Domino anionic-pericyclic processes

As expected, some sequences also occur where a domino anionic/pericyclic process is followed by another bond-forming reaction. An example of this is an anionic/per-icyclic/anionic sequence such as the domino iminium ion formation/aza-Cope/ imino aldol (Mannich) process, which has often been used in organic synthesis, especially to construct the pyrrolidine framework. The group of Brummond [450] has recently used this approach to synthesize the core structure 2-885 of the immunosuppressant FR 901483 (2-886) [451] (Scheme 2.197). The process is most likely initiated by the acid-catalyzed formation of the iminium ion 2-882. There follows an aza-Cope rearrangement to produce 2-883, which cyclizes under formation of the aldehyde 2-884. As this compound is rather unstable, it was transformed into the stable acetal 2-885. The proposed intermediate 2-880 is quite unusual as it does not obey Bredf s rule. Recently, this approach was used successfully for a formal total synthesis of FR 901483 2-886 [452]. 

Another anionic pericyclic domino process is a Pummerer-type rearrangement/... 

Several other anionic/pericyclic domino processes use a Horner-Wadsworth-Emmons (HWE) or a Wittig olefmation as the first step. 

An elegant approach to polyquinanes has recently been described by Paquette et aL1281 By means of an anionic-pericyclic domino reaction four new C-C bonds are formed in one process. First, a two-fold addition of a vinyl lithium reagent, e.g. cyclopentyl lithium to diisopropyl squarate takes... 

An interesting pericyclic-anionic-pericyclic domino reaction showing a high stereoselectivity is the cycloaddition-aldol-retro-ene process depicted in scheme 20.1581 The procedure presumably starts with a [4+2]-cycloaddition of diene 98 and S02 in presence of a Lewis acid. After opening of the formed adduct reaction with (Z)-silyl vinyl ether 99 leads to a mixture of alk-2-enesulfinic acids 101. It follows a retro-ene reaction which affords a 7 3 mixture of the products 102 and 103. The reaction described by Vogel et al is a nice example for the efficient generation of polypropionate chains with the stereoselective formation of three stereogenic centers and one (0-double bond in a three-component domino reaction in its strict definition. 

Besides the numerous examples of anionic/anionic processes, anionic/pericyclic 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. 

Domino radical reactions consisting of radical/cationic, radical/anionic, radi-cal/radical, and radical/pericyclic processes have become increasingly important... 

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 morefold anionic domino processes with one pericyclic reaction, domino sequences combining two initiating anionic with two pericyclic steps have also been developed. For example, the group of Nesi and Turchi reported on the synthe-... 

Abstract In this chapter different types of domino-processes are described which consist of the combination of cationic, anionic, radical, pericyclic and transition metal-catalyzed as well other reactions. The methodology is used for the highly effective synthesis of carbocycles and heterocycles as well as of natural products and other interesting materials. It is also employed as an efficient tool in combinatorial chemistry. 

There are, however, also many examples of mixed domino processes , such as the synthesis of daphnilactone (see Scheme 0.6), where two anionic processes are followed by two pericyclic reactions. As can be seen from the information in Table 0.1, by counting only two steps we have 64 categories, yet by including a further step the number increases to 512. However, many of these categories are not - or only scarcely - occupied. Therefore, only the first number of the different chapter correlates with our mechanistic classification. The second number only corresponds to a consecutive numbering to avoid empty chapters. Thus, for example in Chapters 4 and 6, which describe pericyclic and transition metal-catalyzed reactions, respectively, the second number corresponds to the frequency of the different processes. 

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. 

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