Optically Pure 7-Oxabicyclo[2.2.1]hept-5-en-2-yl Derivatives ("Naked Sugars") as New Chirons

All articles of this category Zinc iodide catalyzed cycloaddition of furan to 1-cyanovinyl (1′ S )-camphanate or 1-cyanovinyl (1′ R )-camphanate led to optically pure (1 R ,2 S ,4 R )-2- exo -cyano-7-oxabicyclo[2.2.1]hept-5-en-2-yl (1′ S )-camphanate and (1 S ,2 R ,4 S )-2- exo -cyano-7-oxabicyclo[2.2.1]hept-5-en-2-yl (1′ R )-camphanate, respectively. Saponification afforded (1 R ,4 R )- and (1 S ,4 S )-7-oxabicyclo[2.2.1]hept-5-en-2-one, respectively, allowing concurrent recovery of the chiral auxiliaries, (1 S )- and (1 R )-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 2-cyclohexen-1-ones. Moreover, conditions have been found under which the bicyclic ethers can be rearranged stereoselectively into polysubstituted cyclopentane-carbaldehydes. 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 sequentially 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" are available, either enantiomer of a target compound can be reached with equal ease. 1. Introduction 2. The Carbonyl Group as Homoconjugated Electron-Releasing Substituent 3. Total Synthesis of 2,5-Anhydro-3-deoxy- and -4-deoxy-D-hexonic Acids and Related Deoxyadenosines-C 4. Total Syntheses of ribo -C-Nuleosides 5. De Novo Syntheses of D- and L-Hexoses and Analogues 6. Acid-Catalyzed Rearrangements of 5- exo -6- exo -Epoxy-7-oxa-2-norbornyl Derivatives 7. Stereoselective trans -Amino-hydroxylation of 7-Oxa-5-norbornen-2-one 8. The 7-Oxa-2-norbornanone/2-Cyclohexen-1-one Rearrangement: Total Syntheses of Conduritols 9. Conclusion and Perspectives