(CO)R6; wherein R6 is a saturated or unsaturated acyclic hydrocarbon group having from 1 to about 20 carbon atoms, or ???(CH2)mR7 wherein m is 0-10, preferably 0-4; and R7 is an aliphatic ring from about 3 to about 7 carbon atoms, or an aryl or heteroaryl ring, as defined above; or a pharmaceutically acceptable salt thereof.