3,6‐Thioanhydro sugar derivatives. An enantiospecific synthesis of (2 R ,3 R ,4 S )‐3‐benzyloxy‐4‐hydroxy‐2‐[( R )‐1‐benzyloxy‐4‐hydroxybutyl]thiolane as the key intermediate for thioswainsonine

Methyl 2‐ O ‐benzyl‐3,6‐thioanhydro‐α‐D‐mannopyranoside ( 9 ) was obtained in eight steps from the commercially available methyl α‐D‐glucopyranoside. Compound 9 was transformed into (2 R ,3 R ,4 S )‐3‐benzyloxy‐4‐hydroxy‐2‐[( R )‐1‐benzyloxy‐4‐hydroxybutyl]thiolane ( 14 ) by acid hydrolysis of its 2,4‐di‐ O ‐benzyl derivative 10 followed by reaction of the not isolated 2,4‐di‐ O ‐benzyl‐3,6‐thioanhydro‐D‐mannose ( 11 ) with ethoxycarbonylmethylenetriphenylphosphorane to give an = 1:1 E/Z mixture of the corresponding α,β‐unsaturated ester ( 12 ). Finally, catalytic hydrogenation of 12 to ethyl ( R )‐4‐benzyloxy‐4‐[(2′ R )3′ R ,4′ S )‐3′‐benzyloxy‐4′‐hydroxythiolan‐2′‐yl]butanoate ( 13 ) and subsequent reduction with lithium aluminum hydride gave the title compound 14 .