Crystal structure of N-{N-[N-(tert-butoxycarbonyl)-L-α-aspartyl]-L-α-aspartyl}-L-α-aspartic acid 1⁴,2⁴,3⁴-trimethyl ester 3¹-2-oxo-2-phenylethyl ester {Boc-[Asp(OMe)]₃-OPac}