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