Diels-Alder additions 4+2 cycloadditions

Dramatic rate accelerations of [4 + 2]cycloadditions were observed in an inert, extremely polar solvent, namely in5 M solutions oflithium perchlorate in diethyl ether(s 532 g LiC104 per litre ). Diels-Alder additions requiring several days, 10—20 kbar of pressure, and/ or elevated temperatures in apolar solvents are achieved in high yields in some hours at ambient pressure and temperature in this solvent (P.A. Grieco, 1990). Also several other reactions, e.g, allylic rearrangements and Michael additions, can be drastically accelerated by this magic solvent. The diastereoselectivities of the reactions in apolar solvents and in LiClO EtjO are often different or even complementary and become thus steerable. 

Note Added in Proof. The stereochemistry of the Diels-Alder addition reactions shown in Scheme 1 to give products 20-22 has been examined by 400 MHz lH-NMR. These data indicate that the cycloadditions occur with a high degree of stereoselectivity (having diastereoselectivities of greater than 94 6). The chiral Fe environment of the dienophile 17 strongly influences the direction of the facial attack of the diene reactants. 

Hetero substituted 2-cyclopropylideneacetates are ring-strain activated acrylates, highly reactive dienophiles in Diels-Alder reactions, but also powerful Michael acceptors. The reactivity of these compounds is enhanced by the same strain release in the Diels-Alder cycloadditions as well as in the 1,4-additions, and indeed the borderline between tandem Michael-cyclization and Diels-Alder-type cycloaddition is not well defined in many cases. 

Cycloaddition.2 In combination with a triarylphosphine, 1 catalyzes the cyclization of a 1,6-enyne (2) to a 1,3-diene (3), and it effects [2+2+2] cycloaddition of 2 with an alkyne to give 4, which is isomeric with the product (5) of Diels-Alder addition of 3 with an alkyne. 

Moving on to vinylallene (2) and its derivatives as substrates, the situation becomes rapidly more complex - and more interesting. The possibility of using vinylallenes as diene components in Diels-Alder additions was recognized many years ago [5]. As shown in Scheme 5.44, the [2+ 4] cycloaddition of a generalized dienophile 283 to 2 yields 3-methylencyclohexene adducts 293. 

Strategies based on two consecutive specific reactions or the so-called "tandem methodologies" very useful for the synthesis of polycyclic compounds. Classical examples of such a strategy are the "Robinson annulation" which involves the "tandem Michael/aldol condensation" [32] and the "tandem cyclobutene electrocyclic opening/Diels-Alder addition" [33] so useful in the synthesis of steroids. To cite a few new methodologies developed more recently we may refer to the stereoselective "tandem Mannich/Michael reaction" for the synthesis of piperidine alkaloids [34], the "tandem cycloaddition/radical cyclisation" [35] which allows a quick assembly of a variety of ring systems in a completely intramolecular manner or the "tandem anionic cyclisation approach" of polycarbocyclic compounds [36]. 

A fairly general route to 1,2-dihydroazocines (64) is provided by the [2+2] cycloaddition of DMAD to 1,2-dihydropyridines (62) (74JCS(Pi)2496,77JOC2903). The reaction gives good yields of the dihydroazocine-6,7-dicarboxylates with N- alkyl, -aryl or functionally substituted removable protective groups. Other substituents can be present at C-3 or -4 in the pyridine, but carboxyl substituents at N-l or C-5 of the pyridine reduce the enamine character, and Diels-Alder addition of the acetylene occurs at the diene system. The [4.2.0] bicyclic intermediates (63) can be detected at -10 to 0°C by NMR warming to 20 °C causes complete conversion to (64). 

Cyclobutenes. This derivative of ketene undergoes [2 + 2]cycloaddition with ethyl propiolate in refluxing methylene chloride to produce the cyclobutene 1 in 65% yield. The ester group activates 1 sufficiently for Diels-Alder addition with the silyl enol ether 2 to give the I I adduct 3 under mild conditions. Hydrolysis of 3 can... 

Cycloaddition of diphenyl ketene with benzoquinone diimides (44) gives quinoxalinones of type 45.45 Analogous additions of olefins lead to tetrahydroquinoxalines these reactions are classified as Diels-Alder additions with inverse electronic demand. 

Several cycloaddition reactions of 2,5-dihydrothiophene derivatives have been reported. Compounds possessing an enamine system undergo [2 + 2] cycloaddition with acetylene-dicarboxylic ester (Scheme 215) (77AHC(2l)253). Diels-Alder addition of the 2,5-di-hydrothiophene-3-carboxylic ester (557) with butadiene, followed by desulfurization, leads to the trisubstituted cyclohexane (558) (B-74MI31404). 

We have not given you much evidence to decide why it is that some thermal [2 + 2] cycloadditions occur but not others. What is special about fluoroalkenes, allenes, and ketenes in these reactions One possibility is that Mobius rather than the Hiickel transition states are involved, but the Mobius transition states are expected to suffer from steric hindrance (Section 21-10B). It is also possible that [2 + 2] cycloadditions, unlike the Diels-Alder additions, proceed by stepwise mechanisms. This possibility is strongly supported by the fact that these reactions generally are not stereospecific. Thus with tetrafluoroethene and trans,trans-2,5-bexadiene two products are formed, which differ in that the 1-propenyl group is trans to the methyl group in one adduct, 45, and cis in the other, 46 ... 

The facially perturbed enantiopure (.S, .S )-2-(p-tolylsulfinyl)norborncno-/7-bcnzoquin-ones (119), undergo asymmetric Diels-Alder additions with cyclopentadiene to yield the four possible adducts (120) and (121). The endo-syn cycloadducts (121) can be used in the synthesis of the cage compound garudane (122) (Scheme 44).234 The antiaromatic compound 1,4-biphenylenequinone (123) has been synthesized and trapped by Diels-Alder reaction with cyclopentadiene (Scheme 45).235 The 4 + 2-cycloadditions of 4-methylene-5-(bromomethylene)-4,5-dihydrothiazole with 2- and 3-bromonaphtha-quinones are highly regiospecific.236... 

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