0-Mannopyranosides synthesis

Scheme 3.10 Reversal of anomeric stereoselectivity in (i-mannopyranoside synthesis.

Selective Sulphonylation with A -Tosylimidazole. A One-Step Preparation of Methyl 2,3-Anhydro-4,6-0-benzylidene-a-D-mannopyranoside. Synthesis 1974, 203. 

Because of this situation, the prospect of synthesizing D-rhamnosyl cardenolides containing the unnatural, a-D-linkage was investigated. D-Rhamnose has not been reported to occur naturally, but it has been synthesized by Hudson and co-workers (4) in six steps, starting with methyl a-D-mannopyranoside. An intermediate in the synthesis is methyl... 

This section is completed with a brief review of the synthesis and properties of this epimer (20) of the precursor of thiazole in bacteria. This pentulose is conveniently accessible by an unconventional route (Scheme 19). Methyl 2,3 4,6-di-O-isopropylidene-a-D-mannopyranoside, readily available from methyl ot-D-mannopyranoside, is converted to the ketonic glycoside by butyllithium in 91% yield, following a method first published by Klemer and Rodemeyer43 and scaled up by Horton and Weckerle.44 This was converted by means of lithium hydroxide in a water-ether mixture into 3,5-0-benzylidene-l-deoxy-D-eryf/iro-2-pen-tulose in 55% yield. Hydrolysis to the free pentulose (20) proceeded in 73% yield in aqueous acetic acid. This product was obtained as a syrup with a characteristic absorption band at 1705 cm 1 as a film. Thus, there is a fair proportion of the open-chain ketone under these conditions, as with the D-threo epimer.45... 

SCHEME 8. Alternative strategy toward the synthesis of hexakis mannopyranosides.82... 

The synthesis of D-mannotetrazole 58 and D-rhamnotetrazole 57 from mannopyranoside 54 (Scheme 3), and L-rhamnotetrazole 59 from L-rhamnose (Scheme 4), provides the first examples of tetrazole analogs of carbohydrates... 

Stereoselective synthesis of beta-D-mannopyranosides with reactive mannopyranosyl donors possessing a neighboring electron-withdrawing group. Angew. Chem. Int. Ed. 2002,41,2972-2974. 

Srivastava, V. K. Schuerch, C., Synthesis of Beta-Deuterium Mannopyranosides and Regioselective Ortho-Alkylationof Dibutylstannylene Complexes. Tetrahedron Lett. 1979, 3269-3272. 

Furstner, A. Konetzki, L, A practical synthesis of beta-D-mannopyranosides. 

Ivanova, I. A. Nikolaev, A. V., Synthesis of beta-D-talopyranosides and beta-D-mannopyranosides via intramolecular nucleophilic substitution. J. Chem. Soc. Perkin Trans. 1 1998, 3093-3099. 

Example 19 Crich and Dudkin have used phosphoroamidite containing two different protecting groups 0-benzyl-0-2-cyanoethyl-iV,Ar-diisopropyl-phosphoroamidite [48]. This phosphitylating reagent was prepared in excellent yield from 2-cyanoethyl NyN diisopropylchlorophosphoroamidite, which was immediately used for coupling with an appropriate alcohol in the presence of tetrazole and oxidized without delay with TBHP. This kind of phosphorylation procedure was used in the synthesis of 4,8,12,16,20-pen-tamethyl-pentacosylphosphoryl / -o-mannopyranoside, an unusual / -man-nosyl phosphoisoprenoid from Mycobacterium avium. 

In 1985, Vasella and co-workers provided an efficient synthesis of COTC ((4R,5R,6R)-2-crotonyloxymethyl-4,5,6-trihydroxycyclohex-2-enone 52), a gly-oxalase I inhibitor.Methyl a-D-mannopyranoside was transformed in three synthetic steps into the protected all cis-trihydroxycyclohex-2-enone 163 (34 %, overall) which was then transformed into COTC (52) in four steps. Thus, transformations of our "naked sugar" 32 to 137 — 138 — 163 realize a total, asymmetric synthesis of COTC. Another total synthesis of this natural compound has been reported recently by Koizumi and co-workers, as mentioned earlier. 

Stoicheiometric RuOyCCl was also used to oxidise several furanoses, partially acylated glycosides and l,4 3,6-dianhydrohexitols [317] pyranosides to pyrano-siduloses [313] methyl 2,3,6-tri-O-benzoyl-a-D-glucopyranoside and its C-4 epimer to the a-D-xy/o-hexapyranosid-4-ulose (Table 2.3) [317], and methyl 2,3,6-trideoxy-a-D-e 7f/tro-hexapyranoside to the -a-D-,g/yceri9-hexa-pyranosid-4-ulose, an intermediate in the synthesis of forosamine [318], It was also used to oxidise benzyl 6-deoxy-2,3-0-isopropylidene-a-L-mannopyranoside to the a-L-/yxo-hexapyranosid-4-ulose [319] and for oxidation of isolated secondary alcohol functions, e.g. in the conversion of l,6-anhydro-2,3-0-isopropylidene-P-D-man-nopyranose to the-P-D-/yxo-hexa-pyranos-4-ulose mannopyranose (Fig. 2.16, Table 2.3 [20, 320, 324]). 

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