Esters 2- benzothiazole synthesis

One of the first published microwave-assisted synthesis of benzothiazoles is the condensation of a dinucleophile such as 2-aminothiophenol, with an ortho-ester (neat) in the presence of KSF clay in a mono-mode microwave reactor operating at 60 W under a nitrogene atmosphere [ 12] (Scheme 12). Traditional heating (oil bath, toluene as solvent and KSF clay) gave the expected products in similar yields compared to the microwave experiments but more than 12 h were required for completion. Solvent-free microwave-assisted syntheses of benzothiazoles was also described by attack of the dinucleophiles cited above on benzaldehydes and benzaldoximines [13] (Scheme 12). This methodology was performed in a dedicated monomode microwave reactor... 

Malononitrile and cyanoacetamide only give aminopyrans. However, in reaction with cyanoacetic ester 27b aminopyran 233 has been isolated from the mother liquor and also side product 234 from the cooled mixture. Mechanistic considerations have been proved by the isolation of a deace-tylated intermediate (95M615). Similar transformations occur in the synthesis of spiropyrans 235 from benzothiazole 236 (97G605) (Scheme 90). 

Synthesis of the title compounds from benzothiazole precursors has also been accomplished by cyclocondensation of 2-chlorobenzothiazole (282) with anthranilic acids or their esters (34JIC463 53JA712, 53JOC1380 75JIC886 79CB3424, 79PHA138). 

Acetic formic anhydride is a versatile reagent in organic synthesis (review [3823]), for example, when it is stirred with an o-aminothiophenol. a 2-unsub-stituted benzothiazole is formed. Aminothiophenols, like their oxygen counterparts [2111], are cyclized by carboxylic acids or their esters, and also by dichloromethyleneammonium salts (review [2549]) to give good yields of benzo-thiazoles [2548]. The use of a dithioic ester is shown in Section 1.2 [2860]. 

H-Pyrido[2,1 -6]benzothiazole-4-carboxylic acid, 1-oxo-2-phenyl-ethyl ester synthesis, 5, 151... 

A large volume of work continues to be published in the area of thiazole chemistry. Physicochemical aspects continue to receive their share of attention, and interest in meso-ionic compounds has been sustained. Asinger and Offerman s versatile synthesis provides routes to reduced thiazoles (75, 212—217). Other notable methods employ thiirans (75, 218, 219), thiocarbonylamino-acid silyl esters (75, 220), and enamines (75, 221) as starting materials. Reduced thiazoles play an important role in peptide and protein studies (75, 234—239), and generate interest because of their occurrence as structural units in compounds of physiological interest, such as the antibiotics bacitracin A (75, 147 - 149) and thiostrepton. Benzothiazole derivatives have been employed to separate amidines for the first time into their syn- and fl/itZ-isomers (74, 49). The reaction of citric acid with penicillamine provides an interesting entry into the thiazolo-[3,2-fl]pyridine system (74, 152). 

By analogy to the standard synthesis of benzoxazoles (cf. p. 180), benzothiazoles are synthesized [360] by cyclocondensation of (2-amino)thiophenols with carboxylic acids or esters, with orthoesters in the presence of the acidic montmorillonite KSF clay catalyst, practicable also for the synthesis of benzoxazoles and benzimidazoles [361] or with aldehydes in the presence of an oxidant [362] ... 

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