A1 is --C(R7)--, and R7 and R1 together comprise a heterocyclic, 6-membered, oxygen- or sulfur-containing ring including N' and A1 ; A2 is --CF--; A3 is --CH--; R4 is OH and pharmaceutically-acceptable salts; R6 is H; and R3 is a 3-[mercapto(thioxomethyl)amino]-1-pyrrolidinyl group, a 3-[mercapto(thioxomethyl)aminomethyl]-1-pyrrolidinyl group, a 3-[ethyl[mercapto(thioxomethyl)]aminomethyl]-1-pyrro