Sulfoxide and trichloroacetimidate

Almost all types of leaving groups have been explored in sialyl donors with, until recently, two notable exceptions the venerable imidates and the sulfoxides. While the preparation of stable, isolable sialyl trichloroacetimidates has still to be demonstrated, the recent innovation of the Biao Yu group—the (V-phenyltrifluoroacctimidatcs—has proven equal to the task and has provided access to new potent class of sialyl donors [27], Stable sulfoxides of sialyl thioglycosides have also been described recently and employed as glycosyl donors on activation with triflic anhydride (Tf20) [28], The S-benzoxazolyl (.S -Box) thiosialosides constitute another recent addition to the armory of sialyl donors [29, 30],... 

R. Weingart and R. R. Schmidt, Can preferentialjS-mannopyranoside formation with 4,6-O-benzyli-dene protected mannopyranosyl sulfoxides be reached with trichloroacetimidates Tetrahedron Lett., 41 (2000) 8753-8758. 

Garegg,95,96 and others97,98 involved the activation of alkyl halides over S -alkyl/aryl glycosides. Subsequently, other examples of suitable LGa for selective activations have become available trichloroacetimidates,99,100 phosphite,101 phosphate,102 thioi-midate,103-106 thiocyanate,107,108 hemiacetal,109,110 sulfoxide,111,112 selenoglyco-side,113-116 orthoester,76,117-119 O-hetaryl,120-122 and others.123-129 Many of these couplings were performed with the use of S -alkyl/aryl, fluoro, or O-al kenyl/hetary 1 moieties as LGb. 

Trichloroacetimidates and glycosyl sulfoxides are now commonly used for coupling (95, 96). Other coupling functional groups include phosphites, phosphates, thioglycosides, and pentenyl glycosides (97-99). 

---