SmI2 Reduced Thioesters as Synthons of Unstable Acyl Radicals: Direct Synthesis of Potential Protease Inhibitors via Intermolecular Radical Addition
摘要:
Aromatic alpha-heterosubstituted thioesters were found to undergo radical 1,4-addition reactions to a series of alpha,beta-unsaturated amides and one ester when subjected to the single electron reducing agent, samarium diiodide, at -78 degrees C. These thioesters derived from alpha-amino acids represent a synthetically useful synthon of unstable acyl radicals. This reaction conveniently provides access to gamma-ketoamides and esters in yields up to 90%, structures that are common in various protease inhibitors derived from peptides. Examples with acryloyl and methacryloyl derivatives of alpha-amino acids and dipeptides lead directly to tri- and tetrapeptide mimetics possessing the gamma-ketoamide functionality. No epimerization was observed with the mild conditions used for these reactions.
Peptides bearing an acryloyl residue at their N-terminus were coupled with various C-glycosides in an equimolar ratio via cross-metathesis. The newly formed olefin was obtained with high E/Z selectivity in satisfying to high yields with low homodimerization of the starting materials. The posttranslational cross-metathesis approach was shown to be suitable for the combinatorial synthesis of a small library of C-glycopeptides.