieved in high yields, giving partially benzoylated carbohydrates of synthetic importance such as 15, 22, 35, 38, 40, 42, and 44. Similar regioselective alkylation of the partially stannylated monosaccharides was achieved. For example, 31 was heated in allylbromide to afford diallyl ether 45 in 71% yield. 45 was further transformed into dibenzyl ether 47 of which structure was confirmed by the transformation into dibenzoate 32. Compound 47 could be regarded as the key intermediate for the synthesis of cell surface glycans such as 49 and 50. In fact, 47 was successfully employed for the synthesis of model glycans 51, 52 and 53. Key monosaccharide synthons for these branching mannopentaosides and mannohexaoside are 47, 54 and 55. The glycosyl donors 54 and 55 could readily be prepared from 56 and 57. The reaction of 54 and 47 gave a 78% yield of 58. Deacetylation and subsequent glycosidation of 59 with 54 led to the isolation of 60 in 79% yield which was deprotected to give free mannopentaoside 51. In a similar manner, branching mannopentaoside 52 and hexaoside 53 were synthesized from the same synthons, demonstrating the efficiency of our approach to the synthesis of branching glycan chains.
