The aim of stereocontrolling of glycosylation has been approached in a variety of ways and the participation of a neighboring acyl-type group has been widely used to obtain 1,2-trans glycosides.1 Recently, our group has expanded methods available for 1,2-trans glycosylation by developing the neighboring 2-O-picolinyl (2-pyridylmethyl, Pic) participating group. It was demonstrated that 2-O-picolinyl-assisted glycosylations proceed via a formal six-membered ring intermediate leading to the formation of 1,2-trans glycosides with complete stereocontrol.2,3
Herein, we report the study of a series of novel glycosyl donors equipped with picolinyl and picoloyl (2-pyridinecarbonyl, Pico) groups at remote positions (C-3, C-4, or C-6). The major emphasis of this study is to investigate the effect that these remote substituents may have on the stereoselectivity of glycosylation. We demonstrated that O-picolinyl and O-picoloyl groups at remote positions can mediate glycosylation reactions by providing high or even complete facial syn-selectivity for the attack of the glycosyl acceptor. The set of data presented herein offers a strong evidence of the inter-molecular hydrogen bond-tethering between the glycosyl donor and glycosyl acceptor counterparts while providing a practically new methodology for the stereocontrolled synthesis.4 Challenging glycosidic linkages including α-gluco, β-manno, and β-rhamno have been obtained with high or complete stereocontrol.
References:
(1) Handbook of Chemical Glycosylation: Advances in Stereoselectivityand Therapeutic Relevance; Demchenko, A. V., Ed.; Wiley-VCH:Weinheim, Germany, 2008.
(2) Smoot, J. T.; Pornsuriyasak, P.; Demchenko, A. V. Angew. Chem. Int. Ed. 2005, 44, 7123-7126.
(3) Smoot, J. T.; Demchenko, A. V. J. Org. Chem. 2008, 73, 8838-8850.
(4) Yasomanee, J. P.; Demchenko, A. V. J. Am. Chem. Soc. 2012, 134, 20097-20102.
