Difficult macrocyclizations: new strategies for synthesizing highly strained cyclic tetrapeptides.

[reaction: see text] Cyclic tetrapeptides are an intriguing class of natural products. To synthesize highly strained cyclic tetrapeptides we developed a macrocyclization strategy that involves the inclusion of 2-hydroxy-6-nitrobenzyl (HnB) group at the N-terminus and in the "middle" of the sequence. The N-terminal auxiliary performs a ring closure/ring contraction role, and the backbone auxiliary promotes cis amide bonds to facilitate the otherwise difficult ring contraction. Following this route, the all-L cyclic tetrapeptide cyclo-[Tyr-Arg-Phe-Ala] was successfully prepared.