Synthesis of an N-Acetylated Heparin Pentasaccharide and its anticoagulant activity in comparison with the heparin pentasaccharide with high anti-factor-Xa activity

✍ Scribed by Hans Peter Wessel; Ludvik Labler; Thomas B. Tschopp

• Publisher: John Wiley and Sons
• Year: 1989
• Tongue: German
• Weight: 714 KB
• Volume: 72
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19890720613

✦ Synopsis

The synthesis of tri-N-acetylated heparin pentasaccharide 2 is described. It was assembled from five suitably blocked monosaccharide units (S7). Glucuronic-acid building block 4 was prepared from glucose by direct Jones oxidation of the 6-0-trityl derivative 18. The resulting acid 16 was esterified to 17 in large amounts using methyl chloroformate/base. Trimethylsilyl bromide proved to he an excellent reagent for the hydrolysis of a prop-I-enyl glycoside (19 + 21). The pentasaccharide 29 was obtained by a [2 + 21 + 1 synthesis, the glycosylation reactions furnished good to very good yields. The identity of protected oligosaccharides was confirmed by 'H-NMR spectroscopy. Sequential deblocking of the pentasaccharide, U-sulfation, and N-acetylation gave 2 which was shown to exhibit ca. 600 times lower anticoagulant activity than pentasaccharide 1.

Introduction. -Heparin is a sulfated glucosaminoglycuronan used in the clinic for over 50 years because of its anticoagulant properties mediated by interaction with antithrombin 111 (AT 111). Degradation of heparin followed by investigation of heparin fragments led to the hypothesis that a specific heparin sequence represents the bindingsite for AT I11 [l] [2]. Since then, three groups [3] have reported syntheses of the heparin pentasaccharide 1, the fragment with high affinity for AT 111. Several analogues have been prepared to study structure-activity relations [4]. In addition to its anticoagulant properties, heparin exerts a number of biological effects, including antilipemic [5], antiangiogenic [6], and antiproliferative activities [7]. Therefore, we turned our attention to non-anticoagulant heparin fractions. A de-N-sulfated, N-acetylated heparin was described that lacked its anticoagulant activity, but retained the anticomplementary activity and the ability to inhibit growth of smooth muscle cells [8]. We now report on the synthesis of the tri-N-acetylated heparin pentasaccharide 2.

Results and Discussion. -For the assembly of the heparin pentasaccharide we adopted the approach of Sin@ et al. [3a] starting with five suitably blocked monosaccharide units, namely 3 [9], 4 [4fl, 5 [lo], 6, and 7 [ll]. For the selective acetylation of 8 [12], we employed equimolar amounts of Ac,O instead of N-acetylimidazole [ 1 13, furnishing monoacetate 7 in 83 % yield, along with diacetate 9 ([13], 6%), 4-0-acetate 10 (6%), and starting material 8 (3 %). Iduronic ester 11 [l I] was prepared from glucose or, in a shorter synthesis, from 6,3-glucuronolactone via alkylating lactone opening [ 141. For temporary protection of the 4-OH group, we preferred a levulinic ester ( = 4-oxopentanoate) which usually is cleaved in excellent yield. Thus, treatment of 11 with levulinic anhydride in pyridine gave 6 . Azido sugar 5 was prepared as described [lo] but using LiN, (cf. [3b])