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