Synthesis of 2-alkylcysteine via phase transfer catalysis

A method of preparing a 2-alkylcysteine derivative represented by Structural Formula (I):

• or a salt thereof, wherein
• R1 is —NH2;
• —N(R5)(R6);
• —NHR7;
• or —N═R8;
• wherein R5, R6, R7, and R8 are, independently, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aromatic group, or a substituted or unsubstituted heterocyclic group;
• R2 and R3 are, independently, —H, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aromatic group, or a substituted or unsubstituted heterocyclic group;
• and
• R4 is a substituted or unsubstituted alkyl group;
• the method comprising,
• reacting, in the presence of a phase transfer catalyst, a cysteine derivative represented by Structural Formula (II):
• or a salt thereof, wherein R1, R2 and R3 are defined as above;
• with a compound having the formula R4-L, wherein R4 is defined as above and L is a leaving group thereby forming the 2-alkylcysteine derivative represented by Structural Formula (I), wherein the phase transfer catalyst is a compound represented by Structural Formula (V):
• wherein
• R9 is a substituted or unsubstituted alkyl group, a substituted or unsubstituted aromatic group, or a substituted or unsubstituted heterocyclic group;
• R10 and R11 are, independently, —H, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aromatic group, or a substituted or unsubstituted heterocyclic group;
• and
• X is a halogen.