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