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