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