Crich David, Banerjee Abhisek, Yao Qingjia
Department of Chemistry, University of Illinois, 845 West Taylor Street, Chicago, IL 60607-7061, USA.
J Am Chem Soc. 2004 Nov 17;126(45):14930-4. doi: 10.1021/ja047194t.
The direct syntheses of a beta-(1-->2)-mannooctaose and of a beta-(1-->4)-mannohexaose are reported by means of 4,6-O-benzylidene-protected beta-mannosyl donors. The synthesis of the (1-->2)-mannan was achieved by means of the sulfoxide coupling protocol, whereas the (1-->4)-mannan was prepared using the analogous thioglycoside/sulfinamide methodology. In the synthesis of the (1-->4)-mannan, the glycosylation yields and stereoselectivities remain approximately constant with increasing chain length, whereas those for the (1-->2)-mannan consist of two groups with the formation of the tetra- and higher saccharides giving yields and selectivities consistently lower than those of the lower homologues. The decrease in yield after the trisaccharide in the (1-->2)-mannan synthesis is attributed to steric interference by the n-3 residue and is consistent with the collapsed, disordered structure predicted by early computational work. The consistently high yields and selectivities seen in the synthesis of the (1-->4)-mannan are congruent with the more open, ordered structure originally predicted for this polymer. The lack of order in the structure of the (1-->2)-mannan, as compared to the high degree of order in the (1-->4)-mannan, is also evident from a comparison of the NMR spectra of the two polymers and even from their physical nature: the (1-->2)-mannan is a gum and the (1-->4)-mannan is a high melting solid.
报道了通过4,6-O-亚苄基保护的β-甘露糖基供体直接合成β-(1→2)-甘露八糖和β-(1→4)-甘露六糖。(1→2)-甘露聚糖的合成通过亚砜偶联方案实现，而(1→4)-甘露聚糖则使用类似的硫代糖苷/亚磺酰胺方法制备。在(1→4)-甘露聚糖的合成中，随着链长增加，糖基化产率和立体选择性大致保持恒定，而(1→2)-甘露聚糖的糖基化产率和立体选择性则分为两组，四糖及更高糖的形成产率和选择性始终低于较低同系物。(1→2)-甘露聚糖合成中三糖之后产率的降低归因于n-3残基的空间干扰，这与早期计算工作预测的塌陷、无序结构一致。(1→4)-甘露聚糖合成中始终较高的产率和选择性与最初预测的该聚合物更开放、有序的结构一致。与(1→4)-甘露聚糖高度有序的结构相比，(1→2)-甘露聚糖结构缺乏有序性，这也从两种聚合物的核磁共振光谱比较甚至从它们的物理性质中明显看出：(1→2)-甘露聚糖是一种树胶，而(1→4)-甘露聚糖是一种高熔点固体。
