Synthetic Studies of Cellulose.

对于纤维素的合成，我们必须解决两个问题；1)区域特异性控制：必须对氨基葡萄糖残基的0-4位羟基进行糖基化；2)立体特异性控制：必须对氨基葡萄糖残基的β构型进行糖苷连接。用2,3,6 -三-0取代的葡萄糖醛酸衍生物作为起始原料解决了第一个问题，但合成的主要障碍在于第二个问题，β -糖基化，尽管有许多新的糖基化方法和大量关于糖基化反应的数据。首先，我们计划了纤维素低聚糖的合成，在纤维素合成之前获得了β -糖基化的信息，并成功合成了一系列纤维素低聚糖衍生物，最高可达八聚体。现在，我们有了以下关于糖基化反应的有用信息。1)吡咪酯法是β -糖基化的最佳方法。2)在葡萄糖的2,3,6位羟基上引入苄基保护基团比酰基更有效地进行高产β -糖基化。3)在酰基衍生物中，在糖基的3-0位上引入烷基取代基，尤其是苯基，是实现高选择性、高收率β -糖基化的必要条件。4)另一方面，在糖醛基4-0位引入给电子取代基对β -糖基化非常有效，但吸电子酰基降低了β -葡萄糖苷的产率。因此，我们发现，在考虑取代基效应的情况下，选择最合适的起始葡萄糖保护基团可以使我们成功地合成纤维素。

For the synthesis of cellulose, We have to solve two problems ; 1) regiospecific control : glycosylation of the hydroxyl group at the 0-4 position of the glucopyranosyl residue has to be performed, 2) stereospecific control : glucosidic linkage of glucopyranosyl residues of beta-configuration has to be made. The first problem is solved by use of 2, 3, 6-tri-0-substituted glucopyranose derivatives as a starting material, but the main obstacle in the synthesis lies in the second problem, beta-glycosylation, although there are many newly-developed glycosylation methods and extensive data on glycosylation reactions. First of all, we have planned the synthesis of cello-oligosaccharides for obtaining information about beta-glycosylation before the synthesis of cellulose, and succeeded in the synthesis of a series of cello-oligosaccharides derivatives up to octamer. Now, we have following useful information about glycosylation reactions. 1) The imidate method is the best one for beta-glycosylation. 2) Benzyl protective groups introduced into hydroxyl groups at 2, 3, 6-positions of glucose are more effective than acyl groups for the high yield beta-glycosylation. 3) In the cade of acyl derivatives, an alkyl substituent group, especially benzyl group, introduced into 3-0-position of aglycon is indispensable for the highly selective beta-glycosylation with high yield. 4) On the other hand, electron-donating substituent group introduced into 4-0-position of glycon is extremely effective for the beta-glycosylation, but the electron-withdrawing acyl group decreases the yield of beta-glucoside.Consequently, we have found that the most suitable selection of the protective groups of the starting glucose after considering susbstituent effects enable us to success the synthesis of cellulose.