{HEAD ~ R6} is lower alkyl, phenyl, R3 phenyl, {TAIL ~ benzyl} or R3 benzyl 
{ HEAD ~ R6 } is lower alkyl , phenyl , R3 phenyl , { TAIL ~ benzyl } or R3 benzyl
HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH