2(1H)-pyrazinone moiety. The absolute configurations of the amino acid residues were determined by application of Marfey’s method. Because 1 did not appear to comprise the normal connection of amino acid units, we confirmed its structure by the total synthesis of 1. Biosynthetically, 1 consists of a unique skeleton connected to the peptide chain at C-5 of the pyrazinone ring.
A catalytic method for the formation of amide bonds was developed in which the amidation of N-hydroxyimino esters with a broad range of amino acid tert-butyl esters is promoted by a niobium catalyst in the absence of solvent. Contrary to the predominant protocol based on reagent control commonly applied to amidation reactions, this study provides insight into an approach based on substrate control