using a two-stage cis-C5-homologation procedure is described. During recent years, much attention has been given to polyprenols, mainly due to their importance as lipophilic, membrane-soluble carbohydrate carriers in the biosynthesis of both bacterial cell wall polysaccharides and prokaryotic and eukaryotic glycoproteins.' Natural sources of polyprenol are rather scarce and therefore much effort has been devoted to the synthesis of these compounus. Additional stimulus for these studies stems from an uncertainty existing in the structure elucidation of some complex polyprenols. Among several alternative synthetic routes to polyprenols, those imitating their biosynthesis',2 seem to be of special importance and versatility since they allow a stepwise prenylation, witn a predetermined sequence of the introduced Z-and E-prenyl units. At present, the problem of trans-C5-homologation of the isoprenoid chain seems to be practically solvea. 3,4 At the same time, similar biomimetic approach to the synthesis of polyprenols with specifically positioned cis trisubstituted C=C bonds is still lacking and depends largely on the development of a stereospecific cis-C5-homologation procedure. It has recently been reported that necessary cisoid geometry could be secured by stereospecific alkylation of senecioic or 2-butynoic acids and the possibilities of this approach were illustrated by the synthesis of (Z,Z>farnesol.' Here we wish to demonstrate an alternative approach based on the prenylation of the easily accessible5 cis isoprenoid synthon 1 followed by the crown ether catalyzed reductive cleavage of the intermediate sulfonami-de6 which led us to the synthesis of triterpenol z containing three Z-prenyl units (Scheme).
Treatment of the dilithium derivative z5 with the bropde 2 /0.9 mol equiv; freshly prepared from (E,E)-farnesol according to / in THF solution at -70' for 20 min (Ar) followed by quenching of the reaction mixture with H20 and flash-column chromatography on silica gel (elution with ether) yielded 6C% of the hydroxy sulfonamide 5 as an oil, Rf 0.458; sgg13: 1.59