Anomeric control

These results demonstrate that O-glycosyl trichloroacetimidate-based oligosaccharide synthesis on solid support may eventually become a valuable alternative to solution-phase synthesis because useful experience is available for the selection of the polymer support and choice of the linker system and the glycosyl donor. Further standardization of the building blocks and the protective group pattern will finally provide the yields and the anomeric control in order to successfully plan automated syntheses of oligosaccharides also in a combinatorial manner. 

SCHEME 4.11 Dual anomeric control using IAD in the synthesis of a,a-trehalose. PCB,... 

In the transformation 16 > 14, water attacks the oxonium ion on the axial face. The electron pair of the cleaved n bond ends up in the axial orbital on the ring oxygen that exerts anomeric effect on the crc () bond that is just formed. An attack from the equatorial site will generate 19, where the new ctc o bond formed is not in anomeric effect with any of the electron pair orbitals of the ring oxygen. Both the formation and the cleavage of a bond under anomeric control are more facile than when the anomeric effect is absent. We shall continue to learn this aspect through the discussions below as well. 

Collectively, these results support the proposed mode of anomeric control by a (S)-(phenylthiomethyl)benzyl moiety at C-2 of a glycosyl donor. The new glycosylation protocol described here in combination with traditional... 

This latter fact has been used as a convenient tool to modulate the regioselec-tivity of glycosylation by switching the configuratitMi at the anomeric center of the glycosidic acceptor. This technique has been commonly denoted as anomeric control (Scheme 2.223). 

Scheme 2.223 Anomeric control in Ar-acetylglucosaminyl transfer onto a- and 3-D-methylglucosides by (3-galactosidase...

The potentially useful insecticide milbemycin P3 (198) has been synthesized in racemic form by Smith and co-workers. Intramolecular closure of the hemiacetal (193), derived from the lactone (192), first gave, under anomeric control, the crystalline aldehyde (194). After chain extension to (195), a coupling reaction... 

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