Diazo-sulfones and -nitriles in oxazole synthesis; three step preparation of a bis-oxazole

Clickable Transformation of Nitriles (RCN) to Oxazolyl Sulfonyl Fluoride Warheads

The protocol for simple, efficient, and mild synthesis of oxazolyl sulfonyl fluorides was developed through Rh₂(OAc)₄-catalyzed annulation of methyl-2-diazo-2-(fluorosulfonyl)acetate (MDF) or its ethyl ester derivative with nitriles. This practical method provides a general and direct route to a unique class of highly functionalized oxazolyl-decorated sulfonyl fluoride warheads with great potential in medicinal chemistry, chemical biology, and drug discovery.

AuBr3 -Catalyzed Chemoselective Annulation of Isocyanates with 2H-Azirine

The chemoselective cyclization of isocyanates with 2H-azirine was achieved with AuBr₃ as catalyst. This transfer sets the stage for the synthesis of aromatic oxazole-ureas in a tandem process. The addition of a catalytic amount of phosphite enhances the process enormously. The reaction can also be performed in a one-pot process using benzoyl azide instead of isocyanate under the same conditions. A detailed study on the role of the phosphite that was applied as an additive revealed that only non-coordinated phosphite can reduce gold(III) and that gold(I) coordinated phosphite is not oxidized. Accompanied by the reduction of gold, HBr is generated in situ, which turned out to be the actual promotor in combination with the remaining AuBr₃ . The positive effect of acid can be explained by a strong N-Au coordination, which tends to break more easily in the presence of small amount of protic acid in the reaction solution.

Oxidation of Oxazolines and Thiazolines to Oxazoles and Thiazoles. Application of the Kharasch-Sosnovsky Reaction

Using a modification of the Kharasch-Sosnovsky reaction, the oxidation of oxazolines and thiazolines bearing a variety of 2-alkyl substituents (chiral and achiral) were smoothly oxidized to their corresponding oxazoles and thiazoles, respectively. The key feature involved in the successful implementation of this important oxidation was the use of a mixture of Cu(I) and Cu(II) salts to enhance the oxidation of the intermediate captodative radical, 24. The main limitation of this method was shown when the oxidation failed with oxazolines/thiazolines lacking the carboalkoxy group at C-4.