Pericydic domino processes

Oxidative or Reductive/Pericyclic/Pericydic Domino Processes... 

Barriault developed a new pericydic domino process for the synthesis of the bioactive diterpenoid vinigrol (4-262), which was isolated from Virgaria nigra [92], The natural product possesses antihypertensive and anti-platelet-aggregating properties. 4-262 contains a unique tricyclo [4.4.4.04a8a]tetradecane framework, which could be obtained by a combination of an oxy-Cope, a Claisen and an ene reaction... 

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. 

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. 

The most common combination of a Heck reaction in a domino process with a non-Pd-catalyzed transformation is that with a pericydic reaction, especially a Diels-Alder cycloaddition. This is reasonable, since a Heck reaction allows an easy formation of a 1,3-butadiene. 

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-... 

The connection of radical and pericydic transformations in one and the same reaction sequence seems to be on the fringe within the field of domino processes. Here, we describe two examples, both of which are highly interesting from a mechanistic viewpoint. The first example addresses the synthesis of dihydroindene 3-326 by Parsons and coworkers, starting from the fiiran 3-321 (Scheme 3.79) [128]. Reaction of 3-321 with tributyltin hydride and AIBN in refluxing toluene led to the 1,3,5-hexatriene 3-324 via the radicals 3-322 and 3-323. 3-324 then underwent an elec-trocyclization to yield the hexadiene 3-325 which, under the reaction conditions, aromatized to afford 3-326 in 51 % yield. 

Since the number of domino processes which start with a Diels-Alder reaction is rather large, we have subdivided this section of the chapter according to the second step, which might be a second Diels-Alder reaction, a 1,3-dipolar cydoaddition, or a sigmatropic rearrangement. However, there are also several examples where the following reaction is not a pericydic but rather is an aldol reaction these examples will be discussed under the term Mixed Transformations . 

Domino transition metal-catalyzed processes can also start with a cross-coupling reaction most often, Suzuki, Stille and Sonogashira reactions are used in this context They can be combined with another Pd-catalyzed transformation, and a number of examples have also been reported where a pericydic reaction, usually a Diels-Alder reaction, follows. An interesting combination is also a Pd-catalyzed borina-tion followed by a Suzuki reaction. 

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... 

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