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