Enantiomerically pure 7‐oxabicylo[2.2.1]hept‐5‐en‐zyl derivatives as synthetic intermediates. Part III . Total synthesis of D ‐ and L ‐ribose derivatives

Enantiomerically pure methyl 5‐bromo‐5‐deoxy‐2,3‐ O ‐ isopropylidene‐β‐ D ‐ ( D ‐ 5b ) and ‐β‐l‐ribofuranoside (l‐ 5b ) have been derived from (−)‐(1 R ,2 S ,4 R )‐2‐ exo ‐cyano‐7‐oxabicylo[2.2.1]hept‐5‐en‐ endo ,‐yl (1′ S )‐camphanate ( 1 ) and (+)‐(1 S ,2 R ,4 S )‐2‐ exo ‐cyano‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐ endo ‐yl(1′ R )‐camphanate ( 2 ), respectively, in 5 synthetic steps and 28% overall yield. Hydrolysis of D‐5 b and L ‐ 5b afforded methyl 2,3‐O isopropylidene‐β‐ D ‐ribofuranoside ( D ‐5 a ) and methyl 2,3‐ O ‐isopropylidene β‐L‐ribofuranoside (L‐ 5a ), respectively. The intermediate (+)‐(1 R ,4 R ,5 R ,6 R ) 5‐ exo ,6‐ exo ‐(isopropylidenedioxy)‐ 7 ‐oxabicyclo[2.2.1]heptan‐2‐one ((+)‐ 7 ) and its enantiomer(–)‐7 were also obtained enantiomerically pure by resolution of (=)‐ 7 by the Johnson ‐ Zeller method. In bothe approaches, the chiral auxiliaries ((–)‐ and (+)‐camphanic acids, or (+)‐( S )‐ N , S ‐dimethyl‐ S ‐phenylsulfoximide) were recovered at an early stage of the synthesis.