A compound having the structure of Formula or a pharmaceutically acceptable salt thereof: wherein: R 1 is H or CO(C 1 -C 6)alkyl, wherein CO(C 1 -C 6)alkyl is optionally substituted at any position with 1-3 substituents selected from halogen, hydroxyl, O-acetyl, NH-acetyl, or -NH 2; R 2 and R 3 are each, independently, H, (C 1 -C 10)alkyl, (C 6 -C 10)aryl(C 0 -C 6)alkyl, (C 3 -C 10)heteroaryl, (C 3 -C 8)cycloalkyl, (C 2 -C 10)alkynyl or (C 2 -C 10)alkenyl; wherein (C 1 -C 10)alkyl, (C 6 -C 10)aryl(C 0 -C 6)alkyl, (C 3 -C 10)heteroaryl, (C 3 -C 8)cycloalkyl, (C 2 -C 10)alkynyl and (C 2 -C 10)alkenyl are optionally substituted at any position with 1-5 substituents selected from (C 1 -C 6)alkyl, (C 1 -C 6)alkoxy, NH 2, -NH(C 1 -C 6 alkyl), -NH (C 1 -C 6 alkyl), -NH (C 1 -C 6 haloalkyl), -NH (C 1 -C 6 alkoxy), -NH (C 1 -C 6 haloalkoxy), -NH (C 2 -C 6 alkenyl), -Si(C 1 -C 6 alkyl), -SO 2 (C 1 -C 6 alkyl), -SH, -B 2, -OTosyl, -CO(C 1 -C 6 alkoxyl), -COH, -COOH, - CO(C 1 -C 6 alkyl) or halogen; or R 2 and R 3 are taken together to form a ring, wherein the ring is optionally substituted with 1-5 additional substituents selected from halogen or (C 1 -C 6)alkyl; and R 4 is (C 1 -C 10)alkyl, (C 3 -C 8)cycloalkyl, (C 6 -C 10)aryl, (C 2 -C 10)alkenyl, (C 2 -C 10)alkynyl, or (C 3 -C 10)heteroaryl, further wherein R 4 is optionally substituted at any position with 1-5 substituents selected from (C 1 -C 10)alkyl, (C 2 -C 10)alkenyl, (C 2 -C 10)alkynyl, (C 3 -C 8)cycloalkyl, (C 1 -C 6)alkoxy, (C 6 -C 10)aryloxy, (C 6 -C 10)aryl, amino, -NH (C 1 -C 6 alkyl), -NH (C 2 -C 6 alkenyl), - NH (C 2 -C 6 alkynyl), -NH (C 1 -C 6 haloalkyl), halo, OCF 3, CF 3, hydroxyl, or a halogen radioisotope. R 1 is H or CO(C 1 -C 6)alkyl, wherein CO(C 1 -C 6)alkyl is optionally substituted at any position with 1-3 substituents selected from halogen, hydroxyl, O-acetyl, NH-acetyl, or -NH 2; R 2 and R 3 are each, independently, H, (C 1 -C 10)alkyl, (C 6 -C 10)aryl(C 0 -C 6)alkyl, (C 3 -C 10)heteroaryl, (C 3 -C 8)cycloalkyl, (C 2 -C 10)alkynyl or (C 2 -C 10)alkenyl; wherein (C 1 -C 10)alkyl, (C 6 -C 10)aryl(C 0 -C 6)alkyl, (C 3 -C 10)heteroaryl, (C 3 -C 8)cycloalkyl, (C 2 -C 10)alkynyl and (C 2 -C 10)alkenyl are optionally substituted at any position with 1-5 substituents selected from (C 1 -C 6)alkyl, (C 1 -C 6)alkoxy, NH 2, -NH(C 1 -C 6 alkyl), -NH (C 1 -C 6 alkyl), -NH (C 1 -C 6 haloalkyl), -NH (C 1 -C 6 alkoxy), -NH (C 1 -C 6 haloalkoxy), -NH (C 2 -C 6 alkenyl), -Si(C 1 -C 6 alkyl), -SO 2 (C 1 -C 6 alkyl), -SH, -B 2, -OTosyl, -CO(C 1 -C 6 alkoxyl), -COH, -COOH, - CO(C 1 -C 6 alkyl) or halogen; wherein (C 1 -C 10)alkyl, (C 6 -C 10)aryl(C 0 -C 6)alkyl, (C 3 -C 10)heteroaryl, (C 3 -C 8)cycloalkyl, (C 2 -C 10)alkynyl and (C 2 -C 10)alkenyl are optionally substituted at any position with 1-5 substituents selected from (C 1 -C 6)alkyl, (C 1 -C 6)alkoxy, NH 2, -NH(C 1 -C 6 alkyl), -NH (C 1 -C 6 alkyl), -NH (C 1 -C 6 haloalkyl), -NH (C 1 -C 6 alkoxy), -NH (C 1 -C 6 haloalkoxy), -NH (C 2 -C 6 alkenyl), -Si(C 1 -C 6 alkyl), -SO 2 (C 1 -C 6 alkyl), -SH, -B 2, -OTosyl, -CO(C 1 -C 6 alkoxyl), -COH, -COOH, - CO(C 1 -C 6 alkyl) or halogen; or R 2 and R 3 are taken together to form a ring, wherein the ring is optionally substituted with 1-5 additional substituents selected from halogen or (C 1 -C 6)alkyl; and R 4 is (C 1 -C 10)alkyl, (C 3 -C 8)cycloalkyl, (C 6 -C 10)aryl, (C 2 -C 10)alkenyl, (C 2 -C 10)alkynyl, or (C 3 -C 10)heteroaryl, further wherein R 4 is optionally substituted at any position with 1-5 substituents selected from (C 1 -C 10)alkyl, (C 2 -C 10)alkenyl, (C 2 -C 10)alkynyl, (C 3 -C 8)cycloalkyl, (C 1 -C 6)alkoxy, (C 6 -C 10)aryloxy, (C 6 -C 10)aryl, amino, -NH (C 1 -C 6 alkyl), -NH (C 2 -C 6 alkenyl), - NH (C 2 -C 6 alkynyl), -NH (C 1 -C 6 haloalkyl), halo, OCF 3, CF 3, hydroxyl, or a halogen radioisotope.