Amino diradicals

The platinum(0) complex [Pt(PhNO)(PPh3)2] reacts with C02 to afford the metallacyclic nitroso species [Pt 0N(Ph)C(0)0 (PPh3)2] (60), the first example of insertion of C02 into a Pt—N bond.186 Other unsaturated carbon compounds such as CS2 and electrophilic alkenes and alkynes react similarly. The diradical peril uoro-/V,/V -dimethylethane-l,2-bis(amino-oxyl) reacts readily by oxidative addition to the platinum(0) precursor Pt(PPh3)4 to afford the corresponding platinum(lI)-nitroso complex containing a seven-membered chelate ring (61). The resulting complex is stable in air for several days at room temperature.187... 

An elimination-addition mechanism has been suggested for the substitution of amino for chloro in chlorobenzene.95 The neutral and symmetrical intermediate is called benzyne although it can not contain an ordinary linear bond system like that of acetylene. It is not known whether it is a distorted acetylene, a triplet (diradical), or a zwitterion. 

Amino ketones 11 isomerize to 2-aminocyclobutanols 12. A geminal dimethyl group on C2 increases the cyclization vs. cleavage rate of the intermediate diradical.10... 

Photoreaction of l-vinyl-4-alkyl-5-aminotriazolines leads to unexpected pyrrolines and pyrroles.459 The 1-vinylaziridine formed by ring closure of the 1,3-diradical undergoes a selective ring cleavage at one of the C—N bonds (bearing the amino group) followed by cyclization with the j -vinyl carbon to give the products (Scheme 157)459... 

Wessig P. Synthesis and modifications of amino acids and peptides via diradicals. In Renaud P, Sibi M, eds. Radicals in Organic Synthesis. Wiley, 2001. 

Synthesis and Modifications of Amino Acids and Peptides via Diradicals... 

Peptides and proteins represent, apart from the nucleic acids, the most important class of compounds governing the basic biochemical principles in nature. During the last hundred years the synthesis of natural and unnatural amino acids as well as peptide synthesis has experienced a breathtaking development. The interest in this process has grown as the knowledge about the relationship between the structure of peptides and proteins and their physiological effects has increased. Nowadays the chemist may refer to a variety of synthetic methods to prepare enantiomerically enriched or pure a-amino acids (for an overview see [1]). Nevertheless, the need for amino acids with a very special substitution pattern often reveals the limits of the established methods. Consequently, the development of new synthetic routes to a-amino acids (and, naturally, also to 8-amino acids, which have enjoyed increased attention over the last years) is playing an important role in the current chemical research. This chapter reviews the application of a special part of radical chemistry in the synthesis and modification of amino acids and peptides, namely reactions that proceed via diradicals. 

The concept of intramolecular alkylation of AT-substituted amino acid derivatives via 1,5-diradicals also turned out to be an excellent system for studying the different stereochemical course of spinisomers as discussed in Section 6.2.2. Thus, the a-ketoester 9, which contains an alanine moiety, was prepared. In contrast to aryl ketones, a-ketoesters are not completely converted into the triplet state after photochemical excitation. Upon addition of either a triplet quencher (naphthalene) or a triplet sensitizer (benzophenone), each of the two spin states may be forced (Scheme 3, Table 1). The chiral center at the d-position with respect to the keto carbonyl group raises the question whether a memory effect of chirality may be observed during the cyclization. The results summarized in Table 1 amply demonstrate the specific properties of spinisomeric biradicals. In the presence of naphthalene, which probably acts not only as a triplet quencher but also as a singlet sensitizer, the chiral information of the reactant 9 is almost entirely conserved in the helical diradical 10 because of its very short lifetime. In contrast, the addition of benzophenone results in almost complete racemization, and also the cis/trans selectivity is... 

The successful synthesis of amino acids by intramolecular alkylation of A-substituted natural amino acids via diradicals brought up the question whether this concept may be applied to peptides. The ability to influence peptide conformation... 

---