Abstract
| - Two complementary routes, A and B, have been followed for the stepwise iterative assembly ofβ-d-(1,6)-glucopyranose and galactopyranose residues through methylene bridges. In route A thebuilding block was constituted by 2,3,4-tri-O-benzyl-6-O-tert-butyldiphenylsilyl (O-TBDPS) β-linkedgalactosylmethylenephosphorane, while in route B the building block was a β-linked formylC-glycopyranoside with a similar orthogonal protection of hydroxy groups. In route A each cycleconsisted of the reaction of the phosphorane building block with a sugar residue bearing a formylgroup at the C-5 carbon atom (coupling) and transformation of the O-TBDPS-protected primaryalcohol into the formyl group (arming). Accordingly, route A is defined as the aldehyde route. Onthe other hand, each cycle in route B involved the coupling of the sugar aldehyde building blockwith a substrate bearing a phosphorus ylide at C-6 and introduction of the phosphonium group inthe arming step as a precursor of the ylide functionality. Accordingly, route B is defined as theylide route. The efficiency of route A proved to be seriously hampered by the 1,2-elimination ofBnOH under the basic reaction conditions of the Wittig olefination, giving rise to the formation ofsubstantial amounts of enopyranose. On the other hand, the ylide route B proved to be more efficientsince very good yields (70−93%) of the isolated Wittig products were obtained throughout fourconsecutive cycles. Individual olefins and polyolefins obtained by routes A and B using gluco andgalacto substrates were reduced and debenzylated in one pot by H2/Pd(OH)2 to give thecorresponding β-d-C-(1,6)-linked oligosaccharides up to the pentaose stage. The latter compoundswere fully characterized by high-field NMR spectroscopy (500 MHz).
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