Silicon-Containing Dipeptidic Aspartame and Neotame Analogues

A series of seven silicon-containing dipeptides ((S,R)-5, (S,S)-5, (S,R)-6, (S,R)-7, (S,S)-7, (S,R)-8, and (S,S)-8) was prepared in multistep syntheses, starting from the enantiopure silicon-containing alpha-methylated alpha-amino acids (R)- or (S)-alpha-[(trimethylsilyl)methyl]-alanine ((R)-4 or (S)-4). For the preparation of (R)-4 and (S)-4, a robust synthesis on a multigram scale was developed, using an enzyme-catalyzed stereoselective ester hydrolysis as the key step. Coupling of (R)-4 and (S)-4 with different S-configured aspartate residues yielded a series of dipeptides that can be best described as silicon analogues of the artificial sweeteners aspartame and neotame. The identity of these dipeptides was established by elemental analyses and NMR spectroscopic studies (H-1, C-13, N-15, Si-29). Some of the title compounds and some of their precursors were structurally characterized by single-crystal X-ray diffraction. The silicon-containing dipeptides were shown to display attractive ADME properties, including good solubility and plasma protein binding capabilities, no CYP inhibition, and no metabolic stability liabilities. Thus, the silicon-containing alpha-amino acids (R)-4 and (S)-4 proved to be promising building blocks for the design of biologically active peptides. Attempts to characterize the title compounds for their T1R2/R3 activating properties by testing their ability to induce GLP-1 secretion from the intestinal endocrine cell line STC-1 failed for unknown reasons. Sensory evaluation of the silicon-containing dipeptides demonstrated the aspartame analogue (S,R)-5 and the neotame analogue (S,R)-8 to be very potent artificial sweeteners, whereas the corresponding diastereomers tasted bitter. The silicon compounds (S,R)-5 and (S,R)-8 are approximately SO and 600 times, respectively, as sweet as sucrose on a weight basis.