Direct Chemical Synthesis of the β-d-Mannans: The β-(1→2) and β-(1→4) Series
journal contributionposted on 17.11.2004, 00:00 by David Crich, Abhisek Banerjee, Qingjia Yao
The direct syntheses of a β-(1→2)-mannooctaose and of a β-(1→4)-mannohexaose are reported by means of 4,6-O-benzylidene-protected β-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.