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