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