Compound for adjusting activity of NMDA receptor, and pharmaceutical composition and use thereof

A compound represented by formula I:

• or a pharmaceutically acceptable salt, ester, or stereoisomer thereof wherein,
• Ring A is selected from
• R1 is selected from H, C1-C6 alkyl, aryl-C1-C4 alkyl, C2-C6 acyl, —CONRR′, and natural amino acid fragments;
• when Ring A is
• R2 is selected from H, C1-C6 alkyl, C1-C6 alkoxycarbonyl, C2-C6 acyl, —CONRR′, and natural amino acid fragments;
• when Ring A is
• R2 is selected from C1-C6 alkoxycarbonyl, C2-C6 acyl, —CONRR′, and natural amino acid fragments;
• R3 is selected from H, cyano, 3- to 8-membered nitrogen-containing aliphatic heterocycle-C1-C4 acyl, C1-C6 alkoxycarbonyl, and —CONHR4;
• R and R′ are each independently selected from H and C1-C6 alkyl;
• R4 is selected from H, natural amino acid fragments and carboxylic acid derivatives of the said natural amino acid fragments;
• optionally, the C1-C6 alkyl, aryl-C1-C4 alkyl, C2-C6 acyl, C1-C6 alkoxycarbonyl, —CONRR′, —CONHR4, and natural amino acid fragments are each independently substituted by one or more substituents selected from:
• halogen, amino, hydroxyl, cyano, carboxyl, nitro, C1-C6 alkyl, and C1-C6 alkoxy;
• 3- to 8-membered nitrogen-containing aliphatic heterocycle-C1-C4 acyl is substituted by one or more substituents selected from:
• halogen, amino, hydroxyl, carboxyl, nitro, C1-C6 alkyl, and C1-C6 alkoxy;
• wherein the natural amino acid fragments are selected from the carboxyl or amino residues of the following amino acids:
• Thr, Ser, Val, Gly, Ala, Ile, Leu, Phe, Tyr, Asp, and Gln;
• and, R1, R2 and R3 satisfy:
• (1) R1, R2 and R3 are not H at the same time;
• (2) when R1 and R3 are H, R2 is not Boc;
• (3) when formula I is
• R1 is
• and R3 is H, R2 is not
• (4) when formula I is
• R1 is
• and R3 is H, R2 is not C2-C6 acyl or —CONRR′;
• (5) when formula I is
• R1 and R3 are H, R2 is not acetyl,
• (6) the compound of formula I is not