Fraser-Reid pentenyl glycosylation

Building on the observations of Kahne and Takahashi, Chenault and Castro relied on the selective activation of an isopropenyl glycoside donor with TMSOTf, followed by subsequent activation of a second n-pentenyl glycoside with NIS/TESOTf.12 This one-pot glycosylation offers proof of the concept that Fraser-Reid s glycosides can be incorporated into one-pot syntheses. (See Scheme 11.7.)... 

Introduced by Fraser-Reid in 1988 [17], this method (Scheme 5) depends on electrophilic addition to the olefin, followed by intramolecular displacement by the ring oxygen and eventual expulsion of the pentenyl chain to form an oxonium species. Trapping with a glycosyl acceptor then leads to the desired glycoside. 

Mach, M, Schlueter, U, Mathew, F, Fraser-Reid, B, Hazen, K C, Comparing n-pentenyl orthoesters and n-pentenyl glycosides as alternative glycosyl donors. Tetrahedron, 58, 7345-7354, 2002. 

Allen, J G, Fraser-Reid, B, -Pentenyl glycosyl orthoesters as versatile intermediates in oligosaccharide synthesis. The proteoglycan hnkage region, J. Am. Chem. Soc., 121,468-469,1999. 

Fraser-Reid et al. [21,22] reported the synthesis of polymer supported oligosaccharides using n-pentenyl glycosides, while Seeberger et al. [23] accomplished the synAesis of p-(l- 4)- and p.(2 6)-linked ohgosaccharides using glycosyl phosphates combined with an octenediol linker. 

P. Konradsson and B. Fraser-Reid, Conversion of 4-pentenyl glycosides into glycosyl bromides, J. Chem. Soc. Chem. Commun. p. 1124 (1989). 

Konradsson and Fraser-Reid [44] reported the conversion of NPGs into glycosyl bromides, e. g. 76, by treatment of the corresponding pentenyl glycoside with a dilute dichloromethane solution of bromine, conditions that favor unimolecular reaction. The reaction was shown to be compatible with acetals, benzyl, silyl, and allyl protecting groups in the NPG (O Scheme 11a). 

Arasappan and Fraser-Reid described the preparation of w-pentenyl galactofuranosides and evaluated their prospects as glycosyl donors (O Scheme 21) [81]. Fischer glycosidation of D-galactose under kinetic conditions using -pentenyl alcohol and DMSO as co-solvent [82]... 

K. N. Jayaprakash and B. Fraser-Reid, A study of n-pentenyl orthoesters having manno, gluco, and galacto configurations in regioselective glycosylations, Carbohydr. Res., 342 (2007) 490-498. 

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. 

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