S-Glycosylation
3.5
685
⊡ Scheme 52
3.1.2 Trichloroacetimidates as Donors
Trichloroacetimidates function well as donors also with saccharide thiol acceptors. A number
of thiodisaccharides have accordingly been prepared using this method [169,170,171,172,173].
Possible by-products are various elimination products and orthoester formation. The stereochemical outcome is not easy to predict or control and, as so often in carbohydrate chemistry,
an optimization of conditions is usually necessary for each glycosylation. For example, the use
of an acetylated α-trichloroacetimidate glucose donor can yield not only an almost exclusively
β-linked disaccharide product but also preferentially an α-linked product, depending on the
acceptor and conditions used ( > Scheme 53) (see also > Sect. 3.3).
⊡ Scheme 53
3.1.3 Glycals as Donors
Addition reactions of saccharide thiols to glucals have been performed ( > Scheme 54, compare with > Scheme 26). The α-linked thiooligosaccharide was the main product but the
β-anomer and the product resulting from a displacement of the 3-O-acetate were also isolated
(ratio 5 : 1 : 1) [174].
3.5
685
⊡ Scheme 52
3.1.2 Trichloroacetimidates as Donors
Trichloroacetimidates function well as donors also with saccharide thiol acceptors. A number
of thiodisaccharides have accordingly been prepared using this method [169,170,171,172,173].
Possible by-products are various elimination products and orthoester formation. The stereochemical outcome is not easy to predict or control and, as so often in carbohydrate chemistry,
an optimization of conditions is usually necessary for each glycosylation. For example, the use
of an acetylated α-trichloroacetimidate glucose donor can yield not only an almost exclusively
β-linked disaccharide product but also preferentially an α-linked product, depending on the
acceptor and conditions used ( > Scheme 53) (see also > Sect. 3.3).
⊡ Scheme 53
3.1.3 Glycals as Donors
Addition reactions of saccharide thiols to glucals have been performed ( > Scheme 54, compare with > Scheme 26). The α-linked thiooligosaccharide was the main product but the
β-anomer and the product resulting from a displacement of the 3-O-acetate were also isolated
(ratio 5 : 1 : 1) [174].