Cationic domino processes

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.

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-... 

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],... 

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. 

This is the only example of a radical/cationic domino process, which has been found in the literature. 

In the cationic domino process which is a synonym for an electrophilic reaction a carbocation is formed first, either formally or in reality, which under bond formation reacts with a nucleophile to form a new carbocation. In most of the known... 

A cationic-cationic domino process of a 3-hydro-xytetrahydro-P-carboline which leads to a stereo-seledive preparation of ( )-ulein was described by Blechert et al.1151 Although the intermediates of this fast proceeding sequence could not be identified definitely, it is acceptable to assume that the process is initiated by the formation of a 2-indolyl carbocation, which cyclizes after tautomerization to a more stable iminium ion. 

---