Anionic-pericydic process

Anionic/Pericydic Processes Followed by Further Transformations... 

An example for the combination of mechanistically different reactions is the anionic-pericydic process such as the domino-Knoevenagd-hetero-Diels-Alder reaction. In the inter-intramolecular version of this process an aldehyde 1 containing a dienophile moiety and a 1,3-dicarbonyl compound 2 can be mixed together to give unusual heterocycles of diversified structures such as 4 via the intermediate formation of an 1-oxa-l,3-butadiene 3 (scheme 1).[41... 

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 this process the primary step is the formation of an anion, which is a synonym for a nucleophile, mostly by deprotonation using a base. It follows a reaction with an electrophile to give a new anion which in the anionic-anionic process again reacts with an electrophile The reaction is then completed either by addition of another electrophile as a proton or by elimination of an X group. Besides the anionic-anionic process there are several examples of anionic-pericydic domino reactions as the domino-Knoevenagel-hetero-Diels-Alder reaction in which after the first step an 1-oxa-l,3-butadiene is formed. 

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

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 pericydic 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 pericydic and transition metal-catalyzed reactions, respectively, the second number corresponds to the frequency of the different processes. 

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