Synthesis of a 3,4‐Di‐O‐substituted Heptose Structure: A Partial Oligosaccharide Expressed in Neisserial Lipooligosaccharide

Abstract

AbstractWe have synthesized a tetrasaccharide containing a 3,4‐dibranched L‐glycero‐D‐manno‐heptose (Hep), β‐lactosyl‐(1⇄4)‐[L‐α‐D‐Hep‐(1⇄3)]‐L‐α‐D‐Hep 19, by using a mannose (Man) derivative as an acceptor. Prior to the construction of the branched Hep, we confirmed that the 3,4‐dibranched Man structure could be synthesized using a 3‐branched Man 6 as an acceptor. Glycosylation of the acceptor 6 using hepta‐O‐acetyl‐α‐lactosyl trichloroacetimidate (7) gave the desired 3,4‐dibranched structure, β‐lactosyl‐(1⇄4)‐[α‐Man‐(1⇄3)]‐α‐Man 8. As expected, β‐lactosyl‐(1⇄4)‐[L‐α‐D‐Hep‐(1⇄3)]‐α‐D‐Man 14 was also obtained by glycosylating the 4‐OH acceptor 13 with 7 in a similar manner. The Man residue of 14 was converted into the Hep unit by Swern oxidation, Grignard reaction, and oxidative cleavage followed by reduction. Thus, we constructed the 3,4‐dibranched Hep structure 19 by using the 3‐branched Man 13 as an acceptor. The current results demonstrate that the gauche orientation of the O‐3 and O‐4 units of the Man configuration does not prevent the formation of the 3,4‐di‐O‐substituted structure. This approach should provide an alternative method to synthesize the 3,4‐dibranched Hep structure expressed in LOS produced by pathogenic Gram‐negative bacteria such as the Neisserial and Haemophilus species. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)