Synthesis of Rare Carbohydrates and Biomolecules from Furan

Zinc iodide‐catalyzed cycloaddition of furan to 1‐cyanovinyl (1′S)‐camphanate or 1‐cyanovinyl (1′R)‐camphanate led to optically pure (1R,2S,4R)‐2‐exo‐cyano‐7‐oxabicyclo[2.2.1]‐hept‐5‐en‐2‐yl (1S')‐camphanate and (1S,2R,4S)‐2‐exo‐cyano‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐yl (1R')‐camphanate, respectively. Saponification afforded (1R,4R)‐ and (1S,4S)‐7‐oxabicyclo[2.2.1]‐hept‐5‐en‐2‐one, respectively, and allowing recovery of the chiral auxiliaries (1S)‐ and (1R)‐camphanic acid, respectively. The 7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐yl derivatives (”naked sugars“) so‐obtained can be substituted at C(3), C(5) and C(6) by direct techniques with high stereoselectivity. The methods exploit the exo‐facial selectivity of the reactions of these bicyclic systems and the regiochemical control exerted by the remote substituents at C(2) in electrophilic attack at C(5) and C(6). The polysubstituted 7‐oxabicyclo[2.2.1]heptan‐2‐ones so‐obtained are chirons that can be converted into D‐ or L‐hexose derivatives and analogues, into 2,5‐anhydrohexonic acid derivatives (precursors for C‐nucleosides and C‐glycosides), or into polysubstituted cyclohex‐1‐ones. Moreover, conditions have been found under which the bicyclic ethers can transformed into (+) and (−)‐methyl 8‐epinonactate and (+)‐ and (−)‐methyl nonactate (precursors of nonactine), into octoses, into branched carbohydrates such as calditoses and calditols, and into (+)‐castanospermine. Compared with natural carbohydrates, the ”naked sugars“ are chirons with the following advantages: (1) the problem of selective protection and deprotection of polyalcoholic systems does not exist; (2) the functional moieties are grafted onto C(3), C(5), and C(6) of the 7‐oxabicyclo[2.2.1]heptane systems together with the appropriate protective groups; (3) since both enantiomeric forms of the ”naked sugars“ an available, either enantiomers of a targeted compound can be reached with the equal ease.