A new and expeditious synthesis of all enantiomerically pure stereoisomers of rosaprostol, an antiulcer drug.

Four enantiomerically pure stereoisomers of rosaprostol (1), an antiulcer drug, were efficiently synthesized from the enantiomers of 2-(dimethoxyphosphoryl)-3-hexylcyclopentanone (3) as chiral substrates. The latter were obtained by resolution of racemic 3 with (+)-(R)-1-(1-naphthyl)ethylamine. The conversion of (+)-3 into rosaprostol stereoisomer (-)-1a was accomplished in four steps in 56% overall yield. According to the same protocol, the second stereoisomer (+)-1c was obtained from (-)-3 in 55% overall yield. A slightly improved procedure of the last two steps of the transformation of (+)-3 into (-)-1a allowed an increase in the overall yield to 64%. The remaining two stereoisomers, (-)-1b and (+)-1d, were obtained from (-)-1a and (+)-1c in 71 and 68% yield, respectively, by a two-reaction sequence, in which a Mitsunobu inversion of configuration at C-5 was the key step.