Synthesis of the ?-Rhamnopyranosides and the 6-Deoxy-?-mannoheptopyranosides

Abstract

A review is presented of the development of methods for the synthesis of ?-rhamnopyranosides and the related 6-deoxy-?-mannoheptopyranosides based on the 4,6-O-alkylidene directed synthesis of mannopyranosides and their 6-thia derivatives followed by selective reduction at the 6-position. Applications to total synthesis of complex bacterial oligosaccharides containing the title moieties are presented.

Intramolecular Aglycon Delivery Enables the Synthesis of 6-Deoxy-β-d-manno-heptosides as Fragments of Burkholderia pseudomallei and Burkholderia mallei Capsular Polysaccharide

“…In the core and surface polysaccharides of Gram-negative bacteria, l - and d - glycero - d - manno -heptoses of the α-anomeric configuration (1,2- trans ) are ubiquitous . Stereoselective syntheses of complex bacterial oligosaccharides incorporating these α- manno -heptoses have been achieved by taking advantage of the so-called neighboring group participation effect of having an acetyl at the C2 position. − Nevertheless, the synthesis of β-linked- manno -heptosides (1,2- cis ) still represents a major challenge. As far as we are aware, Crich and co-workers − are the only group to have reported the stereocontrolled synthesis of 1,2- cis - manno -heptosides in either the d - or l - glycero configurations.…”

Methodology Development and Physical Organic Chemistry: A Powerful Combination for the Advancement of Glycochemistry

“…91 This methodology, which has been reviewed, 67b,92 was subsequently applied to the synthesis of a β-1,3-D-rhamnan 93 and to the 6-deoxy-β-D-mannoheptosides. 94 Seeking a perhaps more practical third generation method we turned to 6-deoxy-6-thia mannosyl donors with the 4,6-hydroxythiol unit tied up in a monothioacetal.…”

“…After glycosylation treatment with Raney nickel in hot methanol cleanly affected desulfurization to give the β-rhamnosides while at the same time removing any benzyl protecting groups. 92,95 The cyano group present in the monothioacetal function of donors 43 and 44 is a critical component and functions to prevent activation of the cyclic thioacetal by the BSP and triflic anhydride mixture used for glycosylation while at the same time improving stereoselectivity because of its electron-withdrawing nature. 92,95 …”

Efficient routes toward the synthesis of the D-rhamno-trisaccharide related to the A-band polysaccharide of Pseudomonas aeruginosa

“…Deoxygenation at the C-6 position of D-mannose derivatives bearing conventional or Crich’s modified 4,6- O -benzylidene protection [19–23] has been the principal philosophy behind the synthesis of the D-rhamnose [19–23 30,33–39] motif. This is a natural choice not only because it allows simultaneous selective blockage of the O-4 and O-6 positions but also sets up the model on which various deoxygenation protocols may be tried.…”

“…The problem of β-D-rhamnoside synthesis has been greatly addressed by Crich et al [19–23]. But, Crich’s global deoxygenating strategy, despite its ultimate efficiency, still requires synthetic modification on the conventional 4,6- O -benzylidene framework involving reagents which are rather expensive.…”