Compounds and method of treating COVID-19

1. A compound of Formula I

wherein

R 1 is selected from the group consisting of hydrogen, -C(O)C 1 -C 6 alkyl, -C(O)OC 1 -C 6 alkyl and -C 1 -C 6 alkylOC(O)OC 1 -C 6 alkyl;

when R 1 is hydrogen then R 2 is selected from the group consisting of ethyl, isopropyl, 2-methylpropyl, C 1 -C 6 alkyl substituted with one to three R 3 , cyclopropyl substituted with one to three R 4 , C 4 -C 7 cycloalkyl unsubstituted or substituted with one to three R 4 , C 5 -C 12 bicycloalkyl unsubstituted or substituted with one to three R 4 , four- to seven-membered heterocycloalkyl comprising one to three heteroatoms selected independently from N, O and S and which is unsubstituted or substituted with one to three R 4 , five- to ten-membered heteroaryl comprising one to four heteroatoms selected independently from N, O and S and which is unsubstituted or substituted with one to three R 4 , 2-methylphenyl, 2-methoxyphenyl, 2-chlorophenyl, 2-methoxymethylphenyl, 2-(dimethylamino)methylphenyl, 2-(dimethylamino)methyl-6-methylphenyl, 2-methoxymethyl-6-methylphenyl, 4-cyano-2,6-dimethylphenyl, 2-methoxy-6-methylphenyl, 2-fluoro-6-methylphenyl, 2-chloro-6-methylphenyl, 4-chloro-2,6-dimethylphenyl, 2,6-dimethyl-4-methoxyphenyl, 2,6-dimethyl-4-fluorophenyl and 2,6-dimethyl-3-fluorophenyl;

when R 1 is-C(O) C 1 -C 6 alkyl, -C(O)OC 1 -C 6 alkyl or -C 1 -C 6 alkylOC(O)OC 1 -C 6 alkyl then R 2 is selected from the group consisting of C 1 -C 6 alkyl unsubstituted or substituted with one to three R 3 , C 3 -C 7 cycloalkyl unsubstituted or substituted with one to three R 4 , C 5 -C 12 bicycloalkyl unsubstituted or substituted with one to three R 4 , four-to seven-membered heterocycloalkyl comprising one to three heteroatoms selected independently from N, O and S and which is unsubstituted or substituted with one to three R 4 , C 6 -C 10 aryl unsubstituted or substituted with one to three R 4 , and five-to ten-membered heteroaryl comprising one to four heteroatoms selected independently from N, O and S and which is unsubstituted or substituted with one to three R 4 ;

R 3 at each occurrence is independently selected from the group consisting of halo, cyano, hydroxy, di(C 1 -C 6 alkyl)amino, (C 1 -C 6 alkyl)amino and amino; and

R 4 at each occurrence is independently selected from halo, hydroxy, cyano, C 1 -C 3 alkyl, C 1 -C 3 alkoxy, C 1 -C 3 alkoxyC 1 -C 3 alkyl, di(C 1 -C 3 alkyl)amino, (C 1 -C 3 alkyl)amino, amino, di(C 1 -C 3 alkyl)amino-C 1 -C 3 alkyl, (C 1 -C 3 alkyl)amino-C 1 -C 3 alkyl, amino-C 1 -C 3 alkyl and four-to seven-membered heterocycloalkyl comprising one to three heteroatoms selected independently from N, O and S;

or a pharmaceutically acceptable salt thereof.

2. The compound of claim 1 wherein R 1 is hydrogen; or a pharmaceutically acceptable salt thereof.

3. The compound of claim 2 wherein R 2 is selected from the group consisting of ethyl, isopropyl, 2-methylpropyl, C 1 -C 6 alkyl substituted with one to three R 3 , cyclopropyl substituted with one to three R 4 , C 4 -C 7 cycloalkyl unsubstituted or substituted with one to three R 4 , and C 5 -C 12 bicycloalkyl unsubstituted or substituted with one to three R 4 ; or a pharmaceutically acceptable salt thereof.

4. The compound of claim 3 selected from the group consisting of:

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl N, N-dimethylglycinate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl N, N-dimethyl-D-alaninate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl N,N,2-trimethylalaninate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-(dimethylamino)-2,2-dimethylbutanoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-(dimethylamino) butanoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl propanoate (3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-methylpropanoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl cyclohexanecarboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methylcyclohexanecarboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methylcyclobutanecarboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl (2S)-2-methylbutanoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl (2R)-2-methylbutanoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl bicyclo[1.1.1]pentane-1-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methylcyclopropanecarboxylate; and

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl L-valinate;

or a pharmaceutically acceptable salt thereof.

5. The compound of claim 1 wherein R 2 is a four-to seven-membered heterocycloalkyl comprising one to three heteroatoms selected independently from N, O and S and which is unsubstituted or substituted with one to three R 4 ; or a pharmaceutically acceptable salt thereof.

6. The compound of claim 5 wherein R 2 is selected from the group consisting of azetidinyl, pyrrolidinyl, piperidinyl and morpholinyl; each of which is unsubstituted or substituted with one to two R 4 ; or a pharmaceutically acceptable salt thereof.

7. The compound of claim 6 wherein R 2 is selected from the group consisting of azetidinyl, pyrrolidinyl, piperidinyl and morpholinyl; each of which is substituted with one to two methyl; or a pharmaceutically acceptable salt thereof.

8. The compound of claim 1 which is selected from the group consisting of:

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1,4-dimethylpiperidine-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1,3-dimethylazetidine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl (3R)-1-methylpyrrolidine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl (3S)-1-methylpyrrolidine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methyl-D-prolinate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methyl-L-prolinate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methylpiperidine-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl (2R)-4-methylmorpholine-2-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl (2S)-1-methylpiperidine-2-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl (2S)-4-methylmorpholine-2-carboxylate; and

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl (2R)-1-methylpiperidine-2-carboxylate;

or a pharmaceutically acceptable salt thereof.

9. The compound of claim 1 wherein R 2 is a five-to ten-membered heteroaryl comprising one to four heteroatoms selected independently from N, O and S and which is unsubstituted or substituted with one to three R 4 ; or a pharmaceutically acceptable salt thereof.

10. The compound of claim 9 wherein R 2 is an imidazolyl, pyrazolyl, isoxazolyl, pyridinyl or pyrimidinyl, each of which is unsubstituted or substituted with one to three R 4 ; or a pharmaceutically acceptable salt thereof.

11. The compound of claim 10 wherein R 4 at each occurrence is independently selected from methyl, methoxy, ethoxy, methoxymethyl, methylamino, (dimethylamino)methyl and tetrahydropyranyl; or a pharmaceutically acceptable salt thereof.

12. The compound of claim 1 selected from the group consisting of:

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-methylpyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-methylpyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl pyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl pyridine-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl pyridine-2-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1,3-dimethyl-1H-pyrazole-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1,5-dimethyl-1H-imidazole-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methyl-1H-pyrazole-5-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methyl-1H-pyrazole-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methyl-1H-pyrazole-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methyl-1H-imidazole-5-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methyl-1H-imidazole-2-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-methoxy-1-methyl-1H-imidazole-5-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1,3,5-trimethyl-1H-pyrazole-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1,4-dimethyl-1H-pyrazole-5-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1,4-dimethyl-1H-pyrazole-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 1-methyl-1H-imidazole-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-(methoxymethyl)-4-methylpyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-(methoxymethyl)-6-methylpyrimidine-5-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-(methoxymethyl)-2-methylpyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-[(dimethylamino)methyl]-4-methylpyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-[(dimethylamino)methyl]-6-methylpyrimidine-5-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-[(dimethylamino)methyl]-2-methylpyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2,4-dimethylpyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-methyl-2-(methylamino)pyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-ethoxy-4-methylpyridine-3-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 3,5-dimethylpyridine-4-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2,4,6-trimethylpyrimidine-5-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4,6-dimethylpyrimidine-5-carboxylate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4,6-dimethyl-2-(tetrahydro-2H-pyran-4-yl)pyrimidine-5-carboxylate; and

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 3,5-dimethyl-1,2-oxazole-4-carboxylate;

or a pharmaceutically acceptable salt thereof.

13. The compound of claim 2 selected from the group consisting of:

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-methylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-methoxybenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-chlorobenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-(methoxymethyl)benzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-[(dimethylamino)methyl]benzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-[(dimethylamino)methyl]-6-methylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-(methoxymethyl)-6-methylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-cyano-2,6-dimethylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-methoxy-6-methylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-chloro-6-methylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-chloro-2,6-dimethylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 2-fluoro-6-methylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-methoxy-2,6-dimethylbenzoate;

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 4-fluoro-2,6-dimethylbenzoate; and

(3S)-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl 3-fluoro-2,6-dimethylbenzoate;

or a pharmaceutically acceptable salt thereof.

14. The compound of claim 1 wherein R 1 is -C(O)C 1 -C 6 alkyl, -C(O)OC 1 -C 6 alkyl and -C 1 -C 6 alkylOC (O) OC 1 -C 6 alkyl; or a pharmaceutically acceptable salt thereof.

15. The compound of claim 14 wherein R 1 is -C(O)CH 3 , -C(O)OCH 3 or -CH(CH 3 )OC(O)OCH 3 ; or a pharmaceutically acceptable salt thereof.

16. The compound of claim 14 wherein R 2 is a C 6 -C 10 aryl which is unsubstituted or substituted with one to three R 4 ; or a pharmaceutically acceptable salt thereof.

17. The compound of claim 16 wherein R 2 is phenyl which is unsubstituted or substituted with one to three R 4 ; or a pharmaceutically acceptable salt thereof.

18. The compound of claim 17 wherein R 2 is 2,6-dimethylphenyl; or a pharmaceutically acceptable salt thereof.

19. The compound of claim 18 selected from the group consisting of:

(3S)-4-[(3S)-1-acetyl-2-oxopyrrolidin-3-yl]-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxobutyl 2,6-dimethylbenzoate;

methyl (3S)-3-[(2S)-4-[(2,6-dimethylbenzoyl)oxy]-2-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-3-oxobutyl]-2-oxopyrrolidine-1-carboxylate; and

(3S)-4-[(3S)-1-{(1S)-1-[(methoxycarbonyl)oxy]ethyl}-2-oxopyrrolidin-3-yl]-3-({N-[(4-methoxy-1H-indol-2-yl) carbonyl]-L-leucyl}amino)-2-oxobutyl 2,6-dimethylbenzoate;

or a pharmaceutically acceptable salt thereof.

20. A pharmaceutical composition comprising a therapeutically effective amount of a compound of claim 1 or a pharmaceutically acceptable salt thereof together with a pharmaceutically acceptable carrier.

21. The pharmaceutical composition of claim 20 further comprising one or more additional therapeutic agents.

22. The pharmaceutical composition of claim 21 wherein the additional therapeutic agent is selected from azithromycin and remdesivir.

23. A method of treating COVID-19 in a patient, the method comprising administering a therapeutically effective amount of a compound of claim 1 or a pharmaceutically acceptable salt thereof to a patient in need thereof.