Concise and Effective Synthesis of 1→2 α‐Linked Mannopyranosyl Oligosaccharides and Related Antigenic Factor 34 and Dominant of Antigenic Factor 13

Abstract

the cell wall m n a n . 5-R Synthesis of 1-2 a-linked miinnose oligosaccharides has been achieved via multistep selective protection and deprotection using orthogonal masking groups. ' In continuation of our previous orthoester-based research'&'' here we report a new effective strategy for highly regio-and stereoselective synthesis of 1-2 a-linked mannopyranosyl oligosaccharides uk condensation of benzoylated manno-orthoesters .As outlined in Scheme 1, 2.3,4,6-tetra-O-acetyla-D-mannopposyl bromide ( 1) was transformed to the corresponding allyl orthoester in the presence of 2.4-11.1-tidine and allyl alcohol, and subsequent ZemplCn deacetylation ( 4 2 ) and benzoylation furnished the or-4 thoester 3. In the presence of catalytic amount of TMSOTf, 3 was transformed to allyl 2-0-acetyl-3,4,6-tri-O-benzoyI-a--mannopyranosyl-( I -+ 2 ) -3 , 4, 6-tri-0-benzoyl-a-D-mannopyranoside ( 4 ) in a good yield (66%) while the reamngement product allyl 2-0-acetyl-3,4,6-tri-0-benzoyl-a-D-mannopyranoside was the minor product ( 18% ) . l3 Formation of ( 1 -2 )-linked miinnose disaccharide from acetylated mannose orthoester was firstly reported by Lindhorst.14 However, the rather low yield ( < 30%) and difficulty for further transformation limited the use of this finding. We were gratified to find that replacement of the acetyl of the mannose orthoester with benzoyl made a substantially different result for the TMS promoted transformation of the mannose orthoester, i . e . the condensation product disaccharide 4 was obtained as the major product (66%).Owing to the high yield and easy separation, large quantity preparation of 4 was carried out and further transformation of 4 was successfully achieved. Thus deallylation followed by tri~hloroacetimidation~~ gave the disaccharide donor 5 (82%) while selective deacetylation16 at C-2 afforded acceptor 6 (93 % .