Benzothiazoline ring

The asymmetrical anhydrobase (20) constitutes the first step of the formation of trimethine thiazolocyanine when a 2-methylthiazolium salt reacts either with a benzothiazolium or its opened form [which is bis-o-(formylmethylamino)(diphenyl disulfide] (Scheme 26). In a second step, 20 is protonated by a second molecule of 2-methylthiazolium. It results in cleavage of the benzothiazoline ring, which gives 21 together with the formation of the monomeric anhydrobase (22). Cleavage of the C-S bond of 20 can be explained by the important electronic desaturation of the C atom observed in NMR spectrum and the great polarizability of the C-S bond in this type of ring (48). 

Triethylamine 1,4-Benzothiazine ring and N-condensed benzothiazoline ring from o-aminomercaptans... 

The configuration is that of an asymmetrical dimer of 2-methylene-V-methylbenzothiazoline with the sulphur atoms in trans positions. The stability of the benzothiazoline moiety of the molecule is impaired by the presence of the methyl group in position 2. The abnormally long S-1—C-2 bond (1.86 A) explains the ready hydrolytic cleaving of the thiazoline ring. The planes of the benzothiazole and benzothiazoline rings subtend an angle of 91°. ... 

Benzothiazoline ring closure with S -acyl group migration... 

Ring-opening is not a mandatory direction of the complexation reaction. Thus, the ring-closed form of the benzothiazoline 395 ligand was found to be preserved in its complex 396 with BuSnCla, whose structure was determined with X-rays (Scheme 146) [93JOM(454)67]. 

Benzothiazoline, cleavage of ring, 42 polarizabilite of CS bond, 42 Benzothiazolium salts, action on anhydro-base, 43... 

Over the years, many spiropyran structures have been prepared. The pyran component consists of benzopyran or naphthopyran and the heterocyclic part consists of indoline, benzothiazoline, benzoxazoline, benzoselen-azoline, phenanthridine, acridine, quinoline, benzopyran, naphthopyran, xanthene, benzodithiole, benzoxathiole, and saturated heterocyclic rings such as pyrolidine and thiazolidine. 

The condensation of o-aminobenzenethiol with pyridine-2,6-dicarbaldehyde does not form the corresponding Schiff base but a polyheterocyclic compound which behaves as a tridentate ligand towards nickel(II).2365 In contrast the reaction of 2-(2-pyridyl)benzothiazoline with nickel(II) results in the opening of the heterocyclic ring and in the formation of a Schiff base ligand which coordinates to nickel(II) as a mononegative tridentate ligand.2366,2367... 

Acetylation of benzothiazepine 85 (R = aryl, 2 -styryl) in the presence of pyridine caused a ring contraction, affording 2,2-disubstituted-3-acetylated benzothiazoline derivatives 86 (Equation 7) <1997LA995, 2003ARK19, 2004JHC399>. 

Figure 5-81. The copper-mediated ring-opening of 2-(2-pyridyl)benzothiazoline gives the complex of an iminothiol ligand.

This metal-directed ring-opening of thiazolines, oxazolines and related compounds is a widespread reaction. In many cases, equilibria are set up between the heterocyclic compound and the imine. The position of these equilibria are metal-ion dependent. A typical example is seen in Fig. 5-81. The reaction of 2-(2-pyridyl)benzothiazoline (5.36) with copper(n) salts leads to the formation of complexes of the ring-opened tridentate form. 

Benzothiazoles are rather more prone to reductive decyclization than are thiazoles, although it is possible to form benzothiazolines (224) through the sodium borohydride reduction of benzothiazolium salts (223). On hydrolysis these products yield 2-aminothiophenols (225). Prolonged exposure of benzothiazolines to excess sodium borohydride also causes ring opening to aminothiophenols, as does reaction with LAH. In this last case the substrates for reduction may be benzothiazoles (226), which presumably initially form benzothiazolines with the reagent (Scheme 18). ... 

In an I.G. Farben patent [180] the use of disulfur dichloride is described in place of sulfur (method BJ. Possibly the intermediates here are 1,3,2-benzothiazathiolium chlorides (CV) which react with carbon disulfide to give benzothiazoline-2-thiones (CIII). In some instances, the cycUzations are carried out in the presence of sodium hydrosulfite or sodium sulfide. A peculiarity of this reaction is that, simultaneously with the cyclization, chlorination of the arylene ring occurs. Thi.s is not a general rule though. It happens with aniline, o-toluidine, o-anisidine and oc-naphthylamine but not with /)-phenetidine, )S-naphthylamine or benzidine. It seems quite clear therefore that the para position of the aniline must be free if chlorination is to take place, since it is in this position that the chlorine atom is brought in. With anthranilic acid there is formed 4-carbox -6-phenylaminobenzothiazoline-2-thione which shows that the chlorine atom, which is substituted during the reaction, is replaced by a second molecule of anthranilic acid and that decarboxylation occurs at the same time. 

Dunbrook and Zimmermann 113) report still higher yields, even with short reaction times, when sodium polysulfide and carbon disulfide are used (method C ). The essential appears to be the replacement of the chlorine atom by a mercapto group and the reduction of the nitro group to an amino group, followed by ring closure of the resulting o-aminothiophenol with carbon disulfide to the benzothiazoline-2-thione (method A). 

The ring-chain tautomerism of 1,3-thiazolidines and 1,3-benzothiazolines can be considered as a reversible intramolecular addition to the C=N bond. The system highly prefers the ring form (27a) and only in some cases the open-chain tautomer (27b) can detected (Equation (4)). 

One synthesis of benzothiazines via a-epoxy ketones83 involves acid-catalyzed ring expansion of a benzothiazoline intermediate (37) to give the benzothiazine derivative 38. 

Prota et al. and a group in Japan independently reported ring expansion reactions of benzothiazoline S-oxides, which are mechanistically similar to the... 

This type of tautomerism is principally important for the preparation of chelates with five-membered metallacycles of composition MN2S2 from benzothiazoline-ligating compounds (Equation (16)). Ring-chain... 

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