Manganese dioxide, active

Hydroxyandrosta-4,6-dien-3-one. A suspension of 42 g of crude androsta-4,6-diene-3j ,17j -diol in 2000 ml of chloroform is treated with 250 g of activated, manganese dioxide. The mixture is then shaken vigorously for 15 min in a stoppered flask. The mixture is filtered and the manganese dioxide washed well with chloroform in order to elute material which initially remains adsorbed on the solid phase. The filtrate is concentrated to a pale yellow, crystalline residue. Recrystallization from acetonitrile gives 38 g (90%) of 17/ -hydroxyandrosta-4,6-dien-3-one as plates mp 211-214°. 

Nickel peroxide is a solid, insoluble oxidant prepared by reaction of nickel (II) salts with hypochlorite or ozone in aqueous alkaline solution. This reagent when used in nonpolar medium is similar to, but more reactive than, activated manganese dioxide in selectively oxidizing allylic or acetylenic alcohols. It also reacts rapidly with amines, phenols, hydrazones and sulfides so that selective oxidation of allylic alcohols in the presence of these functionalities may not be possible. In basic media the oxidizing power of nickel peroxide is increased and saturated primary alcohols can be oxidized directly to carboxylic acids. In the presence of ammonia at —20°, primary allylic alcohols give amides while at elevated temperatures nitriles are formed. At elevated temperatures efficient cleavage of a-glycols, a-ketols... 

Oxidation of di-n-butyl sulphide with activated manganese dioxide in light petroleum gave di-n-butyl sulphoxide exclusively126. However, the reaction was very slow at room temperature. This reagent is also suitable for oxidation of diallyl sulphides although, after 76 h, diallyl sulphoxide was isolated in 13% yield only. 

A reverse reaction, i.e. oxidation of -hydroxysulphoxides to -ketosulphoxides, can be performed using active manganese dioxide ... 

The oxidation of a thiazolidine derivative to the corresponding thiazole using activated manganese dioxide in dichloromethane at 100 °C is shown in Scheme 6.100. Further manipulation of this molecule led to dimethyl sulfomycinamate, a methano-lysis product of the thiopeptide antibiotic sulfomycin I [203]. 

This photoaffinity labelling analogue of all-fraws-retinal, 95b, has been tritium labelled80 by reduction of unlabelled aldehyde 95a with [3H]-NaBH4 and subsequent oxidation of the obtained tritium-labelled retinol with activated manganese dioxide. The product 95b (specific activity 38.3 mCimmol-1) has been isolated by preparative TLC (equation 36). 

Silica reinforced rubber, 22 703 Silica sheets, 22 383-385 Silica-silane system, 22 377-378 Silica sol-gel fiber processing, 23 80 Silica sols, 22 383, 473-474 applications of, 22 394 modification of, 22 393-394 preparation of, 22 392-393 properties of, 22 391-392 purification of, 22 393 Silica, solubility in steam, 23 212-213 Silica-supported activated manganese dioxide, 76 568... 

The reaction of the complex salt 6a with the arylamine 12 affords by regio-selective electrophilic substitution the iron complex 13 [88] (Scheme 11). The oxidative cyclization of complex 13 with very active manganese dioxide provides directly mukonine 14, which by ester cleavage was converted to mukoeic acid 15 [89]. Further applications of the iron-mediated construction of the carbazole framework to the synthesis of 1-oxygenated carbazole alkaloids include murrayanine, koenoline, and murrayafoline A [89]. 

The total synthesis of the carbazomycins emphasizes the utility of the iron-mediated synthesis for the construction of highly substituted carbazole derivatives. The reaction of the complex salts 6a and 6b with the arylamine 20 leads to the iron complexes 21, which prior to oxidative cyclization have to be protected by chemoselective 0-acetylation to 22 (Scheme 13). Oxidation with very active manganese dioxide followed by ester cleavage provides carbazomycin B 23a [93] and carbazomycin C 23b [94]. The regioselectivity of the cyclization of complex 22b to a 6-methoxycarbazole is rationalized by previous results from deuterium labeling studies [87] and the regiodirecting effect of the 2-methoxy substituent of the intermediate tricarbonyliron-coordinated cyclo-hexadienylium ion [79c, 79d]. Starting from the appropriate arylamine, the same sequence of reactions has been applied to the total synthesis of carbazomycin E (carbazomycinal) [95]. 

The carbazole-1,4-quinol alkaloids are also accessible by the iron-mediated arylamine cyclization (Scheme 14). Electrophilic substitution reaction of the arylamine 24 with the complex salts 6a and 6b affords the iron complexes 25. Protection to the acetates 26 and oxidative cyclization with very active manganese dioxide leads to the carbazoles 27, which are oxidized to the carbazole-... 

Using this method, the electrophilic aromatic substitution of the electron-rich arylamine 578 by the molybdenum-complexed cation 577 affords regio- and stereoselectively the molybdenum complexes 579. Cyclization with concomitant aromatization and demetalation using activated manganese dioxide leads to the carbazole derivatives 568 (8,10,560) (Scheme 5.26). 

An alternative access to murrayanine (9) was developed starting from the mukonine precursor 609. The reduction of the ester group of 609 using diisobutylaluminum hydride (DIBAL) afforded the benzylic alcohol 611. In a one-pot reaction, using very active manganese dioxide, 611 was transformed to murrayanine (9) (574) (Scheme 5.36). 

Electrophilic aromatic substitution of 708 with the iron-coordinated cation 602 afforded the iron-complex 714 quantitatively. The iron-mediated quinone imine cyclization of complex 714, by sequential application of two, differently activated, manganese dioxide reagents, provided the iron-coordinated 4b,8a-dihydrocarbazole-3-one 716. Demetalation of the iron complex 716 with concomitant... 

Electrophilic substitution at the arylamine 709 using the complex salt 602, provided the iron complex 725 quantitatively. Sequential, highly chemoselective oxidation of the iron complex 725 with two, differently activated, manganese dioxide reagents provided the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727) via the non-cyclized quinone imine 726. Demetalation of the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727), followed by selective O-methylation, provided hyellazole (245) (599,600) (Scheme 5.70). 

Electrophilic aromatic substitution of the arylamine 780a using the iron-complex salt 602 afforded the iron-complex 785. Oxidative cyclization of complex 785 in toluene at room temperature with very active manganese dioxide afforded carbazomycin A (260) in 25% yield, along with the tricarbonyliron-complexed 4b,8a-dihydro-3H-carbazol-3-one (786) (17% yield). The quinone imine 786 was also converted to carbazomycin A (260) by a sequence of demetalation and O-methylation (Scheme 5.86). The synthesis via the iron-mediated arylamine cyclization provides carbazomycin A (260) in two steps and 21% overall yield based on 602 (607-609) (Scheme 5.86). 

The total synthesis of carbazomycin C (262) was achieved by executing similar reaction sequences as in the iron-mediated arylamine cyclization route described for the synthesis of carbazomycin B (261) (see Scheme 5.87). The electrophilic substitution of the arylamine 780b using the complex salt 779 afforded the iron complex 797, which was transformed to the corresponding acetate 798. Using very active manganese dioxide, compound 798 was cyclized to O-acetylcarbazomycin C (799). Finally, saponification of the ester afforded carbazomycin C (262) (four steps and 25% overall yield based on 779) (611) (Scheme 5.90). 

The reaction of the iron-complex salts 602 and 779 with the arylamine 973 afforded the iron complexes 976 and 977, both in 96% yield. Subsequent O-acetylation provided the corresponding acetates 978 and 979 in almost quantitative yield. The iron-mediated arylamine cyclization of the O-acetyl derivative 978 using very active manganese dioxide provided the carbazole 971 in 72% yield. Under similar reaction conditions, the O-acetyl derivative 979 gave a mixture of the carbazoles 972 and 980 in 35% and 17% yield, respectively (650,651) (Scheme 5.136). 

Reaction of the 5-aminochromene 1044 with the complex salt 577 provided via an electrophilic aromatic substitution regio- and diastereoselectively the molybdenum complex 1050. The oxidative cyclization of complex 1050 with concomitant aromatization and demetalation using activated manganese dioxide led directly to girinimbine (115) in 50% yield. Oxidation of girinimbine (115) with DDQ in methanol afforded murrayacine (124) in 64% yield (660) (Scheme 5.161). 

Among the oxidative procedures for preparing azo compounds are oxidation of aromatic amines with activated manganese dioxide oxidation of fluorinated aromatic amines with sodium hypochlorite oxidation of aromatic amines with peracids in the presence of cupric ions oxidation of hindered aliphatic amines with iodine pentafluoride oxidation of both aromatic and aliphatic hydrazine derivatives with a variety of reagents such as hydrogen peroxide, halogens or hypochlorites, mercuric oxide, A-bromosuccinimide, nitric acid, and oxides of nitrogen. 

Other oxidative methods have recently been reported whereby amines yield enamines either as intermediates [130] or as isolatable chloranil [131], 2,3-dichloronaphtha-l,4-quinone [131], or 2,5-dichloro-3,6-dimethoxybenzo-quinone [132, 133] adducts. Benzoyl peroxide [131] and active manganese dioxide [134] have been reported as effective oxidizing agents in the reactions above. (See Eqs. 49, 50.)... 

Using activated manganese dioxide oxidations, even 2-aminopyridine has been converted into 2-azopyridine (m.p. 85°C). When mixtures of two anilines are oxidized together, the products isolated are only the symmetrically substituted azobenzenes [69b]. 

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