{TAIL ~ R4} represents {HEAD ~ a heteroaromatic group} selected from the formulae: ##STR26## R5 represents C1 3 alkyl; R6 represents H, C1 4 alkyl, phenyl or benzyl, and 
{ TAIL ~ R4 } represents { HEAD ~ a heteroaromatic group } selected from the formulae : # # STR26 # # R5 represents C1 3 alkyl ; R6 represents H , C1 4 alkyl , phenyl or benzyl , and
TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH