{HEAD ~ L1} is {TAIL ~ ##STR24}## where R5 and R6 are independently a hydrogen, alkyl or an aryl group 
{ HEAD ~ L1 } is { TAIL ~ # # STR24 } # # where R5 and R6 are independently a hydrogen , alkyl or an aryl group
HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH