BENZOTHIAZOLE GROUP

One of the reasons that the 2-lithiobenzothiazole system has received attention is the ability of the benzothiazole group to act as a synthon for an aldehyde or ketone group (78TL5 85H295 88BCJ3637). Thus, after reaction of the anion with an electrophile, the product is alkylated with methyl iodide followed by the nucleophilic addition of either an alkyl group or a hydride ion. Subsequent hydrolysis results in the formation of a ketone or an aldehyde (Scheme 100). 

Sulfenamides. Sulfenamides (4) are often produced by oxidizing an equimolar mixture of MBT and an aliphatic amine. Alternatively, the TV-chl oroamine can react with the sodium salt of MBT. One sulfenamide, OTOS (J), uses a thiocarbamyl functionality in place of the benzothiazole group. 

Symmetrical chromophores with the structure 94 bear as electron-rich aromatic bridge a thiophene moiety substituted at the end with electron-withdrawing benzothiazole groups. In this A-n-A chromophore, TPA significantly... 

The bis(benzothiazolylidene)squaraine derivatives show a sharp band around 645 nm, which became broad with a maximum around 515 nm on addition of acid. The acid-base equilibrium for the dye with a pKa of 6.4 was attributed to protonation/deprotonation of the nitrogen in the benzothiazole group [66]. 

In the same year. Cinchona alkaloid derivative 24 was shown to catalyse a Mannich-type reaction of imines bearing a benzothiazole group with diethyl malonate. A 10 mol% eatalyst loading and imine malonate ratio of 1 1.2, at room temperature provided for the first time p-amino esters in good yields and high enantioseleetivities (Seheme 19.31). 

Cinchona alkaloids with a thiourea group have also been applied for the asymmetric preparation of amino acid derivatives. Recent examples include an asymmetric Mannich reaction leading to P-amino ester derivatives with a benzoxazole moiety [77] and the formation of P-amino esters with a benzothiazole group [78]. The latter reaction was catalyzed efficiently by a C9 thiourea derivative (26) with a 2,6-dichloro-4-(trifluoromethyl)phenyl group connected to the thiourea group as shown in Scheme 6.34. 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

The same isomerization also occurs with diarylthiazoles, but when two adjacent phenyl groups are present, even in the final product, a photochemical cyclization gives rise to a polycyclic benzothiazole (Scheme 3) (213,218,219). 

The piC values of polymethine dyes depend on terminal group basicity (64) thus the protonation abHity diminishes if the basic properties of the residues decrease, passing from benzimidazole, quinoline, benzothiazole, to indolenine. On the other hand, the piC of higher homologues increases with chain lengthening. The rate constant of protonation is sensitive to other features, for example, substituents and rings in the chain and steric hindrance for short-chain dyes. 

After reaction for 15 min, the band near 3295 cm (not shown in Fig. 13) decreased significantly in intensity, indicating that the mono-substituted acetylene groups were reacting. New bands also appeared near 1539 and 1512 cm . After reaction for 30 min, several additional bands appeared near 1011, 1030, 1085, 1232, 1320, 1430, and 1515 cm. The bands near 1011, 1030, 1085, 1232, 1320, and 1430 cm were clearly related to the benzothiazole sulfenamide fragment of DCBS while the band near 1539 cm was related to zinc stearate. 

The relative ease of preparation of condensed thiazole derivatives is a consequence of facile thiazole ring closure, and therefore also benzothiazole amines with an amino group on the benzene ring (except for the weakly regioselective nitration of benzothiazoles) are very easily accessible and useful substrates for the Gould-Jacobs reaction. 

Methylphenyl)benzothiazole (80IC762) and 2-benzylbenzothiazole (95ICA(239)125) can be cyclopalladated. In the latter case, cylopalladation occurs upon reaction with palladium(II) acetate and gives the product 80. With lithium chloride, sodium bromide, or sodium iodide, a series of three products of substitution of the acetate group 81 (X = C1, Br, I) results. Pyridine, 2- and 3-methylpyridine, 2,6- and 3,5-dimethylpyridine cause the transformation of the chelate complexes 81 (X = C1, Br, I) and formation of the mononuclear products 82 (R = z= R" = = R = H, X = Cl, Br, I ... 

Chloro-/3-(trifluoromethyl)acroleins reacted with 2-aminothiophenol at room temperature to give a mixture of a benzothiazole and quinoline. This was in contrast to the /3-methyl analogue, which gave a benzothiazep-ine, the difference being due to the electronic influence of the CF3 group (91TL643). 

Presumably the final step involves alkylation of the (allyloxy)phosphonium ion as shown above. 2-(Allyloxy)benzothiazoles react with y regioselectivity, which is also due to the coordi-native effect of the leaving group towards the organometallic reagents41. 

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