Manganese triacetate

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]. 

Meciarova M, Toma S, Luche J-L (2001) The sonochemical arylation of malonic esters mediated by manganese triacetate. Ultrason Sonochem 8(2) 119-122... 

The initially formed nitronate radical reacts with olefin R C=CR" to give an elongated radical A whose successive cyclization and oxidation affords target nitronates (5g) in 18% to 81% yields. Manganese triacetate is regenerated by electrochemical methods. 

It should be noted that nonmetallic redox reactions also experience the sonication influence. The preparation of y lactons from olefines by oxidation of manganese triacetate is an example. 

Dihydropyridinones react with acetic acid or diethyl malonate to give partially reduced furo[3,2-f]pyridine derivatives in low to moderate yields (Scheme 34) <2002CHE368>. More highly reduced products are formed with the acid starting material in the presence of manganese triacetate than with the diester under the same conditions. 

Figure 4.11. Examples of redox-initiated radical reactions. Samarium diiodide reduction of the bromide gives a radical that cyclizes faster than the second reduction reaction. Manganese triacetate oxidation of the P-keto ester gives an enol radical that is not further oxidized by the manganese reagent. The IBX oxidizes anilides to the corresponding radicals. Hexamethylphosphoramide = HMPA and Tetrahydrofuran = THE.

It should be noted that nonmetallic redox reactions also experience the sonication influence. The preparation of y-lactons from olefines upon manganese triacetate oxidation is an example. The reaction with monomethyl malonate in acetic acid, which does not occur at 0-10°C, proceeds smoothly when sonication is applied (Allegretti et al. 1993). From cyclohexene, only the cis ring fusion in the bicyclic lactone is observed the product is formed at 80% yield for 15 min at 10°C. The overall scheme of the reaction, without the detailed mechanism is shown in Scheme 8-18. 

One-electron oxidation systems can also generate radical species in non-chain processes. The manganese(III)-induced oxidation of C-H bonds of enolizable carbonyl compounds [74], which leads to the generation of electrophilic radicals, has found some applications in multicomponent reactions involving carbon monoxide. In the first transformation given in Scheme 6.49, a one-electron oxidation of ethyl acetoacetate by manganese triacetate, yields a radical, which then consecutively adds to 1-decene and CO to form an acyl radical [75]. The subsequent one-electron oxidation of an acyl radical to an acyl cation leads to a carboxylic acid. The formation of a y-lactone is due to the further oxidation of a carboxylic acid having an active C-H bond. As shown in the second equation, alkynes can also be used as substrates for similar three-component reactions, in which further oxidation is not observed [76]. 

Another oxidation reactions have been reported that led to C-C bond formation. They deal with treatment of A-allylsilane-substituted SMAs with CAN, lead tetraacetate or manganese triacetate to form methylene piperidines and azepines. Yields are in the same range as those obtained using SET-photolytic or electrolytic processes.359... 

A combination of radical and electron transfer steps mediated by manganese triacetate has been used in the synthesis of 5-acetoxyfuranones 21 through carbox-ymethyl radical addition to mono- and disubstituted alkynes 20, followed by oxidative cyclization of the resulting vinyl radicals 22 (Scheme 2.4). The cyclic intermediate 24 is transformed into the furanone 21 through stepwise one-electron oxidation and capture of the resulting aUyl cation 26 by acetate. 

Salts of other transition metals including vanadium, cerium, chromium and manganese have been used for a-oxygenation, although rarely applied in synthesis. Manganese triacetate has been used for the efficient a -oxidation of enones (Section 2.3.2.2.1.i), but appears not to have been used for the a-hydrox-ylation of saturated ketones des]Hte its known ability to form the corresponding a-keto radicals. Similarly the use of Lewis acid assisted enolization in the oxidative process appears to have been limited to the LTA-mediated examples. 

Manganese triacetate has been specifically reported as a reagent for a -oxidation.Mechanistic dualism analogous to LTA (vide supra) is observed, although the radical process may be more dominant. Watt and coworkers used this technique during the synthesis of quassinoids. Enone (113) was con-... 

The technique has been applied to the total synthesis of ( )-oxylubimin (134) through hydroxylation of dienol ether (135).The stereoselectivity is good, particularly when compared to the equivalent select -vities obtained using MCPB A, MoOPH or manganese triacetate. 

Reaction with Sodium Azide and Manganese Triacetate... 

In the presence of manganese triacetate and the azide anion in refluxing acetic acid alkenes are converted to 1,2-diazides in satisfactory yield, most probably through a ligand transfer-oxidation mechanism74. 

Manganic acetate (manganese triacetate), Mn(OCOCH3)3, is prepared by refluxing a solution of manganese acetate tetrahydrate in acetic acid with potassium permanganate [S03]. This oxidant hydroxylates ben-zylic methylene groups [416] and forms lactones from terminal alkenes [803, 804] (see equation 88). 

Heating alkenes and cycloalkenes with manganese triacetate yields lactones resulting from an oxidative addition of acetoxyls across the double bonds (equation 88) [50J]. 

The retrosynthesis of Aara-e-viniferin (445) by Thomas et al. indicated that the y-butyrolactone (823) was a key intermediate, which later converted from the corresponding stilbene by manganese triacetate-mediated oxidative lactonization (Scheme 18) [407]. In subsequent papers, they described the FeCb-promoted tandem pericyclic synthesis of catechol analogs of restrytisol C. 3,4-Dimethoxy-12-acetoxy stilbene (824), which was synthesized by Heck coupling, was treated with FeCb to give the unnatural stilbene dimers 585 and 586 (Scheme 19) [288]. 

Manganese triacetate oxidation in acetic acid of an anisole having a... 

It is interesting that the clorination of aromatic compounds using an acetyl chloride - manganese triacetate system is significantly accelerated when performed under sonication [61],... 

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