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