Synthetic Application of Benzothiazole Ring System as an On-Off Type of Leaving Group. Synthesis of Ketones and Carboxylic Acid Derivatives from 2-(1-Substituted 1-Hydroxyalkyl)- and 2-(1-Hydroxyalkyl)benzothiazoles

Abstract

Abstract A synthetic and systematic use of benzothiazole ring system as an “on-off” type of leaving group was developed for the preparation of ketones and carboxylic acid derivatives from a variety of 2-(1-substituted 1-hydroxyalkyl)- and 2-(1-hydroxyalkyl)benzothiazoles (off state) via the corresponding benzothiazolium salts (on state) obtained by the quaternization (on switch). A variety of 3-methyl-2-(1-substituted 1-hydroxyalkyl)benzothiazolium iodides (1) underwent a carbon–carbon bond cleavage at the 2-position to give the corresponding ketones on the simple treatment with a base under mild reaction conditions. The similar reaction of 3-methyl-2-(1-hydroxy-alkyl)benzothiazolium iodides (2) to aldehydes proceeded less efficiently. The oxidative reaction of a variety of 2 with base and nonactivated MnO2 in ethanol afforded the corresponding ethyl esters. This type of oxidative reaction could be also achieved in THF with different nucleophiles such as alcohol, water, thiol, and amine to give the corresponding ester, carboxylic acid, thioester, and amide, respectively.