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