[...]... identify both the number of p electrons involved in the electronic rearrangement and the number of atoms originating the unsaturated six-membered ring The subscript s indicates that the reaction takes place suprafacially on both components There are other [p 4s ‡p 2s ] reactions, and therefore it is the term Diels Alder which specifies this particular type of reaction The Diels Alder reaction can be... state [16] (the formation of one s bond proceeds in advance of the other) have been suggested, and the pathway of the reaction depends on the nature of the reagents and the experimental conditions [17] O + O O O H O O O O H …1:7† O Most Diels Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17] The reaction between... rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels Alder reaction 1.4 IONIC AND RADICAL DIELS ALDER REACTIONS Conjugated cations, anions and radicals can give the Diels Alder reaction In such a case, the two s bonds are formed in two separate steps (stepwise 6 Ionic and Radical Diels Alder Reactions... have appeared in the last decade The classical Diels Alder reaction is a cycloaddition between a conjugated diene and a second component, called dienophile, which has at least a p bond (Equation 1.1) When one or more heteroatoms are present in the diene and/or dienophile framework, the cycloaddition is called a hetero -Diels Alder reaction …1:1† Diels Alder Reaction: General Remarks 3 The reaction is classified... bond They are more numerous and more variegated than dienes [1] Typical dienophiles are illustrated in Table 1.2 The simplest dienophile, ethene, is poorly reactive Electron-withdrawing and electron-donating groups, on the carbon atom double bond, activate the double bond in normal and inverse electron-demand Diels Alder reactions, respectively 1.3 PERICYCLIC DIELS ALDER REACTION The Diels Alder reaction. .. Domino Diels Alder reactions [9] Ph O + H 100 O Ph ЊC N Cl H O H O N H O O Ph Ph Ph Ph Scheme 1.2 The main purpose of this chapter is to introduce the various aspects of the Diels Alder cycloaddition and the terminology employed Since its discovery in 1928 [10], more than 17 000 papers have been published concerning synthetic, mechanistic and theoretical aspects of the reaction and about half of these... cycloaddition in sequence with another Diels Alder reaction or with other reactions without isolating the intermediates (domino, tandem, cascade, consecutive, etc., reactions) [4±6] Scheme 1.2 illustrates some examples Domino Knoevenagel hetero -Diels Alder reactions [7] O t-Bu O CHO + O N N Ph t -Bu EDDA, MeCN Rfx, 16 h, 94% H t -Bu H N O O N N N O Ph Ph Cascade Diels Alder acylation reactions [8] OSMDBT OSMDBT... …1:9† The first evidence of an anionic Diels Alder reaction was given by Rickborn [25a] The reaction of anthrone with N-methylmaleimide in CHCl3 or THF occurs with low yield [26] (Equation 1.10), while in DMF or in the presence of catalytic amounts of amine (Et3 N, Py) the reaction is completed in a few minutes [25] 8 Ionic and Radical Diels Alder Reactions The cycloaddition is ascribable to the oxyanion... regioisomer will be the 1-thiophenyl-2-methoxy-4-acetyl-cyclohexen-1-ene because the SPh is the regiodirector group In fact, this regioisomer is the main reaction product (80 %) [39] Exceptions to the ortho±para rule have been observed, so the prediction of the regiochemistry is still a stimulating challenge The regioselectivity of simple Diels Alder reactions has been explained on the basis of the electronic... electronic effects of the substituents which orient the attack of reagent species by generating partial positive and negative charges in the diene and dienophile Generally, the more powerful the electronic effect of the substituents, the more regioselective the reaction Although this explanation has some merit, the FMO theory [41] and the matching of complementary reactivity surfaces of the diene and dienophile . Microwave-Assisted Diels Alder Reaction 158 4.4 Photo-Induced Diels Alder Reaction 163 4.5 Diels Alder Reaction in Molecular Cavities 170 4.6 Micelle-Promoted Diels Alder Reaction 174 4.6.1 Diels Alder Reactions. Hetero -Diels Alder Reactions 66 2.3.1 Heterodienes 66 2.3.2 Heterodienophiles 70 2.4 Intramolecular Diels Alder Reaction 74 2.5 Outlined Diels Alder Reactions 83 References 92 The Diels Alder Reaction: . Outlined Diels Alder Reactions 237 References 246 6 Diels Alder Reaction in Unconventional Reaction Media 251 6.1 Diels Alder Reaction in Aqueous Medium 251 6.1.1 Uncatalyzed Diels Alder Reaction