Abstract
AbstractA major problem in the synthesis in solution of the gastrointestinal peptide VIP is the solubility of C‐terminal intermediates. Already in the tripeptide stage, the solubility in the usual solvents is very low and gives rise to low coupling yields. The C‐terminal asparagine amide, in combination with leucine and isoleucine, seems to play an important role in this phenomenon. We synthesized the protected C‐terminal segments, VIP(27‐28), VIP(22‐28) and VIP( 14‐28), via the temporary substitution of 28‐asparagine by a β‐tert‐butyl protected aspartic acid. This substitution resulted in a dramatic improvement in solubility. The synthesis could easily be performed via the repetitive excess mixed anhydride method. In the last stage of the synthesis, the β‐tert‐butyl protection of 28‐aspartic acid could be removed selectively. The deprotected aspartic acid was converted into asparagine by amidation using the mixed anhydride or the carbodiimide method.
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