{HEAD ~ R2} is H, C1 4 alkyl, {TAIL ~ C2 6 alkenyl}, or C2 6 alkynyl 
is
{ HEAD ~ R2 } is H , C1 4 alkyl , { TAIL ~ C2 6 alkenyl } , or C2 6 alkynyl
HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH