Substituierte Pyryliumsalze, die leicht erhaltlich sind, eignen sich zur Herstellung zahlreicher, oft schwer zuganglicher aromatischer Verbindungen der heterocyclischen und isocyclischen Reihe. Am langsten bekannt ist die Baeyer-Umwandlung in Pyridin- und Pyridinium-Derivate. lhr schliest sich die Wizinger-Reaktion zur Bildung von Thiopyryliumsalzen an. Besonders reichhaltig sind die Umwandlungsmoglichkeiten in aromatische Verbindungen, wobei nach alteren Verfahren Phenole und alkylierte Amine, nach neueren Nitro-Verbindungen, Ketone, Carbonsauren, Nitrile, Kohlenwasserstoffe, Phenolcarbonsauren oder Aminonitrile in meist guten Ausbeuten gewonnen werden konnen. Auch fur die Synthese von Azulenen nach Hafner bilden Pyryliumsalze geeignete Ausgangsprodukte. Im Anhang wird ein Uberblick uber die Moglichkeiten zur Synthese von Pyryliumsalzen gegeben.

Neuere Methoden der präparativen organischen Chemie III. 3. Aromatische Verbindungen aus Pyryliumsalzen

Previously, 2,4,6-trimethylpyridine (collidine), due to steric shielding around the N-atom, was found to be an efficient base for effecting peptide segment coupling via azabenzotriazole-based onium-style coupling reagents. A number of even more highly hindered bases, including 2,3,5,6-tetramethylpyridine, 2,6-di-tert-butyl-4-(dimethylamino)pyridine, triisopropylamine, and N-tert-butylmorpholine, have been compared with collidine in such reactions. Some of the newer bases showed advantages in terms of convenience in handling and maintenance of configuration during segment coupling processes, although dramatic differences based on steric effects were not observed. On the basis of results with a number of test peptides and many base-coupling reagent combinations, it was noted that most efficient results are obtained if 1 equiv of HOAt is present as an additive during the coupling process. For rapid activation of onium-style coupling reagents during stepwise solid-phase coupling reactions, the stronger base 2...

Peptide Coupling in the Presence of Highly Hindered Tertiary Amines

Publisher Summary The pyrylium salts represent one of the fundamental heterocyclic systems and are the basis of important natural products such as the anthoeyanins. This chapter reviews the scope, nomenclature, formulation, and syntheses of pyrylium salts. The syntheses of pyrylium salts are classified into three groups, according to the number of building blocks required to form the ring in a one-step process. The three groups are: (1) one-component syntheses, (2) two-component syntheses, and (3) three-component syntheses. Pyrylium salts bearing aromatic or heterocyclic substituents, or possessing fused saturated rings and considered as monocyclic pyrylium salts are discussed. A number of reactions starting from pyrylium salts and involving substitution at the pyrylium ring, or modification of existing substituents are also described. Pyrylium salts are useful intermediates in the conversion of aliphatic starting materials into aromatic carbocyclic or heterocyclic compounds. Some practical applications of pyrylium salts emerged are: fluorescent dyes, photosensitizers for silver halides, or photographic emulsions.

Pyrylium Salts Part I. Syntheses

A total synthesis of FK-506 (1) is presented. The synthesis features a highly convergent approach utilizing a block coupling strategy. Top and bottom half sections of the molecule are coupled by addition of a vinyl cuprate with a spiroenone. The alpha-allyl aldol functionality is revealed by a reductive opening of the spiroenone system. The labile alpha,beta-diketoamide hemiketal portion of the molecule is prepared by a late stage generation and oxidation of a masked enediol. Top and bottom half segments are themselves derived by coupling of smaller subunits, resulting in a very convergent route.

L. Carroll King

Papers

Preparation of Substituted Mercaptopyrylium Salts1

Reactions of pyrylium and pyridinium salts with amines

Formation and Kinetics of Formation of Substituted Mercaptopyrylium Salts1