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