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