I agree with Rhodium.
But if one had 3,4-methylenedioxy-phenylalanine wouldn't it be a good idea to alkylate with a methyl group at the alpha position followed by the nitrogen, decarboxylating along the way.
1) R1-CH2-CH(NH2)-COOH + EtOH --> R1-CH2-CH(NH2)-CO.OEt (Fisher esterification, refux with 5% HCl added via gassing)
2) R1-CH2-CH(NH2)-CO.OEt + R2-CHO --> R1-CH2-CH(N=CH-R2)-CO.OEt
3) R1-CH2-CH(N=CH-R2)-CO.OEt + MeX --> R1-CH2-C(Me)(N=CH-R2)-CO.OEt [strong basic conditions with a PTC]
4) R1-CH2-C(Me)(N=CH-R2)-CO.OEt --> R1-CH2-C(Me)(NH2)-CO.OEt [hydrolysis]
5) R1-CH2-C(Me)(NH2)-CO.OEt --> R1-CH2-C(Me)(NH2)-COOH [hydrolysis]
6) R1-CH2-C(Me)(NH2)-COOH - CO2 --> MDA [decarboxylation]
R1 : 3,4-MD-phenyl-
R2 : substituted-phenyl- [eg. para-chloro-phenyl]
X : halogen (bromide / iodide)
Me : Methyl-
Et : Ethyl-
I appreciate that it looks like a large number of reactions but these are all very high yield and quite simple. It's an easier and higher yielding route than what you propose, which anyway, wouldn't work as you plan.