R 19, at each occurrence, is independently selected from H, methyl, and ethyl; R 19b is H, mehyl, ethyl, propyl, butyl, cyclopropyl, cyclobutyl, cyclopentyl, phenyl, benzyl or phenethyl ;.
R 18 and R 19, when substituents on the same atom, may be combined to form a 3 to 7 membered heterocyclic ring.