A compound of formula IIb: ** Formula ** or a pharmaceutically acceptable salt thereof, in which: X is SCH2； X1 is O or S； R1 is H, C1-8 alkyl, heteroaryl or C3-10 cycloalkyl, in which each R1 group is substituted with 0-4 R1a； R1a, at each occurrence, is selected from C1-6 alkyl substituted with 0-3 R1b； C2-6 alkynyl substituted with 0-2 R1b； aryl substituted with 0-2 R1b, heteroaryl substituted with 0-2 R1b10, C3-10 cycloalkyl substituted with 0-2 R1b and a heterocycle substituted with 0-2 R1b； R1b, at each occurrence, is selected from OH substituted with 0-1 R1c； SH substituted with 0-1 R1c； Cl； F； NH2 substituted with 0-2 R1c； -CN； NO2； methanesulfonyl； C1-4 alkyl substituted with 0-3 R1c； C2-4 alkenyl substituted with 0-2 R1c； aryl substituted with 0-2 R1c； heteroaryl substituted with 0-2 R1c； C3-6 cycloalkyl substituted with 0-2 R16； and a heterocycle substituted with 0-2 R1c； R1c, at each occurrence, is selected from OH, SH, Cl, F, NH2, -CN, NO2, C1-4 alkyl and C2-4 alkenyl； aryl substituted with 0-2 R1d, heteroaryl substituted with 0-2 R1d, C3-6 cycloalkyl substituted with 0-2 R1d and heterocycle substituted with 0-2 R1d； R1d, at each occurrence, is selected from OH, SH, Cl, F, NH2, -CN, NO2, C1-4 alkyl； and C2-4 alkenyl； R2a is selected from Ch alkyl substituted with 0-2 R2b, aryl substituted with 0-2 R2b and heteroaryls substituted with 0-2 R2b； R2b, at each occurrence, is selected from OH substituted with 0-1 R2c, SH substituted with 0-1 R2c, Cl, F, NH2 substituted with 0-2 R2c, -CN, NO2, methanesulfonyl, C1-4 alkyl substituted with 0-2 R2c, C2-4 alkenyl substituted with 0-2 R2c, aryl substituted with 0-2 R2c, heteroaryl substituted with 0-2 R2c, C3-6 cycloalkyl substituted with 0-2 R2c and a heterocyl substituted with 0-2 R2c； R2c, at each occurrence, is selected from OH, SH, Cl, F, NH2, -CN, NO2, C1-4 alkyl, C2-4 alkenyl, aryl substituted with 0-2 R2d, and heteroaryl substituted with 0-2 R2d ； R2d, at each occurrence, is selected from OH, SH, Cl, F, NH2
