Radical cyclizations manganese acetate

Keywords Cascade sequences Indole Manganese(III) acetate Radical cyclizations Radical reactions Tributyltin hydride Xanthates... 

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]. 

Among the oxidants that have been used to generate radicals, manganese (HI) acetate has emerged as a powerful reagent to mediate radical cyclizations.147 The manganese(III) acetate-mediated oxidation of enolizable carbonyl compounds is one of the best methods available for the cyclization of electrophilic radicals. The substrates are vety easily prepared by standard alkylation and acylation reactions. Radicals are formed with high selectivity by oxidation of acidic C—H bonds, and, because the reaction is an oxi-... 

Radical Cyclization of ) -Halo Allylic Acetal with a Crignard Reagent in the Presence of Manganese(ll) Chloride or Iron(ll) Chloride... 

Manganese(III)-mediated radical reactions have become a valuable method for the formation of carbon-carbon bonds over the past thirty years since the oxidative addition of acetic acid (1) to alkenes to give y-butyrolactones 6 (Scheme 1) was first reported by Heiba and Dessau [1] and Bush and Finkbeiner [2] in 1968. This method differs from most radical reactions in that it is carried out under oxidative, rather than reductive, conditions leading to more highly functionalized products from simple precursors. Mn(III)-based oxidative free-radical cyclizations have been extensively developed since they were first reported in 1984-1985 [3-5] and extended to tandem, triple and quadruple cyclizations. Since these additions and cyclizations have been exhaustively reviewed recently [6-11], this chapter will present an overview with an emphasis on the recent literature. 

Oxidative radical cyclization sequences have also been used to generate 1,2-fused indoles. Treatment of amides 152 and 154 with dimethyl methylmalonate in the presence of manganese(III) acetate and sodium acetate in acetic acid, gave the expected cyclized product in 63% and 40%, respectively [97]. The proposed mechanistic sequence involves the intermolecular addition of the dimethyl methylmalonate radical to the tethered exocyclic alkene followed by cyclization and finally rearomatization. Byers and coworkers also achieved a similar cyclization on the C-2 position of the indole when a 3-acylindole was subjected to these oxidative cyclization conditions. 

Snider BB, Merritt JE, Dombroski MA, Buckman, BO. Solvent effects on manganese(III)-based oxidative free-radical cyclizations ethanol and acetic acid. J. Org. Chem. 1991 56 (19) 5544-5553. 

The manganese(UI) acetate mediated oxidative cyclization of fl-ketoesters has been utilized to construct a pentacyclic compound in low yield [95CC403]. The intermediate radical 107 formed from an initial 6-endo-trig reaction undergoes further lactonization in the presence of copper(II) acetate. The compound 108 has the basic skeleton found in fungal metabolite sesquiterpene phenols. 

Despite the development of various intermolecular radical addition methods, those studies have rarely accommodated additional functionality, our discovery of the manganese-mediated photolysis conditions notwithstanding. Prior to that discovery, we began to elaborate an alternative strategy which employs temporary tethers ([115, 116] reviews of silicon-tethered reactions [117-120]) (silyl ether or acetal linkages) linking radical and acceptor. In this scenario the C-C bond is constructed via cyclization, in which internal conformational constraints can control diaster-eoselectivity. The tether itself would be converted to useful functionality upon cleavage, and once the tether is cleaved the net result may be considered as formal acyclic stereocontrol. ... 

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. 

Reactions of 2,4-pentanedione or ethyl acetoacetate with ethene in the presence of manganese(III) acetate and copper(II) acetate in an autoclave under 50 atm at 60 °C give a mixture of 443 and 444 (equation 150). This reaction involves an oxidative 1,3-cyclization of alkyl radicals . 

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