Benzothiazolines, formation

Scheme 97). Stepanov has thoroughly studied this nucleophilic reactivity some examples are given in Refs. 423 and 424. The formation of 5-benzylidene derivatives involves the same nucleophilic reactivity (422). 5-Benzothiazoline and 5-benzose enazoline derivatives of 2-diphenylaminothiazoline-4-one, have been obtained by nucleophilic addition of the thiazolone on the corresponding benzothiazolium or ben-zoselenazolium salts (433). 

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). 

In the presence of various metal ions, 2-(fluoroenone)benzothiazoline has been found to rearrange to A-2-mercaptophenylenimine, while a free radical mechanism involving the homolysis of C-S and C-N bonds has been invoked to explain the formation of 3-phenyl-1,2,4-triazole derivatives from the thermal fragmentation and rearrangement of 2-(arylidenehydrazino)-4-(5//)-thiazolone derivatives. The cycloadducts (36) formed from the reaction of 3-diethylamino-4-(4-methoxyphenyl)-5-vinyl-isothiazole 1,1-dioxide (34) with nitric oxides or miinchnones (35) have been found to undergo pyrolytic transformation into a, jS-unsaturated nitriles (38) by way of pyrrole-isothiazoline 1,1-dioxide intermediates (37). 

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... 

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. 

Precursors of azomethinic complexes with live-member metal-cycles, of the type MN2S2, are the benzothiazoline compounds represented by structure 461 [337a,805,823,824] they can easily be transformed into chelates of type 462 [806-812] under proper conditions for complex formation (2.12) [9,269,804,823,824] ... 

Methods available before 1971 for the preparation of thiol esters are briefly summarized in a review article.4 Since then, several newer techniques have been developed, to meet a certain set of criteria required for recent synthetic operations. This development may be summarized as follows. Whenever an acid chloride is available, the reaction of the T1(I) salt of a thiolate of virtually any kind, including alkane-, benzene-, 2-benzothiazoline-, and 2-pyridinethiol, proceeds efficiently and near-quan-titatively. However, if selective thiol ester formation in the presence of hydroxy or other functional groups in the same molecule is required, three main procedures are available. First, reaction of an acid (1), with... 

More recently, similar reactions have been used extensively in the formation of macrocyclic compounds. Imine or Schiff base complexes (R j N=CHR2) have been extensively studied. In this case, the compounds can be formed without complexation, but the reaction is much faster in the presence of metal ions. An example is shown in Figure 12-17. In the absence of copper, benzothiazoline is favored in the final step rather than the imine very little of the Schiff base is present at equilibrium. 

Benzothiazolin-2-one hydrazones. The oxidation of 3-methylbenzo-thiazolin-2-one hydrazone (55) by potassium ferricyanide produces, apart from nitrogen, the compounds (56), (57), and (58) in relative proportions which vary with the prevailing pH and the order of the addition of the reagents yields of (58) are consistently minute. Oxidation, under various conditions, of the 2- N-labelled reactant yields di-labelled nitrogen, mono-labelled 3-methyl-2-benzothiazoline azine (56), and unlabelled 2-imino-3-methylbenzothiazoline (57). The formation of the individual products and the results of the labelling studies are consistent with the mechanism shown in Scheme 3. ... 

A simple one-step synthesis of 1,4-benzothiazines (59) is found in the ring-expansion of the chlorosulphonium salts of the readily available benzothiazolines (58) (Scheme 5). Treatment of 3-methylbenzothiazolium ion with phenacyl bromide in the presence of triethylamine results in ring-expansion and formation of the 1,4-benzothiazine (61), possibly via the thermally unstable intermediate... 

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