Hetero inverse-electron-demand

Fig. 8.11 An FMO diagram of the normal and inverse electron-demand hetero-Diels-Al-...

Inverse Electron-demand Hetero-Diels-Alder Reactions... 

The final class of reactions to be considered will be the [4 + 2]-cycloaddition reaction of nitroalkenes with alkenes which in principle can be considered as an inverse electron-demand hetero-Diels-Alder reaction. Domingo et al. have studied the influence of reactant polarity on the reaction course of this type of reactions using DFT calculation in order to understand the regio- and stereoselectivity for the reaction, and the role of Lewis acid catalysis [29]. The reaction of e.g. ni-troethene 15 with an electron-rich alkene 16 can take place in four different ways and the four different transition-state structures are depicted in Fig. 8.16. 

Various substituted unsaturated acylphosphonates participate in highly dias-tereoselective and enantioselective cycloadditions with vinyl ethers, Eqs. 177 and 178. It is intriguing to note that catalysts [(.V,.Y)-f-Bu-box]Cu (OTf)2 (269c) and [(.V,.S )-Ph-box]Cu (OTf>2 (269d) possessing the same sense of chirality afford opposite antipodes of the cycloadduct in comparable selectivities. Cyclopentadiene was found to react with acylphosphonates to give a mixture of the normal Diels-Alder adduct and the inverse electron demand hetero-Diels-Alder adduct (35 65), Eq. 179. This result may be contrasted with crotonylimide, which furnishes the normal demand Diels-Alder adduct exclusively. 

Diels-Alder Reactions The organocatalytic Diels-Alder reaction of a,P-unsaturated carbonyl compounds can be performed either via iminium (see Section 11.3) or enamine catalysis. The first highly selective enamine-promoted cycloaddition reaction was reported by Jprgensen and coworkers, who developed an amine-catalyzed inverse-electron-demand hetero-Diels-Alder (HDA) reaction (Scheme ll.lOa). ... 

Furthermore, Akiyama and coworkers applied phosphoric acid (/ )-3m (10 mol%, R = 9-anthryl) to the asymmetric inverse-electron-demand hetero-Diels-Alder reaction of A-2-hydroxyphenyl-protected aldimines 8 with vinyl ethers 99 (Scheme 38) [61], Tetrahydroquinolines 100 were obtained in good yields (59-95%), excellent yyn-diastereoselectivities (24 1-99 1), and high enantioselec-tivities (87-97% ee). 

Recently, the first examples of catalytic enantioselective preparations of chiral a-substituted allylic boronates have appeared. Cyclic dihydropyranylboronate 76 (Fig. 6) is prepared in very high enantiomeric purity by an inverse electron-demand hetero-Diels-Alder reaction between 3-boronoacrolein pinacolate (87) and ethyl vinyl ether catalyzed by chiral Cr(lll) complex 88 (Eq. 64). The resulting boronate 76 adds stereoselectively to aldehydes to give 2-hydroxyalkyl dihydropyran products 90 in a one-pot process.The diastereoselectiv-ity of the addition is explained by invoking transition structure 89. Key to this process is the fact that the possible self-allylboration between 76 and 87 does not take place at room temperature. Several applications of this three-component reaction to the synthesis of complex natural products have been described (see section on Applications to the Synthesis of Natural Products ). 

Dihydrofuran reacts with /3,7-unsaturated a-keto esters with copper or zinc complex catalysts to generate furo[2,3-/ ]pyran derivatives in good yields with high stereoselectivity. The synthesis proceeds via an inverse electron demand hetero-Diels-Alder reaction <2000CC459>. 

Hanessian and Compain have also reported a Lewis acid-promoted inverse electron demand hetero-Diels-Alder reaction between dihydrofurans and dihydropyrans with a-keto-/3,7-unsaturated phosphonates to give stmcturally related products <2002T6521>. High-pressure OTr/o-selective hetero-Diels-Alder reactions between a,/3-unsaturated aldehydes and enol ethers in the presence of lanthanide catalysts have also been reported and give 3,4-dihydro-27/-pyrans. Examples include the use of cyclic enol ethers to give 2,3,4,4a,5,8a-hexahydro-277,577-pyrano[2,3-. ]pyrans <1995T8383>. 

Alternatively, the inverse electron-demand hetero IEDARs of ,( >-unsalur-ated carbonyl compounds with enol ethers serve as a short and attractive route to dihydropyrans. For instance, the use of Lewis acids such as Eu(fod)3 along with high-pressure afforded a high yield of the dihydropyran 56, as shown in Scheme 17 [47]. Analogous and more synthetically extensive results including dihydrothiopyrans were also reported [48-52],... 

A novel formal inverse-electron-demand hetero-Diels-Alder reaction between 2-aryl-a,/3-unsaturated aldehydes and ketones produces dihydropyran derivatives stereo-specifically.161 The inverse-electron-demand Diels-Alder reaction of 3,4-r-butylthio-phene 1-oxide with electron-rich dienophiles shows vyn-jr-face and endo selectivity.162 (g) The inverse-electron-demand Diels-Alder reaction of dimethyl l,2,4,5-tetrazine-3,6-dicarboxylate with a variety of dienophiles produces phthalazine-type dihydrodiol and diol epoxides which were synthesized as possible carcinogens.163... 

Aminoindanol-derived Schiff bases were developed as tridentate ligands for the chromium-catalyzed hetero Diels-Alder reaction between weakly nucleophilic dienes and unactivated aldehydes.24 The generality of the utility of these Schiff bases, readily obtained by condensation of 1 with the corresponding aldehyde, was later demonstrated in the hetero Diels-Alder reaction between Danishefsky s diene and chiral aldehydes,25 in the inverse electron-demand hetero Diels-Alder reaction of a,P-unsaturated aldehydes with alkyl vinyl ethers,26 and in hetero-ene reactions.27... 

Chromium complex 53 was also shown to efficiently catalyze the inverse electron-demand hetero Diels-Alder reaction of a,(3-unsaturated aldehydes with alkyl vinyl ethers (Scheme 17.19).26 Although the uncatalyzed process required elevated temperatures and pressures to give dihydropyrans in good yields but poor endo. exo selectivities, the reaction proceeded at room temperature in the presence of 5 mol% of ent-53 and 4A molecular sieves in dichloromethane of tert-butyl methyl ether with excellent diastereoselectivity (endo. exo >96 4) and promising enantioselectivities (72-78% ee). Optimal results were achieved using a solvent-free system and excess vinyl ether. 

Crystal structure of 60 revealed that the complex existed as a dimeric species where the two chromium centers are bridged through the indane oxygen and each chromium metal bears one molecule of water (Figure 17.6). It was therefore proposed, as for the inverse electron-demand hetero Diels-Alder reaction, that the barium oxide desiccant removed one molecule of bound water from the catalyst dimer, which opened one coordination site for binding of the substrate carbonyl.26... 

The inverse electron demand hetero Diels-Alder reaction of 1-oxa-l,3-butadienes and electron-rich dienophiles is an extremely versatile tool in natural product synthesis. This cycloaddition represents the key step of numerous approaches not only to carbohydrates, but also to terpenes, alkaloids, polyethers, steroid derivatives and various biologically active metabolites. 

Narsaiah and Krishnaiah (2002) reported that the hetero-Diels-Alder reaction of A(-acetyl perflnoroalkyl substituted 2(1//) pyridones with DMAD on neutral alumina under solvent-free microwave irradiation conditions extended to undergo an inverse electron demand hetero-Diels-Alder reaction, resulting exclusively in E, Z isomers (3 1) of Michael-type A-adducts. 

Krishnaiah, A. and Narsaiah, B. 2002. Studies on inverse electron demand hetero Diels-Alder reaction of perfluoroalkyl 2(1H) pyridones with different dienophiles under microwave irradiation. Journal of Fluorine Chemistry, 113 133-37. 

Inverse electron demand hetero-Diels-Alder reactions of acyl phosphonates or a-keto ester heterodienes and enol ethers are also catalyzed by (5, iS )-t-Bu-box complexes. High levels of enantioselectivity are obtained with y-alkyl-, -aryl-, -alkoxy-... 

A four-component [4 + 2]/[4 + 2]/[3 + 2] cycloaddition reaction, reported by Scheeren et is another application of multicomponent reactions. The interaction of 2-methoxy-l,3-butadiene (92) (1 equiv.) with p-nitrostyrene (81a) (3 equiv.) at 1.5 GPa led to a mixture of nitroso-acetals 93-95. In this reaction, p-nitrostyrene (81a) first reacts as a dienophile in the normal electron-demand Diels-Alder reaction, it then reacts as a heterodiene in the inverse electron-demand hetero-Diels-Alder reaction and, finally, it acts as a dipolarophile in the [3 + 2] cycloaddition (Scheme 7.23). 

Evans et al. reported that in the presence of Cz-symmetric bis(oxazoline)—Cu(II) complex 438 the inverse electron demand hetero-Diels—Alder reaction of the a,/Tunsaturated carbonyl compounds 436 with ethyl vinyl ether gave the chiral dihydropyrans 437 in high diastereo- and enantioselectivities (Scheme 139).199c... 

A strategy developed by Tietze and coworkers early in his independent career involved the application of intramolecular or intermolecular inverse electron demand hetero-Diels-Alder reactions to iridoid total synthesis. The intermolecular [4+ 2]-cycloaddition of ethyl vinyl ether and unsaturated aldehyde 31 provided acetal 32, which underwent double bond isomerization to afford 33 (Scheme 1) An intramolecular variant of this reaction is discussed in detail later (Scheme 10). More recently, Jacobsen and Chavez extended this work with the enantio- and diastereoselective synthesis of a range of iridoid natural products. Utilizing tridentate Cr(III) catalyst 34, acetal 35 was prepared in 98% ee with good diastereoselectivity (Scheme 1)P... 

The Boger pyridine synthesis involves an inverse-electron demand hetero-Diels-Alder [4 + 2] reaction of the electron-deficient 1,2,4-triazine... 

Pyrrolidine has been the common organocatalyst used by Boger and coworkers for the synthesis of highly substituted pyridines based on the inverse-electron-demand Diels-Alder reaction of in situ formed enamines and 1,2,4-triazines. Barbas and coworkers have described the amine-catalysed direct self-Diels-Alder reaction of a,p-unsaturated ketones. The inverse-electron-demand hetero-Diels-Alder reaction of nitrosoalkenes and in situ formed enamines as dienophiles was also shown to be possible. ... 

Swindell CS, Tao M (1993) Chiral Auxiliary-Mediated Asymmetric Induction in a Thermal Inverse Electron Demand Hetero-Diels-Alder Reaction - Enantioselective Synthesis of the Taxol A-Ring Side Chain. J Org Chem 58 5889... 

Tandem inverse electron-demand hetero-Zretro-Diels-Alder reactions for aromatic nitrogen heterocycle synthesis 13CSR63. 

J0rgensen and Juhl reported the first organocatalytic enantioselective inverse-electron-demand hetero-Diels-Alder reaction of aldehydes (e.g., 71) and enones (e.g., 72) with excellent diastereo- and enantioselectivity. Scheme 3.26 [41], The reaction utilizes a chiral enamine intermediate as an alkene in catalytic asymmetric cycloaddition reactions. 

Scheme 3.26 Enantioselective inverse-electron-demand Hetero-Diels-Alder reaction of aldehyde and enone...

Scheme 23.13 Inverse-electron-demand hetero-Diels-Alder reaction reported by Feng and coworkers.

Dixon and co-workers [182] published an enantioselective aryloxylation of aldehydes using an inverse-electron-demand hetero-Diels-Alder reaction of in s/tM-generated enamines and o-quinones. After trying different proline-derived catalysts, imidazolidinone 120 gave the best enantioselectivities (Scheme 12.34). 

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