Carba-furanoses

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 ... 

Scheme 8.7 shows that the same methodology was used for the synthesis of carba-furanoses starting from D-erythrose. As illustrated, compound 18 was cyclized according to the methods described in Scheme 8.6, giving a 71% yield of the carbocycle 19. The ultimate result of this sequence was isolation of 4a-carba- 3-L-arabinofuranose. 

SCHEME 8.7 Synthesis of a carba-furanose using malonates. 

SCHEME 8.9 Synthesis of carba-furanoses using a nitro group. 

A few 4a-carba-furanoses were synthesized using the same methodology. However, after saturation of the alcohol 40 the syntheses diverged when protection of the hydroxyl, cleavage of the acetonide and oxidative cleavage of the diol afforded the aldehyde 44 (Scheme 8.15). This compound underwent an aldolisation reaction whose stereoselectivity could be controlled by the reaction temperature. Specifically, at — 90°C the bicyclic product 46 was obtained in a yield of 71%. Alternately, bicyclic derivative 45 was the principle product obtained when the reaction was run at room temperature (78%). Reduction of the lactone and desilylation afforded 4a-carba-(3-D-xylofuranose from isomer 45, whereas 4a-carba-(3-D-ribofuranose was formed from 46. Isomerisation of compound 40 led to the synthesis of two other isomeric carba-sugars 4a-carba-(3-L-lyxofuranose and 4a-carba-(3-L-arabinofuranose [36]. 

The 2-oxygenated lactone 67 was reduced by means of sodium borohydride to give the protected carbasugar 72, which by hydrolysis yielded carba- -D-manno-furanose (73). Similarly, the 2-amino substituted lactone 70b was reduced to carba-5-amino-5-deoxy-a-L-glucofuranose (74) [98a] (Scheme 15). 

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