Mannopyranosyl trichloroacetimidate

In another variation on the same theme Seeberger and coworkers employed 2-0-acetyl-3,4,6-tri-0-benzyl-a-D-mannopyranosyl trichloroacetimidate 130 to construct the a-(l >2)-mannoheptaose 209 bound to the support by means of a m-4-octenyl linker.49 Cleavage from the resin was achieved with the first-generation Grubb s metathesis catalyst in the presence of ethylene, ultimately leading to the isolation of the heptasaccharide 210 in the form of a pentenyl glycoside ready for further transformations (Scheme 39). 

Neighbouring group participation in trichloroacetimidate glycosylations glycosylation of 4,6-0-benzylidene-l,2-0-(/ /5 )-ethylidene-a-D-glucopyranose with 2,3,4,6-tetra-O-benzoyl-a-D-mannopyranosyl trichloroacetimidate [39]. 

A solution of 3-(per luorooctyl)propanyloxybutenyl-4-0-benzyl-3,6-dihydroxy-2-O-pivaloyl-a-D-mannopyranoside (0.030 g, 0.03 mmol) and 2-0-acetyl-3,4,6-0-tribenzyl-a/fi-D-mannopyranosyl trichloroacetimidate (0.07mg, O.lOmmol) in dichloromethane (3 ml) was cooled to 5°C and TMSOTf (15 tl 0.80mmol) was added [47]. The reaction mixture was stirred for 30min. The reaction mixture was quenched with triethylamine (30 tl) and then concentrated under reduced pressure. The crude product was purified by solid-phase extraction using a FSPE... 

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. 

An interesting twist on the orthoester approach was uncovered by Fraser-Reid and coworkers in which it was revealed that a 4-pentenyl orthobenzoate 123, on activation with ytterbium triflate in dichloromethane, was able to selectively glycosylate the primary hydroxyl group in a mannopyranosyl 2,6-diol 122. Treatment with a 2-O-benzoyl mannosyl trichloroacetimidate 126 in diethyl ether with the same promoter then introduced a second y-mannosidic linkage to the remaining secondary hydroxyl group (Scheme 22).30 This selectivity pattern was central to the development of a very efficient synthesis of a branched pentadecamannan 129. 

---