The disclosure relates to a method for the synthesis of a compound of the following formula (I) in which: R1 and R2 are independently an N-protective group; R3 is a hydrogen atom, a C
1
-C
6
alkyl group, a C
1
-C
6
arylalkyl group, a C
2
-C
6
alkenyl group, or a alkoxycarbonylalkyl group; Y is a —C(HR4)- group in which R4 is a hydrogen atom, a C
1
-C
6
alkyl group, an aryl group, a C
1
-C
6
arylalkyl group, or a C
2
-C
6
alkenyl group; or an orthophenylene group.