R3 is a lower alkyl, a cycloalkyl, phenyl, benzyl, a cycloheteroalkyl, a lower alkyl having one to eight carbons substituted with one or more fluorines or containing 1 or 2 unsaturated bonds; and carbon 15 may be in the R or the S configuration, or a mixture of R and S with the proviso that when R1 is a lower alkyl or lower hydroxyalkyl that carbon 15 is not in the R configuration.