Polyenes cycloaddition reactions

This chapter deals with [2 + 2], [4 + 2], [6 + 4], [8 + 2] and [2 + 2 + 2] cycloaddition reactions of dienes and polyenes. Most attention is devoted to the [4 + 2] cycloaddition reaction (Diels-Alder reaction) which is likely to be the most studied reaction in the chemical literature. 

The reactions of dienes and other polyenes can be broadly classified as either addition reactions, coupling (or substitution reactions) or rearrangements (including metathesis reactions). This chapter will present recent examples from the literature of synthetic transformations involving polyenes. Cycloaddition and ring closing metathesis reactions appeared in volume one of this series and therefore will not be covered in this chapter. Citations for more detailed descriptions of the individual reactions discussed in this chapter and for more comprehensive reviews appear in the text. 

In a cycloaddition reaction, the two active n systems may approach each other in either of two orientations, for example, head to head or head to tail. If one combination dominates, the reaction is said to be regioselective. In the course of the reaction, 4 new saturated centers are formed. With maximum labeling, a total of 16 (=24) stereo-isomeric forms, consisting of 8 enantiomeric pairs of diastereomers if neither polyene is chiral, may be formed. In pericyclic reactions, the stereochemistry is determined by specifying the stereochemical mode in which each component reacts. Each of the two... 

There is a way around the An+ 2 rule that is not very important for substances analogous to benzene, but. is quite important for cycloaddition reactions. Let us see how this works for a cyclic conjugated polyene. 

Besides the common oleftnic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acyclic electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cyclic polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

Although the [2 + 2] photocycloaddition is preferably prevented in the synthesis of PAHs, it has been proven to be a very feasible approach in the selective synthesis of syn-[2.2]cyclophanes [86,87]. Topological reaction control in solution has been achieved for a multistep intramolecular [2 + 2] cycloaddition reaction in the photochemical formation of [n]-ladderanes from pseudo-gem-bis(polyene) substituted [2.2]paracyclophanes [88]. The probably most well-known example of an intramolecular photocycloaddition process is one of the... 

Scheme 10.1. Possible Combinations for Cycloaddition Reactions of Conjugated Polyenes...

In addition to functioning as the polyene component in Diels-Alder reactions of 1//-azepines, a dienophilic character is demonstrated by the ease with which they also function as the dienophile in inverse cycloaddition reactions. In the reaction of ethoxy 1//-azepine-1-carboxylate with 3,4,5,6-tetrachloro-l,2-benzoquinone, both [2- -4]jt and [6- -4]jt adducts are formed in a hetero Diels-... 

Reviews have appeared on the use of the Wittig reaction in industrial practice, the Claisen rearrangement, synthetic applications of the retro-Diels-Alder reaction, organo-palladium intermediates for the alkylation and arylation of olefins, the Prins reaction to give 1,3-dienes, and intramolecular [4 + 2] (Diels-Alder) and [3 + 2] cycloadditions.An interesting discussion of the regiospecificity of the Diels-Alder reaction in terms of frontier orbital overlap favours the Woodward-Katz concept. Useful alkyne and polyene coupling reactions are described in reviews on the chemistry of vitamin the synthesis of insect sex... 

For a review of photochemical cycloaddition reactions of conjugated dienes and polyenes, see Dilling, W. A. Chem. Rev. 1969, 69, 845. 

This chapter deals with the photochemistry of alkenes, alkynes, dienes, polyenes, and related compounds through a choice of the literature published during the period January 2010 — December 2011. Furthermore, recently many researchers are developing the photochemistry of these compounds for energy conversion, e.g. through nanotechnology applications, such as molecular devices, chemomechanics, molecular switches, etc. This chapter also covers the nanotechnology aspects that are based upon the utilization of isomerization/electrocyclization/cycloaddition reactions of the title compounds. 

Dilling, W. L. "Photochemical Cycloaddition Reactions of Nonaromatic Conjugated Hydrocarbon Dienes and Polyenes." Chem. Rev., 69,845 (1969). 

It has been proposed that these complex structures arise from various electro-cyclisation and cycloaddition reactions of linear polyene side-chains, undergone post-pyrone formation. For example, Trauner pointed out the wide variety of skeletal types that can be theoretically obtained from pericyclic transformations of a relatively simple E,Z,Z,E)-tQttait Q 21, further observing that isomerisations of the alkene geometries lead to an even greater number of potential cyclisation products (Scheme 1.4) [14],... 

Scheme 34)P Diels-Alder cycloaddition reactions of conjugated polyenes to tetracyclic ring-fused, facially dissymmetric maleic anhydrides occur exclusively on... 

Let us now examine tlie properties of the next higher cyclic polyene analog of benzene, 1,3,5,7-cyclooctatetraene, another 4n tt cycle (n = 2). Is it antiaromatic, like 1,3-cyclobutadiene First prepared in 1911 by Willstatter, this substance is now readily available from a special reaction, the nickel-catalyzed cyclotetramerization of ethyne. It is a yellow liquid (b.p. 152 °C) that is stable if kept cold but that polymerizes when heated. It is oxidized by air, eatalytically hydrogenated to cyclooctane, and snbject to electrophilic additions and to cycloaddition reactions. This chemical reactivity is diagnostic of a normal polyene (Section 14-7). 

The terpenes a- and 6-myrcene are classical examples of substrates controlled by factor (ii). In these polyenes the relative reactivity of the nonconjugated C=C double bond (di- versus trisubstituted) controls the first reaction event. 6-Myrcene, a monoterpene triene with a trisubstituted nonconjugated double bond, undergoes preferentially the ene reaction with subsequent [4 + 2]-cycloaddition,whereas a-myrcene 57 solely reacts in a [4 + 2]-mode to give the endoperoxide 58. Alkoxy-substituted 1,3-dienes, which cannot undergo ene reaction, have been shown to be excellent substrates for [2 + 2]-cycloaddi-tion. The l,4-di-t-butoxy-l,3-butadienes were the first substrates that showed nonstereoselective [2 + 2]-cycloaddition reactions. Formation of the endoperoxide 60 was observed only from the ,Z-isomer 59, which indicates a pronounced activation effect by the alkoxy groups for dioxetane formation. 

The polyene character of 1 /7-azcpines makes them susceptible not only to a variety of electro-cyclic reactions, but also to cycloaddition with a variety of dienophiles, and to dimerization by [6 + 4] 7i-pericyclic reactions. 

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