Compound18mimicking lipid A, containing D-glucose instead of D-glucosamine moieties in its gentiobiose skeleton,O-tetradecyl groups at the C-2, C-3, C-2' and C-3' instead of the ester- and amide-linked fatty acids, andO-allyl groups at the C-1α and C-4' replacing the phosphate groups, was synthesized by the Schmidt trichloroacetamidate method in a combined 8% yield of 13 steps. Allyl 4,6-O-(4-methoxybenzylidene)-α-D-glucopyranoside (1) and methyl 4,6-O-benzylidene-α-D-glucopyranoside (4) were starting materials for preparation of the respectiveO-alkylated andO-allylated glycosyl donor and sugar nucleophile. While boron trifluoride etherate in dichloromethane catalysed a highly preferential formation of the required β-(1→6)-glycosidic bond, α-linked lipidodisaccharide was a major product when trimethylsilyl trifluoromethanesulfonate was used as a catalyst, in both cases independently of the anomeric configuration of the starting imidate. Prolonged treatment with acid catalysts in the coupling step was exploited also for a one-pot removal of the intermediate 4-O-(4-methoxybenzyl) protection of the target mimic18of lipid A.
合成了一种类脂A的化合物18,其龙胆二糖骨架中的D-葡萄糖胺基团被替换为D-葡萄糖基团,C-2、C-3、C-2'和C-3'位置上的酯和酰胺连接的脂肪酸被替换为O-十四烷基,C-1α和C-4'位置上的磷酸基团被替换为O-丙烯基,该化合物通过Schmidt三氯乙酰胺法合成,共13步,总收率为8%。丙烯基4,6-O-(4-甲氧基苯甲醛基)-α-D-葡萄糖苷(1)和甲基4,6-O-苯甲醛基-α-D-葡萄糖苷(4)是制备相应的O-烷基化和O-丙烯基化糖基供体和糖核苷的起始物质。在二氯甲烷中,三氟化硼醚催化高度优先形成所需的β-(1→6)-糖苷键,当三甲基硅基三氟甲磺酸盐用作催化剂时,α-连接的脂肪二糖是主要产物,在两种情况下,不管起始亚胺的异构构型如何。在偶联步骤中,长时间使用酸催化剂还可用于一锅法去除目标类脂A的中间体4-O-(4-甲氧基苯甲醇)保护基。