Synthesis of a 2,3‐Di‐O‐substituted Heptose Structure by Regioselective 3‐O‐Silylation of a 2‐O‐Substituted Heptose Derivative

Abstract

AbstractA 3,4‐diol derivative of 2‐O‐benzyl (Bn) heptose (Hep), methyl 6,7‐di‐O‐acetyl‐2‐O‐benzyl‐L‐glycero‐α‐D‐manno‐heptopyranoside (3), was treated with both triethylsilyl (TES) and tert‐butyldimethylsilyl (TBDMS) chlorides to regioselectively form the 3‐O‐silyl ethers 4 and 6, respectively. To examine whether silylation of the 3,4‐diol of a 2‐O‐substituted disaccharide also gives the corresponding 3‐O‐silylated disaccharide, we synthesized α‐GlcN3‐(1⇄2)‐Hep 11a by coupling a Hep 2‐OH acceptor 9 with a GlcN3 trichloroacetimidate 10. As expected from the results obtained using the 2‐O‐Bn Hep 3, treatment of α‐GlcN3‐(1⇄2)‐Hep‐3,4‐diol 14 with TESCl followed by acetylation gave only the 3‐O‐TES 15. Compound 14 was converted into the 3‐OH acceptor 16 by silylation/acetylation — without isolating 15 — and subsequent acid hydrolysis. By coupling the disaccharide 3‐OH acceptor 16 with per‐O‐benzylated β‐lactosyl trichloroacetimidate 17, we obtained the desired 2,3‐branched tetrasaccharide, α‐Lac‐(1⇄3)‐[α‐GlcN3‐(1⇄2)]‐Hep 18a. Hydrogenation of 18a, followed by N‐acetylation, gave α‐Lac‐(1⇄3)‐[α‐GlcNAc‐(1⇄2)]‐Hep 22. Thus, we synthesized the 2,3‐dibranched Hep by utilizing the 2‐O‐substituted Hep. This regioselective O‐3‐silylation of the 2‐O‐substituted Hep provides an intermediate that can be utilized for the synthesis of not only 2,3‐ and 3,4‐dibranched Hep but also the 2,3,4‐tribranched Hep structures present in lipooligo‐ and lipopolysaccharides produced by pathogenic Gram‐negative bacteria. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)