The crystal structure of (4S)-5-(2-methoxy-2-oxoethylamino)-5-oxo-4-(3,4,5-trimethoxybenzamido) pentanoic acid 5 (C18H24N2O9, Mr=412.39) has been determined by single-crystal X-ray diffraction analysis. The crystal belongs to monoclinic, space group P2(1)/c with a=27.665(9), b=5.1444(16), c=13.907(4) Å, β=98.401(5)º, V=1958.0(11) Å3, Z=4, Dc=1.399 g/cm3, μ=0.113mm－1, F(000)=872, R=0.0606 and wR=0.1405 (I>2σ(I)). The results confirmed that 5 could be assigned to the tautomeric form. The intermolecular hydrogen bonds between O(5)-H(5)…O(7), N(2)-H(2)…O(6) and N(1)-H(1)…O(4) have been observed.