R4 represents hydrogen or a linear or branched {HEAD ~ C1 C6 alkyl} optionally substituted by {TAIL ~ hydroxy, mercapto, C1 C3 alkythio}, amino, carboxy or ureido 
R4 represents hydrogen or a linear or branched { HEAD ~ C1 C6 alkyl } optionally substituted by { TAIL ~ hydroxy , mercapto , C1 C3 alkythio } , amino , carboxy or ureido
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH