Pyranose sugars, synthesis

Barnes, J J, Davidson, A H, Hughes, L R, Procter, G, The synthesis of optically active tetrahydropyrans by the addition of a stabilized Wittig reagent to pyranose sugars, J. Chem. Soc., Chem. Commun., 1292—1294, 1985. 

So far, the only examples of O-C migrations have applied to pyranose sugars or derivatives thereof. However, it should be noted that such reactions are applicable to furanose sugars. One particular example, reported by Matsumoto, et al.,101 was used in the total synthesis of gilvocarcin M. The specific reaction, shown in Scheme 2.4.25, utilized the acetoxy activated furanoside shown. Treating this compound with the phenol and using the hafnocene dichloride-silver perchlorate complex as the Lewis acid, the desired C-arylglycoside was obtained in 87% yield with the a anomer favored in a ratio of 8 1. 

The oxidative ring expansion of furyl carbinols can be accomplished with many of the same oxidants discussed earlier for routine oxidation of the furan ring. As a pyran is formed in the reaction, an obvious application would be for the synthesis of pyranose sugars (Scheme 19). 

Sulfinylacrylate 41 has been successfully used in the enantioselective synthesis of pseudo-sugar [46, 47]. Cycloaddition of (S)-3-(2-pyridylsulfinylacrylate) (41) with furan and 3,4-dibenzyloxyfuran under Et2AlCl catalysis afforded cycloadducts 42, 43 and 44 (Equation 3.12) which were converted into pseudo-manno-pyranoses 45, 46 and 47 (Figure 3.5). 

The enzymatic synthesis of sucrose also throws light on the formation of the furanose form of fructose in the sucrose molecule. The fact that sucrose is directly formed from D-glucose-l-phosphate and D-fructose supports Isbell and Pigman s34 and Gottschalk s85 evidence that the latter monosaccharide occurs in solution in an equilibrium mixture of furanose and pyranose forms. This makes it unnecessary to postulate a special mechanism of stabilization of a five membered (furanose) ring before the formation of compound sugars containing the D-fructose molecule.86... 

Aiming at the pyranose form of sugars, normal type hetero-Diels-Alder reactions were extensively used for the synthesis of functionally substituted dihydropyran and tetrahydropyran systems (5-10) (see routes A - D in the general Scheme 1) which are also important targets in the "Chiron approach" to natural product syntheses (2.) Hetero-Diels-Alder reactions with inverse electron demand such as a, p-unsaturated carbonyl compounds (l-oxa-1,3-dienes) as heterodienes and enol ethers as hetero-dienophiles, are an attractive route for the synthesis of 3,4-dihydro-2H-pyrans (11). 

There are several instances where reactive intermediates present during the synthesis of septanosides have reacted intramolecularly to give ring-contracted or bicyclic products.91 Similar intramolecular reactions (such as the formation of 1,6-anhydro sugars) have precedents in the pyranose literature. Such cases of intramolecular reactivity described in the literature notably involve novel ethers or thioethers as the nucleophilic species in the ring contractions. 

Scheme 47 Synthesis of a 2-C-branched-chain sugar via a pyranose-fused butenolide...

The parent compounds of all pyranoses, the 2H- and 4H-pvrans (165 and 166), and their derivatives, are impractical as substrates for the synthesis of sugars. They are usually difficultly available and are rather unstable.60... 

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