CHEMICAL AND CHEMOENZYMATIC SYNTHESIS OF THE -GALPNAC-(1-]4)-BETA-D-GLCPNAC-(1-]2)-ALPHA-D-MANP ELEMENT THAT IS PART OF N-LINKED CARBOHYDRATE CHAINS OF HUMAN LUTROPIN

In the framework of a project aimed at the elucidation of the nature o f the functional importance of the N-glycosylation of the alpha-subuni t of the glycoprotein hormones human lutropin and human chorionic gona dotropin, the structural element alpha-Neu p5Ac-(2 --> 6)-beta-D-GalpN Ac-(1 --> 4)-beta-D-GlcpNAc-(1 --> 2)-alpha-D-Manp, which is part of t he carbohydrate chains of human lutropin, has been prepared by chemica l and chemo-enzymatic synthesis in the form of its propyl glycoside. C ondensation of -2-deoxy-2-phthalimido-alpha/beta-D-glucopyranosyl tric hloroacetimidate with allyl 3,4,6-tri-O-benzyl-alpha-D-mannopyranoside gave after deacetylation allyl yl-2-deoxy-2-phthalimido-beta-D-glycop yranosyl)-(1 --> 2)-3,4,6-tri-O-benzyl-alpha-D-mannopyranoside. Ethyl -deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside was converted into the galacto-derivative ethyl 4,6-di-O- deoxy-2-phthalimidio-1-thio-bet a-D-galatopranoside was converted into the galacto-derivative ethyl eo xy-2-phthalimido-1-thio-beta-D-galactopyranoside via an oxidation-redu ction route, as well as via S(N)2-type substitution with acetate. The use of this gabacto thioglycoside, after its conversion into the corre sponding bromide, as GalN donor for condensation with the mentioned di saccharide derivative yielded after deacetylation allyl yl-2-deoxy-2-p hthalimido-beta-D-glucopyranosyl)-(1 --> yl-2-deoxy-2-phthalimido-beta -D-glucopyranosyl)-(1 --> 2)-3,4,6-tri-O-benzyl-alpha-D-mannopyranosid e. Methylsulfenyl bromide-silver triflate promoted sialylation of this trisaccharide derivative with O-ethyl S-[methyl -glycero-alpha-D-gala cto-non-2-ulopyranosyl)onate] dithiocarbonate and subsequent deprotect ion resulted into the aimed tetrasaccharide structural element. Altern atively, this compound was prepared via a block synthesis, which, howe ver, was not superior to the linear strategy. Finally, a stereoselecti ve sialylation of synthetically prepared beta-D-GalpNAc-(1 --> 4)-beta -D-GlcpNAc-(1 --> 2)-alpha-D-Manp-(1 --> 0)CH2CH2CH3 with CMP-Neu5Ac a nd rat liver alpha-2,6-sialyltransferase was accomplished affording th e same tetrasaccharide structural element. (C) 1997 Elsevier Science L td.