The use of the N(π)-phenacyl group for the protection of the histidine side chain in peptide synthesis

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Abstract
N(α)-Benzyloxycarbonyl-N(τ)-phenacyl-L-histidine (5) and N(α)-benzyloxycarbonyl-N(π)-phenacyl-L-histidine (6) have been prepared. Under carboxy-activating conditions which cause gross racemisation of (5), (6) yields optically pure peptide. N(α)-t-Butoxycarbonyl-N(π)-phenacyl-L-histidine (7) has also been prepared and the practicability of the N(π)-phenacyl protective group (which is unchanged by strong aqueous or anhydrous acids and essentially unchanged by aqueous alkali, but which is quantitatively cleaved on treatment with zinc–acetic acid or on photolysis) has been demonstrated in a synthesis of thyroliberin.