Oxidative domino processes

Figure 9.2 The proposed mechanism for 1,6-enyne cyclization/oxidation domino process.

As was the case in our earlier publications, the total syntheses described herein were classified according to the first step of the domino process they feature. Hence, the distinction has been made between cationic, anionic, radical, pericyclic, transition-metal-catalyzed, and reductive or oxidative domino processes. 

Formylfurans are obtained in a one-step procedure from propargyl alcohols and electron-deficient alkynes (Scheme 4-289). This domino process involves the nucleophilic addition of the propargyl alcohol to the electron-deficient alkyne forming vinyl propargyl ethers. An iron(lIl)-catalyzed cyclization, rearrangement and oxidation domino process follows to furnish the furan-2-carbaldehydes. ... 

Selective transformations Selective styrene ring opening [103] One-pot domino process for regioselective synthesis of a-carbonyl furans [104] Tandem process for synthesis of quinoxalines [105] Atmospheric oxidation of toluene [106] Cyclohexane oxidation [107] Synthesis of imines from alcohols [108] Synthesis of 2-aminodiphenylamine [109] 9H-Fluorene oxidation [110] Dehydrogenation of ethane in the presence of C02 [111] Decomposition of methane [112] Carbon monoxide oxidation [113]... 

Another beautiful example of an early domino process is the formation of daphnilactone A (0-19), as described by Heathcock and coworkers [17]. In this process the precursor 0-17 containing two hydroxymethyl groups is oxidized to give the corresponding dialdehyde, which is condensed with methylamine leading to a 2-azabutadiene. There follow a cycloaddition and an ene reaction to give the hexacycle 0-18, which is transformed into daphnilactone A (0-19) (Scheme 0.6). 

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... 

Besides the numerous examples of anionic/anionic processes, anionic/pericydic domino reactions have become increasingly important and present the second largest group of anionically induced sequences. In contrast, there are only a few examples of anionic/radical, anionic/transition metal-mediated, as well as anionic/re-ductive or anionic/oxidative domino reactions. Anionic/photochemically induced and anionic/enzyme-mediated domino sequences have not been found in the literature during the past few decades. It should be noted that, as a consequence of our definition, anionic/cationic domino processes are not listed, as already stated for cationic/anionic domino processes. Thus, these reactions would require an oxidative and reductive step, respectively, which would be discussed under oxidative or reductive processes. 

In an anionic/radical domino process an interim single-electron transfer (SET) from the intermediate of the first anionic reaction must occur. Thus, a radical is generated which can enter into subsequent reactions. Although a SET corresponds to a formal change of the oxidation state, the transformations will be treated as typical radical reactions. To date, only a few true anionic/radical domino transformations have been reported in the literature. However, some interesting examples of related one-pot procedures have been established where formation of the radical occurs after the anionic step by addition of TEMPO or Bu3SnH. A reason for the latter approach are the problems associated with the switch between anionic and radical reaction patterns, which often do not permit the presence of a radical generator until the initial anionic reaction step is finished. 

This procedure is not a domino process in its strictest definition, but since the oxidant tert-butyl hydroperoxide is added after allylation is complete, it is a very impressive and useful transformation for the rapid assembly of three contiguous stereogenic centers, including a tertiary alcohol moiety. 

Recently, Carreno, Urbano and coworkers were also able to synthesize almost enantiopure [7]helicene bisquinones 4-58 and 4-59 (96% ee) by reaction of the sulfoxide (S,S)-4-55 with the diene 4-56 in dichloromethane at -20 °C (Scheme 4.12) [19]. This six-step domino process includes a double Diels-Alder reaction, sulfoxide elimination, and aromatization of the rings B and F of the intermediate 4-57 to give 4-58, which could be oxidized to the fully aromatized 4-59. 

This chapter begins by classifying the combinations of oxidation/reduction processes with subsequent cationic transformations, though to date the details of only two examples have been published. The first example comprises an asymmetric epoxidation/ring expansion domino process of aryl-substituted cyclopropyl-idenes (e. g., 7-1) to provide chiral cyclobutanones 7-3 via 7-2, which was first described by Fukumoto and coworkers (Scheme 7.1) [2]. 

Oxidative or Reductive/Anionic/Anionic Domino Processes... 

Coumarins are known to have pronounced bioactivity. A rather new procedure for the synthesis of pyrrolo[3,2-c]coumarins uses a domino process in which an amine oxide rearrangement [61] is involved (Scheme 7.37) [62]. Reaction of the cou-... 

In 1996, the first successful combination of an enzymatic with a nonenzymatic transformation within a domino process was reported by Waldmann and coworkers [6]. These authors described a reaction in which functionalized bicy-clo[2.2.2]octenediones were produced by a tyrosinase (from Agaricus bisporus) -catalyzed oxidation of para-substituted phenols, followed by a Diels-Alder reaction with an alkene or enol ether as dienophile. Hence, treatment of phenols such as 8-1 and an electron-rich alkene 8-4 in chloroform with tyrosinase in the presence of oxygen led to the bicyclic cycloadducts 8-5 and 8-6 in moderate to good yield (Scheme 8.1). It can be assumed that, in the first step, the phenol 8-1 is hydroxylated by tyrosinase, generating the catechol intermediate 8-2, which is then again oxidized enzy-... 

In a similar manner, 2,4,6-trisubstituted 3-pyridinecarboxylic acid esters have been prepared using a mixture of a, 3-ketoesters, propargylic alcohols, ammonium acetate and Mn02. The reaction proceeds by oxidation of the propargylic alcohol to the corresponding ketone, which reacts with the enamine being formed from the like loos lor and ammonium acetate. For this domino process, normal heating at 70 °C was used. 

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