This invention is directed to compounds of the following formula:wherein when a is 0, b is 1 or 2; when a is 1, b is 1, m is from 0-3, X is O or S, Y is halogeno, R1 is hydrogen C1-4 alkyl, R2 is hydrogen, C1-4 alkyl, or optionally substituted propargyl and R3 and R4 are each independently hydrogen, C1-6 alkyl, C6-12 aryl, C6-12 aralkyl each optionally substituted.