Unsatd. sugars are useful synthetic intermediates for the conversion of common saccharides into complex carbohydrates, as well as for the prepn. of other types of enantiomerically pure derivs. The effectiveness of this new method for the prepn. of hexenopyranoside enol ethers has been further illustrated through some new examples involving: the 4-OH group of two beta-D-talopyranosides carrying different types of ether protecting groups; the 4-OH group of the D-TalNAc unit of a complex trisaccharide; and the 2-OH group of two beta-D-talopyranosides and of a 6-deoxy-alfa-L-talopyranoside. The NaH/Im2SO2 system is a valuable tool for the one-pot dehydration of pyranosides having an axial OH group. The method presents several advantages with respect to the alternative use of triflates, which requires two distinct reactions, purifn. of the intermediate triflate, and use of more expensive and less stable reagent. Moreover, the activation-elimination with NaH/Im2SO2 system requires short reaction times, generally permitting the prepn. and purifn. of hexenopyranoside enol ethers in only 1 day. The method should be possible to apply to other pyranosides and to other six-membered polyoxygenated systems (inositols). The sole limitation is constituted by the combination of absence of stereoelectronic assistance and the presence of a steric hindrance to the imidazolate attack to the departing proton, as in the case of alfa -D-galacto alcs.
NaH/Im2SO2-Mediated Preparation of Hex-2- and Hex-3-Enopyranoside Enol Ethers
CATELANI, GIORGIO;D'ANDREA, FELICIA;GUAZZELLI, LORENZO;
2011-01-01
Abstract
Unsatd. sugars are useful synthetic intermediates for the conversion of common saccharides into complex carbohydrates, as well as for the prepn. of other types of enantiomerically pure derivs. The effectiveness of this new method for the prepn. of hexenopyranoside enol ethers has been further illustrated through some new examples involving: the 4-OH group of two beta-D-talopyranosides carrying different types of ether protecting groups; the 4-OH group of the D-TalNAc unit of a complex trisaccharide; and the 2-OH group of two beta-D-talopyranosides and of a 6-deoxy-alfa-L-talopyranoside. The NaH/Im2SO2 system is a valuable tool for the one-pot dehydration of pyranosides having an axial OH group. The method presents several advantages with respect to the alternative use of triflates, which requires two distinct reactions, purifn. of the intermediate triflate, and use of more expensive and less stable reagent. Moreover, the activation-elimination with NaH/Im2SO2 system requires short reaction times, generally permitting the prepn. and purifn. of hexenopyranoside enol ethers in only 1 day. The method should be possible to apply to other pyranosides and to other six-membered polyoxygenated systems (inositols). The sole limitation is constituted by the combination of absence of stereoelectronic assistance and the presence of a steric hindrance to the imidazolate attack to the departing proton, as in the case of alfa -D-galacto alcs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
