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