The high level of stereochemical control exhibited in the Sharpless epoxidation of prochiral divinyl alcohols has been exploited in short enantioselective syntheses of the dideoxy sugars D-digitoxose and D-olivose.
Recent communications
have reported that the Sharpless epoxidation of prochiral divinyl alcohols proceeds with enantiotopic group and diastereotopic face selectivity.* This extremely high level of stereocontrol is a fundamental property of the Sharpless epoxidationlkinetic resolution.3 The kinetic resolution aspect of the Sharpless reaction results from the fact that for a dl-pair of an allylic alcohol one face of one enantiomeric olefin is significantly more reactive than the other three olefin faces. Thus, one enantiomer is epoxidized with face selectivity and the other enantiomer is kinetically resolved. As explained by Schreiber,*a,b in the Sharpless epoxidation of divinyl carbinol" with (+)-DIPT the pro-S vinyl group is epoxidized with diastereofacial selectivity whereas the minor enantiomer (resulting from addition to the pro-R olefin) is kinetically resolved via a double addition process. We have carried out this reaction under catalytic conditions5 (10% Ti(OiPr)4, 12% (+)-DIPT,150% tBuOOH, crushed 4A molecular sieves, -2O"C, 90 Hrs., distillative workup, 60% yield) to give 1 and now report the use of this reaction in the enantioselective syntheses of differentially protected D-digitoxose and D-olivose.6 J/ -:J./ \.' 0, 1
The target compound in this study was digitoxose. We required a synthesis that would be applicable to both the natural (D) and unnatural (L) enantiomers. A simplistic retrosynthetic analysis of D-digitoxose