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

Area of Science:

• Organic Chemistry
• Peptide Synthesis
• Natural Product Synthesis

Background:

• Cyclic tetrapeptides are a significant class of natural products with diverse biological activities.
• The synthesis of highly strained cyclic tetrapeptides presents considerable synthetic challenges due to conformational constraints.

Purpose of the Study:

• To develop an efficient macrocyclization strategy for the synthesis of highly strained cyclic tetrapeptides.
• To investigate the utility of the 2-hydroxy-6-nitrobenzyl (HnB) auxiliary in facilitating challenging ring contractions.

Main Methods:

• A novel macrocyclization strategy was designed incorporating the 2-hydroxy-6-nitrobenzyl (HnB) group at the N-terminus and within the peptide sequence.
• The N-terminal HnB auxiliary was employed for ring closure and contraction.
• A backbone HnB auxiliary was used to promote the formation of cis amide bonds, crucial for ring contraction.

Main Results:

• The developed strategy successfully enabled the synthesis of highly strained cyclic tetrapeptides.
• The all-L cyclic tetrapeptide cyclo-[Tyr-Arg-Phe-Ala] was prepared efficiently using this method.
• The HnB auxiliaries played critical roles in both ring closure/contraction and facilitating cis amide bond formation.

Conclusions:

• The novel macrocyclization strategy offers a viable route to synthesize challenging, highly strained cyclic tetrapeptides.
• The strategic placement of HnB auxiliaries is key to overcoming synthetic hurdles in cyclic peptide synthesis.
• This approach provides a valuable tool for accessing complex cyclic peptide structures for further research.