Linear total synthetic routes to beta-D-C-(1,6)-linked oligoglucoses and oligogalactoses up to pentaoses by iterative Wittig olefination assembly.
Two complementary routes, A and B, have been followed for the stepwise iterative assembly of beta-D-(1,6)-glucopyranose and galactopyranose residues through methylene bridges. In route A the building block was constituted by 2,3,4-tri-O-benzyl-6-O-tert-butyldiphenylsilyl (O-TBDPS) beta-linked galactosylmethylenephosphorane, while in route B the building block was a beta-linked formyl C-glycopyranoside with a similar orthogonal protection of hydroxy groups. In route A each cycle consisted of the reaction of the phosphorane building block with a sugar residue bearing a formyl group at the C-5 carbon atom (coupling) and transformation of the O-TBDPS-protected primary alcohol into the formyl group (arming). Accordingly, route A is defined as the aldehyde route. On the other hand, each cycle in route B involved the coupling of the sugar aldehyde building block with a substrate bearing a phosphorus ylide at C-6 and introduction of the phosphonium group in the arming step as a precursor of the ylide functionality. Accordingly, route B is defined as the ylide route. The efficiency of route A proved to be seriously hampered by the 1,2-elimination of BnOH under the basic reaction conditions of the Wittig olefination, giving rise to the formation of substantial amounts of enopyranose. On the other hand, the ylide route B proved to be more efficient since very good yields (70-93%) of the isolated Wittig products were obtained throughout four consecutive cycles. Individual olefins and polyolefins obtained by routes A and B using gluco and galacto substrates were reduced and debenzylated in one pot by H(2)/Pd(OH)(2) to give the corresponding beta-D-C-(1,6)-linked oligosaccharides up to the pentaose stage. The latter compounds were fully characterized by high-field NMR spectroscopy (500 MHz).
Dondoni
Alessandro
A
Laboratorio di Chimica Organica, Dipartimento di Chimica, Università di Ferrara, Via Borsari 46, 44100, Italy. adn@dns.unife.it
Marra
Alberto
A
Mizuno
Mamoru
M
Giovannini
Pier Paolo
PP
D002384:Catalysis; D002625:Chemistry, Organic / Q000379:methods*; D005697:Galactosides / Q000138:chemical synthesis* / Q000737:chemistry; D005960:Glucosides / Q000138:chemical synthesis* / Q000737:chemistry; D006865:Hydrogenation; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009844:Oligosaccharides / Q000138:chemical synthesis* / Q000737:chemistry; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013237:Stereoisomerism
D005697:Galactosides; D005960:Glucosides; D009844:Oligosaccharides