Cycloaddition 2+2 / sigmatropic rearrangements

During 1989-93 lithium perchlorate iethyl ether (LiC104 EtiO, LP-DE) was studied as a reaction medium in organic synthesis when it was observed that cycloadditions, sigmatropic rearrangements, Michael additions and aldol condensations carried out in LP-DE occurred quickly and selectively under mild reaction conditions [33]. In addition, LP-DE allowed the reaction and subsequent work-up to be carried out under essentially neutral conditions. 

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocydic reactions and ene reactions with each other, and also with non-pericyclic transformations, allows a very rapid increase in the complexity of products. As most of the pericyclic reactions run quite well under neutral or mild Lewis acid acidic conditions, many different set-ups are possible. The majority of the published pericyclic domino reactions deals with two successive cycloadditions, mostly as [4+2]/[4+2] combinations, but there are also [2+2], [2+5], [4+3] (Nazarov), [5+2], and [6+2] cycloadditions. Although there are many examples of the combination of hetero-Diels-Alder reactions with 1,3-dipolar cycloadditions (see Section 4.1), no examples could be found of a domino all-carbon-[4+2]/[3+2] cycloaddition. Co-catalyzed [2+2+2] cycloadditions will be discussed in Chapter 6. 

Berube, G. Fallis, A. G. An intramolecular cycloaddition-sigmatropic rearrangement approach to ( ) gascardic add. Tetrahedron Lett. 1989, 30, 4045 1048. 

Many reactions involve a cyclic transition state. Of these, some involve radical or ionic intermediates and proceed by stepwise mechanisms. Pericyclic reactions are concerted, and in the transition state the redistribution of electrons occurs in a single continuous process. In this chapter, we will consider several different types of pericyclic reactions, including electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. 

Before discussing specific reactions, it is important to learn how to describe pericyclic reactions. There are four major classes of pericyclic reactions electro-cyclic reactions (ring openings or ring closings), cycloadditions, sigmatropic rearrangements, and ene reactions. 

Electrocyclic reactions Cycloadditions Sigmatropic rearrangements Ene reactions... 

Pericyclic reactions are classified into four classes. These are electrocyclic reactions, cycloadditions, sigmatropic rearrangements, and group transfer reactions. 

A review detailing computational methods for determining transition-state geometry in stereoselective cycloaddition reactions has been presented. Diels-Alder reactions, 1,3-dipolar cycloadditions, sigmatropic rearrangements, electrocyclic reactions, and ene reactions have been extensively reviewed. Pressure-induced cycloadditions to strained arenes have been reviewed. ... 

Like electrocyclic reactions and cycloadditions, sigmatropic rearrangements are controlled by orbital symmetries. There are two possible modes of reaction migration of a group across the same face of the tt system is suprafacial, and migration of a group from one face of the tt system to the other face is antara-facial (Figure 30.11). 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

Classify the following reactions as electrocyclizations, sigmatropic rearrangements, cycloadditions, etc., and give the correct symbolism for the electrons involved in each concerted process. Some of the reactions proceed by two sequential processes. 

The Diels-Alder adduct isolated from the reaction with 2,3-dimethylbuta-l, 3-diene at elevated temperatures [200] is in fact the product of a two step reaction a 1,3-cycloaddition followed by a 3,2-sigmatropic rearrangement [199] (equaPon 49)... 

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

Keywords Diels-Alder reactions, dipolar cycloadditions, electrocyclic reactions, ene reactions, pericyclic reactions, sigmatropic rearrangements... 

Chapter 6 looks at concerted pericyclic reactions, including the Diels-Alder reaction, 1,3-dipolar cycloaddition, [3,3]- and [2,3]-sigmatropic rearrangements, and thermal elimination reactions. The carbon-carbon bond-forming reactions are emphasized and the stereoselectivity of the reactions is discussed in detail. 

Finally, it must be emphasised that where, in any of the electrocyclic reactions, cycloadditions or sigmatropic rearrangements considered... 

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 cycloaddition, or a sigmatropic rearrangement. However, there are also several examples where the following reaction is not a pericyclic but rather is an aldol reaction these examples will be discussed under the term Mixed Transformations . 

In contrast to the lack of examples of the domino Diels-Alder reaction/l,3-dipolar cycloaddition, the combination of a Diels-Alder reaction with a sigmatropic rearrangement has been used intensively. 

Kanematsu s group used a combination of an intramolecular [2+2] cycloaddition of an allenyl ether 4-202 followed by a [3+3] sigmatropic rearrangement (Scheme 4.44) [70]. The substrate for the domino reaction can be obtained in situ by treat-... 

---