alkyl wherein n is as defined above, or --(CH2)r --NH2 wherein r is an integer of 1 to 9; a is zero or an integer of 1 to 3; R5 is OH, OR6 wherein R6 is as defined above, ##STR24## wherein R6 and R6a are each the same or different and are as defined above for R6, or NH--OR6 wherein R6 is as defined above; R3 and R4 are each the same or different and eac