Summary of cycloaddition reactions

Give mechanisms for these reactions, explaining the stereo- 

Predict the structure of the product of this Diels-Alder reaction. 

Comment on the difference in rate between these two reactions. It is estimated that the second goes about 10fi times faster than the first. 

Justify the stereoselectivity in this intramolecular Diels-Alder reaction. 

Explain the formation of single adducts in these reactions. 

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In summary, the cycloaddition reactions of 1,2-dihydropyridines have proven to be very useful in the total synthesis of natural products. The primary reasons for their utility are that they have electron-rich reactive 7r-systems, they contain a heterocyclic nitrogen atom for alkaloid synthesis, and the stereochemistry is controlled in their cycloaddition reactions. 

In the following we discuss a few of these cycloadditions. A summary of these reactions by Sauer and Sustmann is soon to be published. 

Originally, it was planned to close this chapter with a section on 1,5-dipolar cycloadditions — mainly stimulated by Huisgen s review from 1980. As we could find only few (although interesting) papers on that subject, we add a short summary of such reactions to this section. ... 

In the [2+4] pencyclic cycloaddition reaction known as the Diels-Alder reaction, fluonne-containing compounds have been widely used as dienes, dieno-philes, or both Much of the fundamental work, including many comprehensive and systematic studies, was done before 1972, and Hudlicky provides an exeellent summary of this work [9] Additional sources for early work in this area are reviews in Organic Reactions [61] and Fluorine Chemistry Reviews [62]... 

Summary N-Silylation is a crucial prerequisite for the synthesis of interesting nonnatural amino acids via cycloaddition reactions of unsaturated amines. 

Summary The formation, reactivity, and cycloaddition behavior of neopentylsilenes towards suitable reaction partners is described. Especially l,l-dichloro-2-neopentylsilene. Cl2Si=CHCH2Bu (2) - easily obtained from vinyltrichlorosilane and LiBu - is a useful building block for the synthesis of SiC four membered ring compounds. These can be converted into the isomeric Diels-Alder and retro ene products upon thermolysis reactions. The mode of the silenes cycloaddition reactions ([4+2] vs [2+2] addition) can be directed by either the substitution pattern at the Si=C moiety, the choice of reaction partners or the conditions. Furthermore the products resulting from cycloaddition reactions open up a wide variety of following reactions, which possibly will lead to new organosilicon materials or pharmaceutical compounds. 

In summary, asymmetric cycloadditions are powerful methods for the synthesis of complex chiral molecules because multiple asymmetric centers can be constructed in one-step transformations. Among them, reactions using chiral catalysts are the most effective and promising, and fruitful results have been reported in asymmetric Diels-Alder reactions. 

The elfectiveness of imidazolidinone of type 11 was confirmed by successful application to a broad range of chemical transformations, including cycloadditions, conjugate additions, Friedel-Crafts alkylations, Mukaiyama-Michael additions, hydrogenations, cyclopropanations, and epoxidations. A summary of these enantio-selective iminium catalyzed processes is provided by reaction subclass. 

The article is organized as follows. In the next Section we present a brief outline of the theoretical background for the present work. Section 3 contains summaries of the SC models for the electronic mechanisms of the gas-phase Diels-Alder reaction between butadiene andethene [11] and the 1,3-dipolar cycloaddition of fulminic acid to ethyne [12]. In Section 4 we provide, for the first time, a description of the SC model for the electronic mechanism of the gas-phase disrotatory electrocyclic ring-opening of cyclohexadiene. Conclusions and final comments are presented in Section 5. 

Theoretical work on the gas-phase hetero-Diels-Alder reaction of A -sulfinyl dienophiles was used to study both endo- and o-modes of cycloaddition for both (E)-29 and (Z)-30 dienophiles at the B3LYP/6-31G level (Scheme 2) <2000JOC3997>. In summary, these calculations have predicted that (1) the A -sulfinyl dienophiles prefer the (Z)-30 orientation over (E)-29 stereochemistry by 5-7 kcalmoP, (2) the transition state is concerted but nonsynchronous, and (3) an lYo-transition state with diene 31 is favored over the fvo-approach both kinetically and thermodynamically. 

In summary then we see that Kochi s work provides a direct experimental link between reactivity and reactant excited state energies. While at the present time the only excited state that has been correlated in this way is the charge transfer state, the CM model predicts that similar correlations for additional excited states are likely to exist. The significance of Kochi s work in relation to cycloaddition reactions is discussed on p. 176. 

This chapter begins with an introduction to the basic principles that are required to apply radical reactions in synthesis, with references to more detailed treatments. After a discussion of the effect of substituents on the rates of radical addition reactions, a new method to notate radical reactions in retrosynthetic analysis will be introduced. A summary of synthetically useful radical addition reactions will then follow. Emphasis will be placed on how the selection of an available method, either chain or non-chain, may affect the outcome of an addition reaction. The addition reactions of carbon radicals to multiple bonds and aromatic rings will be the major focus of the presentation, with a shorter section on the addition reactions of heteroatom-centered radicals. Intramolecular addition reactions, that is radical cyclizations, will be covered in the following chapter with a similar organizational pattern. This second chapter will also cover the use of sequential radical reactions. Reactions of diradicals (and related reactive intermediates) will not be discussed in either chapter. Photochemical [2 + 2] cycloadditions are covered in Volume 5, Chapter 3.1 and diyl cycloadditions are covered in Volume 5, Chapter 3.1. Related functional group transformations of radicals (that do not involve ir-bond additions) are treated in Volume 8, Chapter 4.2. 

In summary, the construction of phosphetane or arsetane rings is usually carried out by McBride synthesis or alkylation-cyclization methods. These methods are quite useful, with a good selectivity in most cases. [2+2] Cycloaddition reactions are also available, but this method is limited to particular cases. In addition to these methods, other useful methods based on the transformation from another ring system have been developed. These methodologies are described in Section 2.10.10. 

In summary, it is clear that silver possesses unique abilities to activate both olefinic species and small strained rings for [2+2] cycloaddition reactions. Nevertheless, reports of silver-mediated [2+2] cycloadditions remain rare. 

Pyridines and their benzo-derivatives have played an important role in the synthesis of biologically active synthetic and natural substances. As a result, the construction of this molecular architecture has attracted the attention of a diverse array of synthetic methodologies. Notably, transition metal catalysis, radical reactions and cycloaddition chemistry-based methods have been developed for the construction of this important ring system. Detailed herein is a summary of the methods developed for the synthesis of pyridines, quinolines, isoquinolines and piperidines that were disclosed in the literature in 2002. Rather than survey all existing methods for the construction of these compound classes, this review will serve as a supplement and update to the review published last year in this series. 

Summary Cl2Si=CHCH2/Bu is a versatile building block in organosilicon synthesis which reacts with a wide variety of multiple bond systems. The unique reactivity is shown by the predominance of [2+2] over [4+2] cycloaddition reactions. This unusual behaviour is caused by the two chlorine substituents at the silicon atom. This presentation shows the synthesis of silacyclobutenes from Cl2Si=CHCH2tBu and disubstituted alkynes and some chemistry of these four membered ring compotmds. 

Summary The reaction between in situ formed l,l-dichloro-2-neopentylsilene, Cl2Si=CHCH2tBu (3), and 1,1-dimethylpentaftilvene (8) leads to the formation of exo/endo-isomeric [4+2] cycloadducts 9 and [2+2] stereoisomers 10 in good yields. 2D-NMR spectroscopic investigations on the product mixture prove die different mode of the silene cycloaddition reactions ([4+2] vs [2+2] addition). 

This chapter reviews thermal [2 + 2] cycloadditions of alkenes to form cyclobutanes, extending the coverage provided by earlier summaries of the literature.It docs not deal with [2 + 2] cycloadditions that are achieved photochemically, or that utilize allenes, arynes or ketenes, or that are deliberately catalyzed, or that result in heterocyclic four-membered rings, even though these other varieties of [2 + 2] cycloaddition reactions are important and of substantial interest. 

These woikers have also examined asymmetric induction in the process in some detail. For instance, cycloaddition of sugar aldehyde (167) occurred to afford only adduct (169) at high pressure (equation 80). It was suggested that the Diels-Alder reaction proceeds via diene attack on the aldehyde conformation shown in (168) from the least congested face. Other chiral aldehydes have been investigated by this group, as has the e ect of lanthanide catalysts upon the extent of asymmetric induction. Summaries of this work have recently been published. ... 

A summary of most of the available routes to cyclobutenes and cyclobutenones is given in equations (4H7). These tqiproaches can be divided into two major strategies, (a) Cycloaddition between an alkyne and an alkene to give directly a cyclobutene. In the case where the alkene partner is a ketene, cyclobutenones are obtained (see Section 6.1.5). (b) Introduction of a double bond into a preformed cyclobutane derivative, typically via an elimination reaction. 

Summary The reaction of trichlorosilane with DABCO provides the anion SiCb, which can be used instead of SiCh in cycloaddition reactions. In the presence of the 1,4-diaza-l,3-butadienes la-g, the five membered rings 2a-g are found. 2b was characterized by an X-ray structure determination. 

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