Domino cationic

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

One notable result is the treatment of 2-hydroxy cyclic ether analog, 2-deoxy-D-ribose with aniline in water catalyzed by InCl3 to afford the novel tricyclic tetrahydroquinoline compounds (Eq. 12.60). The reaction can also be catalyzed by recoverable cation-exchange resin instead of indium chloride.132 By using a stoichiometric amount of indium metal, a domino reaction of nitroarenes with 2,3-dihydrofuran generates the same products. 133... 

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... 

The overwhelming number of examples dealing with domino processes are those where the different steps are from the same category, such as cationic/ cationic or transition metal/transition metal-catalyzed domino processes, which we term homo domino processes . An example of the former reaction is the synthesis of progesterone (see Scheme 0.3), and for the latter the synthesis of vitamin E (Scheme 0.7). 

In most of the hitherto known cationic domino processes another cationic process follows, representing the category of the so-called homo-domino reactions. In the last step, the final carbocation is stabilized either by the elimination of a proton or by the addition of another nucleophile, furnishing the desired product. Nonetheless, a few intriguing examples have been revealed in which a succession... 

Scheme 1.1. General scheme of a cationic-cationic domino process.

In working towards the synthesis of nonracemic 3-deoxyschweinfurthin B (1-42), an analogue of the biological active schweinfurthin B, Wiemer and coworkers developed an acid-catalyzed cationic domino reaction to afford the tricyclic diol (f ,f ,i )-l-41 from 1-40 in moderate yield (Scheme 1.11) [11]. 

Scheme 1.11. Cationic domino poly-cyclization for the synthesis of 3-deoxyschweinfurthin B (1-42).

Scheme 1.13. Cationic domino process in the synthesis of pseudomonic acid C analogue.

Another interesting cationic domino process is the acid-induced ring opening of a-cyclopropyl ketones and subsequent endocyclic trapping of the formed carboca-... 

Scheme 1.18. Cationic domino rearrangement for the synthesis of tricyclic thiophenes.

Scheme 1.19. Cationic domino ring-enlargement/annulation process for the synthesis of bicyclo[3.2.0]heptanes.

Cationic olefin polycyclizations represent important transformations in the field of domino-type reactions. 

Scheme 1.24. Lewis acid-catalyzed cationic domino cyclization to give bicyclo[2.2.1]heptane derivatives.

Scheme 1.25. Cationic [4+3]-cycloaddition/nucleophilic trapping domino reaction in the synthesis of halocycloheptynes.

An unusual cationic domino transformation has been observed by Nicolaou and coworkers during their studies on the total synthesis of the natural product azadirachtin (1-105) [30]. Thus, exposure of the substrate 1-106 to sulfuric acid in CHjClj at 0°C led to the smooth production of diketone 1-109 in 80% yield (Scheme 1.27). The reaction is initiated by proto nation of the olefinic bond in 1-106, affording the tertiary carbocation 1-107, which undergoes a 1,5-hydride shift with concomitant disconnection of the oxygen bridge between the two domains of the molecule. Subsequent hydrolysis of the formed oxenium ion 1-108 yielded the diketone 1-109. 

Scheme 1.26. Cationic domino rearrangement/cyclopropanation process for the total synthesis of (+)-isovelleral (1-103).

Scheme 1.27. Cationic domino transformation towards the synthesis of azadirachtin (1-105).

Scheme 1.37. Cationic domino polycyclization in the total synthesis of aegiceradienol (1-150).

An impressive number of eight steps within one cationic domino process was observed by Mulzer and coworkers, when treating the entriol derivative 1-161 with... 

Scheme 1.40. Eight-step cationic domino process.

As discussed earlier, Ila, Junjappa and coworkers used cyclopropyl units as cation-provider in cationic domino processes. Within their interesting approach, the indole derivatives 1-170 could be converted into the unexpected carbazoles 1-171 with 54-69% yield in a five-step transformation using SnCl4 as reagent (Scheme 1.41) [48],... 

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