Cyclization manganese-promoted

Manganese(III)-promoted radical cyclization of arylthioformanilides and a-benzoylthio-formanilides is a recently described microwave-assisted example for the synthesis of 2-arylbenzothiazoles and 2-benzoylbenzothiazoles. In this study, manganese triacetate is introduced as a new reagent to replace potassium ferricyanide or bromide. The 2-substituted benzothiazoles are generated in 6 min at 110°C imder microwave irradiation (300 W) in a domestic oven with no real control of the temperature (reflux of acetic acid) (Scheme 15). Conventional heating (oil bath) of the reaction at 110 °C for 6 h gave similar yields [16]. 

Oxidative cyclization, promoted by manganese dioxide or phenyliodo(m) diacetate (PIDA), amidinobenzothiazoles 303 afford [l,2,4]triazolo[3,2- ]benzothiazoles 304 (Equation 48) (Table 52) <2001IJH315, 19960PP362>. 

There are many studies on manganese(m)-promoted radical cyclizations <1995TL9039, 1998T12029>. The influence of chiral auxiliaries on the stereocontrol of radical cyclizations has been investigated <1995SL915, 1996T489>. Treatment... 

Radical cyclizations catalyzed by 67a require the regeneration of the titanocene catalysts by a stoichiometric reductant, such as manganese. When 10 mol% of substituted cyclopentadienyltitanium complex 47e is applied instead truly catalytic cyclization sequences of epoxides 86 are possible (Fig. 25) [160]. Reductive radical generation from 86 promoted by titanocene chloride 67e and subsequent 5-exo cyclization of radical 86A generates a titanoxy cyclopentylalkyl radical 86B. Since the electron-poor titanocene chloride 67e reduces the tertiary radical 86B only sluggishly, its extended lifetime allows for a 1,5-SHi affording the bicyclic tetrahy-drofuran ring system 87. At the same time catalyst 67e is liberated. The reaction... 

Similarly, a catalyst (10 mol%) generated in situ from Cp2VCl2 and manganese/ chlorotrimethylsilane promotes a radical 5-exo cyclization of (1-iodo allylic ether 68 to tetrahydrofuran 69 in 62% yield (cf. Fig. 22) [153]. The reaction should proceed analogously to the corresponding Cp2TiCl2-catalyzed transformation. 

Synthesis of imidazole-2-carboxaldehyde has previously been reported by manganese dioxide oxidation of the corresponding car-binol,4 by acid-promoted cyclization of 7V-(2,2-diethoxyethyl)-2,2-... 

Manganese (III) acetate or chloride salts [Mn30(0Ac) H0Ac, MnCb] can react with alkenes to afford 1,2-dichlorides and chlorohydrin acetates (equation 16). i The manganese(III) reagent promotes the chlorination of 1,6-heptadiene (42) to afford almost equal amount of open chain and cycliz dichlorides... 

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