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