Oligosaccharide Analogues of Polysaccharides. Part 7. Synthesis of a monosaccharide‐derived monomer for amylose and cyclodextrin analogues

Abstract

AbstractThe synthesis of monomers of type C (Scheme 1) is described. In a first approach, chloro‐acetyl‐addition to the dioxolane 2 (Scheme 2), followed by treatment of the resulting chlorides 3 (α‐D/β‐D 1:3) with excess AgOTf and Bu3SnCCSiMe3 gave the axial C‐alkynyl‐glycoside 4 (31%) and the C‐arylglycoside 5 (29%). The structure of the dialkyne 6, obtained by deacetylation of 4, was established by X‐ray analysis. The yield of the C‐alkynyl‐glycoside was slightly improved by protecting the C(4)‐ethynyl group as the triethysilyl derivative, but not by substituting the benzyl by allyl or 2,6‐difluorobenzyl groups. Silylation of the diol 1 with (chloro)diethyl[2‐(trimethylsilyl)ethynyl]silane (19) resulted in 90% of the monosilyl ether 20. HOC(3) of 20 should favor coordination of a Lewis acid to OC(6), and intramolecular, inverting acetal opening should lead to the product of axial alkynylation. Indeed, treatment of 20 with in situ generated BuAlCl2, followed by treatment of the crude product with 0.1M HCl in MeOH, gave the dialkynylated triol 22 in yields of 85 to 90%. Under similar conditions, the disilyl ether 21 reacted more slowly to 22 (75%). The slower reaction correlates with the assumed intramolecular interaction of the precoordinated Lewis acid with OC(6) in 20.