Carba-pyranose

There are two forms of carba-sugar carba-pyranoses and -furanoses. The former, especially the carba-hexopyranoses, have been extensively studied during the past two decades, ever since their derivatives were found in Nature as components of important antibiotics. However, very little is known about carba-furanoses, except for 4a-carba-)3-L-arabinofuranose ... 

Carba-yS-DL-allopyranose (68) was synthesized from carba-) -DL-gluco-pyranose (33) by epimerization analogous to that employed in the afore-described reaction. O-Isopropylidenation of 33 provided two positional isomers, 63 and 64. Analogous reactions converted 63 into 68 by way of the intermediary compounds 65, 66, and 67. 

The reaction of the aldehyde 174, prepared from D-glucose diethyl dithio-acetal by way of compounds 172 and 173, with lithium dimethyl methyl-phosphonate gave the adduct 175. Conversion of 175 into compound 176, followed by oxidation with dimethyl sulfoxide-oxalyl chloride, provided diketone 177. Cyclization of 177 with ethyldiisopropylamine gave the enone 178, which furnished compounds 179 and 180 on sodium borohydride reduction. 0-Desilylation, catalytic hydrogenation, 0-debenzyIation, and acetylation converted 179 into the pentaacetate 93 and 5a-carba-a-L-ido-pyranose pentaacetate (181). 

Parker has outlined an elegant, enantioselective synthesis of L-vancosamine derivatives commencing from noncarbohydrate precursors (Scheme 17.38) [116]. This approach features a diastereoselective allenylstannane addition and W(CO)5-catalyzed cycloisomerization to construct the pyranose core. Oxidative cyclization of the C4-carba-mate 128 is performed with 10 mol% Rh2(OAc)4 and proceeds stereospecifically to give the crystalline oxazolidinone 129 (86%). All told, synthesis of this useful L-vancosa-mine glycal equivalent covers seven steps from (S)-(-)-ethyl lactate 127 and is accomplished in 44% overall yield. 

In the past, ketose-type pyranose carbasugars were mainly investigated in the search for sucrose mimics to employ as non-nutritive sweeteners [25a,b]. Probably due to this seemingly unappealing potential and the inherent difficulties related to their stereoconstitutional features, up until 1990 only one report was found to have been published dealing with the asymmetric synthesis of D- and L-carba-analogues of fructopyranose [34]. 

SCHEME 8.8 Synthesis of carba-pyranoses using a nitro group. 

SCHEME 8.12 Synthesis of a carba-pyranose using an aldol reaction. 

SCHEME 8.31 Synthesis of carba-pyranoses from the product of the Ferrier-II rearrangement. 

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