Carba-pyranoses, synthesis

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. 

Scheme 27 Ogawa et al. s synthesis of enantiomerically pure carba-pyranoses...

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]. 

A low yielding synthesis of 2-acetamido-5a-carba-2-deoxy-a-D,L-a//o- and gwlo-pyranose tetraacetates and some 3-acetamido-3-deoxy derivatives have been achieved starting from bicyclic acetate 87." By use of the same intermediate the preparations of valienamine and valiolamine have also been achieved." ... 

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