Racemic carba-sugars, synthesis

The Diels-Alder cycloaddition of furan and acrylic acid, in the presence of hydroquinone as a polymerization inhibitor, provided enrfo-7-oxabicy-clo[2.2.1]hept-5-ene-2-carboxylic acid - (29) in a yield of 45%. Compound 29 was found to be the most accessible and important starting-material for the synthesis of various racemic carba-sugars, as well as their enantiomers. 

As 29 had been recognized as the most accessible starting-material for the synthesis of racemic carba-sugars, its resolution was successfully achieved with optically active a-methylbenzylamine as chiral reagent. Reaction of 29 with (-l-)-a-methylbenzylamine gave a mixture of two diastereoisomeric salts [(+)-amine, (—)-29 and (+)-amine, (-l-)-29], which were well separated, and the former salt was converted into (—)-29, [a] -111.8° (ethanol). Analogously, (+)-29, [a] +110.7° (ethanol), was obtained. ... 

All sixteen of the racemic carba-sugars predicted are known, as well as fifteen of the enantiomers. The most accessible starting-material for the synthesis of racemic carba-sugars is the Diels-Alder adduct of furan and acrylic acid, namely, e i o7-oxabicyclo[2.2.1]hept-5-ene-2-carboxylicacid (29). Furthermore, adduct 29 is readily resolved into the antipodes, (—)-29 and (+)-29, by use of optically active a-methylbenzylamine as the resolution agent. The antipodes were used for the synthesis of enantiomeric carba-sugars by reactions analogous to those adopted in the preparation of the racemates. 

The preparation of (-)-carba-P-ribofuranose and (+)-carba-a-ribofuranose by enzymatic resolution of racemic 3P-acetoxy-5P-(t-butyldimethylsiloxymethyl)cyclopentene and racemic 4p-(triphenylmethoxymethyl)cyclopent-2-en-ip-ol, respectively, as the key step has been reported. The preparation of P- and a- carba-psicofuranose derivatives 92 and 93 has been described starting from norbornenol, and the synthesis of the chain-extended carba-sugars 94 and 95 by a multistep process starting from D-allose derivatives has been reported. "... 

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