Robust synthesis of C-terminal cysteine-containing peptide acids through a peptide hydrazide-based strategy

A simple-to-operate and highly efficient strategy for the epimerization-free synthesis of C-terminal Cys-containing peptide acids, which avoids the use of derivatization reagents for resin modification, is developed.

Amide Bond Activation of Biological Molecules

Amide bonds are the most prevalent structures found in organic molecules and various biomolecules such as peptides, proteins, DNA, and RNA. The unique feature of amide bonds is their ability to form resonating structures, thus, they are highly stable and adopt particular three-dimensional structures, which, in turn, are responsible for their functions. The main focus of this review article is to report the methodologies for the activation of the unactivated amide bonds present in biomolecules, which includes the enzymatic approach, metal complexes, and non-metal based methods. This article also discusses some of the applications of amide bond activation approaches in the sequencing of proteins and the synthesis of peptide acids, esters, amides, and thioesters.

Epimerization-free access to C-terminal cysteine peptide acids, carboxamides, secondary amides, and esters via complimentary strategies

We present a convenient method for the diversification of peptides bearing cysteine at the C-terminus that proceeds to form a variety of carboxylic acid, carboxamide, 2° amide, and ester terminated peptides without any detectable epimerization of the α-stereocenter.

Functional Group Interconversion

Chaozhong Li of the Shanghai Institute of Organic Chemistry reported (J. Am. Chem. Soc. 2012, 134, 10401) the silver nitrate catalyzed decarboxylative fluorination of carboxylic acids, which shows interesting chemoselectivity in substrates such as 1. A related decarboxylative chlorination was also reported by Li (J. Am. Chem. Soc. 2012, 134, 4258). Masahito Ochiai at the University of Tokushima has developed (Chem. Commun. 2012, 48, 982) an iodobenzene-catalyzed Hofmann rearrangement (e.g., 3 to 4) that proceeds via hypervalent iodine intermediates. The dehydrating agent T3P (propylphosphonic anhydride), an increasingly popular reagent for acylation chemistry, has been used (Tetrahedron Lett. 2012, 53, 1406) by Vommina Sureshbabu at Bangalore University to convert amino or peptide acids such as 5 to the corresponding thioacids with sodium sulfide. Jianqing Li and co-workers at Bristol-Myers Squibb have shown (Org. Lett. 2012, 14, 214) that trimethylaluminum, which has long been known to effect the direct amidation of esters, can also achieve the direct coupling of acids and amines, such as in the preparation of amide 8. The propensity of severely hindered 2,2,6,6-tetramethylpiperidine (TMP) amides such as 9 to undergo solvolysis at room temperature has been shown (Angew. Chem. Int. Ed. 2012, 51, 548) by Guy Lloyd-Jones and Kevin Booker-Milburn at the University of Bristol. The reaction proceeds by way of the ketene and is enabled by sterically induced destabilization of the usual conformation that allows conjugation of the nitrogen lone pair with the carbonyl. Matthias Beller at Universität Rostock has found (Angew. Chem. Int. Ed. 2012, 51, 3905) that primary amides may be transamidated via copper(II) catalysis. The conditions are mild enough that an epimerization-prone amide such as 11 undergoes no observable racemization during conversion to amide 13. A photochemical transamidation has been achieved (Chem. Sci. 2012, 3, 405) by Christian Bochet at the University of Fribourg that utilizes 385-nm light to activate a dinitroindoline amide in the presence of amines such as 15, which produces the amide 16. Notably, photochemical cleavage of the Ddz protecting group occurs at a shorter wavelength of 300 nm.

A new method for peptide synthesis in the N→C direction: amide assembly through silver-promoted reaction of thioamides

The Ag(i)-promoted coupling of peptide-acids with thioamides generates peptide-imides, which was exploited in the synthesis of peptides in the N→C direction.