1. A compound of formula:or a pharmaceutically acceptable salt thereof, wherein:R1 and R2 are each independently selected from C1-6alkyl, C3-8cycloalkyl, C3-8cycloalkyl-C1-4alkyl, aryl, aryl-C1-4alkyl, heteroaryl and heteroaryl-C1-4alkyl, each of which is optionally substituted with 1, 2 or 3 Rh members selected from F, Cl, Br, I, CN, OH, CF3, NH, CF3O, CH3, CH3O, NO2, cyclopropyl, cyclopropylmethyl, cyclopropylamino, cyclopropylmethylamino, 1-cyanocyclopropyl, methylamino, dimethylamino, methylthio, acetoxy, acetyl, methoxycarbonyl, acetamido, methylcarbamoyl, 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-pyridylmethyl, 3-pyridylmethyl, 4-pyridylmethyl, 2-oxetanyl, 3-oxtetanyl, 2-oxetanylmethyl, 3-oxtetanylmethyl, 2-tetrahydrofuranyl, 3-tetrahydrofuranyl, 2-tetrahydrofuranylmethyl, 3-tetrahydrofuranylmethyl, 1-pyrrolidinyl, 2-pyrrolidinyl, 3-pyrrolidinyl, 2-tetrahydrothiophenyl, 3-tetrahydrothiophenyl, 4-morpholinyl, 2-morpholinyl and 3-morpholinyl; or
R1 and R2 taken together with the nitrogen to which they attach form an optionally substituted 5- or 6-membered heterocycloalkyl ring having from 0-1 additional heteroatoms as ring members selected from O, N and S;
R3 is H;
R4 is hydrogen, F, Cl, CH3, CN, or CF3;
R16 is hydrogen, optionally substituted aryl or optionally substituted C1-6alkyl; and
R19 is hydrogen or 2-(methoxycarbonylamino)propyl.