Compounds having the general formula: ##STR1## wherein R.sub.1 is an alkyl or substituted alkyl; R.sub.2 is selected from the group consisting of H and C(O)R.sub.a ; R.sub.3 is selected from the group consisting of H, protecting groups, R.sub.b, and C(O)R.sub.b ; R.sub.4 is selected from the group consisting of H and C(O)R.sub.c, and wherein R.sub.a, R.sub.b, and R.sub.c are independently selected from the group consisting of alkyls, substituted alkyls, alkenyls, alkynyls, aryls, and substituted aryls; provided that R.sub.a is other than phenyl and 3-hydroxyphenyl.