Carba--fructopyranoses

D-Fructose is the sweetest sugar known in naturally occurring carbohydrates, and its intense sweetness is produced only by ) -D-fructopyranose.  

The sweetness-eliciting units, AH (a proton donor) and B (a proton acceptor) components were respectively assigned to the anomeric OH group on C-2 and the OH group on C-1, and the CH2 group of C-6 was ascribed to a third (hydrophobic) component (X). 

The 6a-carba-y -D-fructopyranose molecule may have this tripartite arrangement (AH, B, and X) in almost the same emplacement as has the y -D-fructopyranose molecule. 

As it has been revealed that replacement of the ring-oxygen atom in a pyranoid sugar by a CH2 group is not detrimental to its sweetness, 6a-carba- -D-fructopyranose may have the same intense sweetness as D-fruc-tose. To substantiate this prediction, the following two reaction routes have been successfully developed for 6a-carba-y -DL-fructopyranose, as well as for the enantiomers. 

Reductive opening of the oxirane ring of 188 with lithium aluminum hydride, and acetylation with acetic anhydride-DMAP, provided the pen- 

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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]. 

The pseudo-sugars 6a-carha-a- (191) and -p-D-fructopyranose (192) have been prepared in six steps from an enzymatically resolved homochiral cyclohexane-triol building block via a common olefinic intermediate 190 by varying the dihydroxylation strategy (osmium tetroxide vs. m-CPBA respectively). Another non-carbohydrate based route to 5a-carba-sugars has been reported which utilizes a 7-norbornanone-derived cyclohexenoid intermediate. The preparation of carba-a- and -p-galacto-, -a-talo- and -a-fucopyranose derivatives were described. ... 

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