A compound of Formula I: ** Formula ** in which R 1 is a 5- or 6-membered aryl, heteroaryl or heterocyclic group wherein each heteroaryl or heterocyclic group contains 1 to 3 heteroatoms independently selected from the group consisting of nitrogen, oxygen and sulfur； and wherein the aryl, heteroaryl or heterocyclic group is optionally substituted with 1 or 2 groups independently selected from the group consisting of -C1-C6 alkyl, -C3-C6 cycloalkyl, C1-C6 alkyl-C3-C6 cycloalkyl, halogen, - C1-C6 haloalkyl, -C1-C6 alkyl, -Chaloalkyl C1-C6, and -C (O) C1-C6 alkyl； R2 and R2 'are in each case independently H or -C1-C6 alkyl； or a set of R2 and R2 'is combined with the carbon atom to which they are attached to form a C3-C6 cycloalkyl group； R3 is H, -C1-C6 alkyl or C3-C6 cycloalkyl, R4 is H, -C1-C6 alkyl, -C1-C6 alkyl, -Chalo-C1-C6 alkyl, -C3-C6 cycloalkyl, or halo； R5 is a 5- or 6-membered aryl, heteroaryl or heterocyclic group optionally substituted with one, two or three groups independently selected from the group consisting of -C1-C6 alkyl, -C2-C6 alkenyl, -C2-C6 alkynyl, C3 cycloalkyl -C6, C1-C6 alkyl-C3-C6 cycloalkyl, -C1-C6 alkyl alkyl, -C1-C6 alkyl, -C1-C6 alkylhaloalkyl, -O-aryl and halo； wherein the cycloalkyl group is optionally substituted with -C1-C6 alkyl, or -C1-C6 alkyl alkyl； and wherein the -O-aryl group of the substituent is optionally substituted with -C1-C6 alkyl, halo, C1-C6 haloalkyl, -C1-C6 alkyl, or -C-C1-C6 alkyl； and wherein two substituents on the aryl, heteroaryl or heterocyclic ring of R5 optionally combine to form an 8-12 membered bicyclic ring optionally containing one or two oxygen atoms； and wherein the bicyclic ring is optionally substituted with 1 or 2 groups independently selected from the group consisting of -C1-C6 alkyl, -C1-C6 alkyl, - C1-C6 alkyl, -C1-C6 haloalkyl, C3-C6 cycloalkyl and halo； R6 is H, -C1-C6 alkyl, -C1-C6 alkyl alkyl, -C1-C6 alkyl, -C1-C6 alkylhaloalkyl or halo； or R6 is combined with the aryl, heteroaryl or heter
