544
3
Chemical Glycosylation Reactions
⊡ Scheme 43
(Ref. [52])
⊡ Scheme 44
(Ref. [53])
Besides these promoters for the activation of glycosyl acetate donors, K-10 montmorillonite
was used as a new inexpensive catalyst in the glycosylation of the simple alcohol such as
benzyl alcohol or methanol to give benzyl or methyl acosaminide with high stereoselectivity ( > Scheme 41) [50], and a catalytic amount of ytterbium(III) tris[bis(perfluorobutylsulfonyl)amide] (Yb[N(O 2 SC 4 F 9 ) 2 ] 3 ) was introduced as a new promoter for the glycosylation
with glucosyl 1-acetate ( > Scheme 42) [51]. And, the combination of trimethylsilyl halides
and Zn(OTf) 2 was also employed to give the corresponding glycoside in good yield from benzyl-protected fucosyl acetate ( > Scheme 43) [52]. In particular, a combination of TMSCl and
Zn(OTf) 2 afforded α-mannosides stereoselectively from benzyl-protected mannosyl acetate
without the participating group at C-2 ( > Scheme 44) [53].
3 Glycosylation with Donors Having Other Anomeric Ester Groups
Besides glycosyl acetate, a variety of other acyl groups were employed as good anomeric
leaving groups. The representative acyl leaving groups used in glycosyl donors for the glycosylation are summarized in > Table 2.
Mukaiyama and coworkers prepared some 1-O-haloacetyl-β-D-glucopyranoses by treating 2,3,4,6-tetra-O-benzyl-D-glucopyranose with haloacetyl chloride in the presence of
cesium fluoride or potassium fluoride and performed the glycosylation reactions with the
1-O-haloacetyl glucoses in the presence of TrClO 4 to obtain the corresponding glycosides
in good yields ( > Scheme 45) [43]. Furthermore, they investigated the reactivity of various
glucoses having substituted acetyl groups at the anomeric center, such as -COCH 2 OMe,
-COCH 2 CH 2 OMe, -COCH 2 SMe, and -COCH 2 OCH 2 CH 2 OMe, in the glycosylation reaction
in the presence of SnCl 4 and AgClO 4 as a promoter system and they reported that the glucopyranose having the (2-methoxyethoxy)acetyl (-COCH 2 OCH 2 CH 2 OMe) group at the anomeric
3
Chemical Glycosylation Reactions
⊡ Scheme 43
(Ref. [52])
⊡ Scheme 44
(Ref. [53])
Besides these promoters for the activation of glycosyl acetate donors, K-10 montmorillonite
was used as a new inexpensive catalyst in the glycosylation of the simple alcohol such as
benzyl alcohol or methanol to give benzyl or methyl acosaminide with high stereoselectivity ( > Scheme 41) [50], and a catalytic amount of ytterbium(III) tris[bis(perfluorobutylsulfonyl)amide] (Yb[N(O 2 SC 4 F 9 ) 2 ] 3 ) was introduced as a new promoter for the glycosylation
with glucosyl 1-acetate ( > Scheme 42) [51]. And, the combination of trimethylsilyl halides
and Zn(OTf) 2 was also employed to give the corresponding glycoside in good yield from benzyl-protected fucosyl acetate ( > Scheme 43) [52]. In particular, a combination of TMSCl and
Zn(OTf) 2 afforded α-mannosides stereoselectively from benzyl-protected mannosyl acetate
without the participating group at C-2 ( > Scheme 44) [53].
3 Glycosylation with Donors Having Other Anomeric Ester Groups
Besides glycosyl acetate, a variety of other acyl groups were employed as good anomeric
leaving groups. The representative acyl leaving groups used in glycosyl donors for the glycosylation are summarized in > Table 2.
Mukaiyama and coworkers prepared some 1-O-haloacetyl-β-D-glucopyranoses by treating 2,3,4,6-tetra-O-benzyl-D-glucopyranose with haloacetyl chloride in the presence of
cesium fluoride or potassium fluoride and performed the glycosylation reactions with the
1-O-haloacetyl glucoses in the presence of TrClO 4 to obtain the corresponding glycosides
in good yields ( > Scheme 45) [43]. Furthermore, they investigated the reactivity of various
glucoses having substituted acetyl groups at the anomeric center, such as -COCH 2 OMe,
-COCH 2 CH 2 OMe, -COCH 2 SMe, and -COCH 2 OCH 2 CH 2 OMe, in the glycosylation reaction
in the presence of SnCl 4 and AgClO 4 as a promoter system and they reported that the glucopyranose having the (2-methoxyethoxy)acetyl (-COCH 2 OCH 2 CH 2 OMe) group at the anomeric