The present invention encompasses structures of the formula ##STR1## wherein: {HEAD ~ X} is oxygen, {TAIL ~ methylene}, or difluoromethylene 
is
The present invention encompasses structures of the formula # # STR1 # # wherein : { HEAD ~ X } is oxygen , { TAIL ~ methylene } , or difluoromethylene
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH