Diels-Alder reaction 3- indoles

Keywords [4+2] Cycloaddition Diels-Alder reaction Indole... 

Keywords Indoles, aromatic amines, oxetanyl aldehydes, asymmetric catalyst A, diethyl ether, room temperature, asymmetric multicomponent reaction, aza-Diels-Alder reaction, indole-alkaloid-type polycycles, diastereoseltivity... 

Benzilic acid rearrangement Benzoin reaction (condensation) Blanc chloromethylation reaction Bouveault-Blanc reduction Bucherer hydantoin synthesis Bucherer reaction Cannizzaro reaction Claisen aldoi condensation Claisen condensation Claisen-Schmidt reaction. Clemmensen reduction Darzens glycidic ester condensation Diazoamino-aminoazo rearrangement Dieckmann reaction Diels-Alder reaction Doebner reaction Erlenmeyer azlactone synthesis Fischer indole synthesis Fischer-Speior esterification Friedel-Crafts reaction... 

Indoles are usually constructed from aromatic nitrogen compounds by formation of the pyrrole ring as has been the case for all of the synthetic methods discussed in the preceding chapters. Recently, methods for construction of the carbocyclic ring from pyrrole derivatives have received more attention. Scheme 8.1 illustrates some of the potential disconnections. In paths a and b, the syntheses involve construction of a mono-substituted pyrrole with a substituent at C2 or C3 which is capable of cyclization, usually by electrophilic substitution. Paths c and d involve Diels-Alder reactions of 2- or 3-vinyl-pyrroles. While such reactions lead to tetrahydro or dihydroindoles (the latter from acetylenic dienophiles) the adducts can be readily aromatized. Path e represents a category Iley cyclization based on 2 -I- 4 cycloadditions of pyrrole-2,3-quinodimcthane intermediates. 

Two types of cycloaddition reactions have found application for the Synthetic elaboration of indoles. One is Diels-Alder reactions of 2- and 3-vinylindoles which yield partially hydrogenated carbazoles. The second is cycloaddition reactions of 2,3-indolequinodimethane intermediates which also construct the carbazole framework. These reactions arc discussed in the following sections. 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

When methyl 2-(indol-2-yl)acrylate derivative (22a) reacted with A-methoxy-carbonyl-l,2-dihydropyridine (8a) in refluxing toluene, in addition to the dimer of 22a (25%), a mixture of the expected isoquinculidine 23a and the product 24a (two isomers) was obtained in 7% and 45% yields, respectively (81CC37). The formation of 24a indicates the involvement of the 3,4-double bond of dihydropyridine. Similarly, Diels-Alder reaction of methyl l-methyl-2-(indol-2-yl)acrylate (22b) with 8a gave, in addition to dimer of 22b, a mixture of adducts 23b and 24b. However, in this case, product 23b was obtained as a major product in a 3 2 mixture of two isomers (with a- and (3-COOMe). The major isomer shows an a-conhguration. The yields of the dimer, 23b, and 24b were 25%, 30%, and 6%, respectively. Thus, a substituent on the nitrogen atom or at the 3-position of indole favors the formation of the isoquinuclidine adduct 23. 

The Diels-Alder reactions of the methyl or ethyl ester of benzenesulfonylindole-2-acrylic acid with several l-alkoxycarbonyl-l,2-dihydropyridines are reported and only a single stereoisomer was obtained, as in the case of l-methoxy(ethoxy)-carbonyl-1,2-dihydropyridines. However, when the Diels-Alder reaction of 17 was carried out with 8g[R = (CHsjsC], a mixture of two stereoisomers 18gand25were obtained in a 1 1 ratio (65% total yield). The bulky rerr-butyl group creates sufficient steric interference with the indole ring to cause the loss of stereochemistry ... 

Phenylsulfonyl)indole 330 was converted to a ketone by a set of standard reactions followed by the selenium dioxide oxidation of the resulting acetyl goup to the ketoaldehyde 332 (Scheme 101). Methylthiosemicarbazide hydroiodide reacted with 332 to the triazine 333 in 83% yield. As Diels-Alder reactions with 1 -pyrrolidinocyclohexene failed, 333 was first oxidized... 

Vinyl- and acetylenic tricarbonyl compounds are reactive dienophilic components in Diels-Alder reactions. Cycloadditions of these compounds with substituted butadienes were recently used to develop a new synthetic approach to indole derivatives [14] (Scheme 2.9) by a three-step procedure including (i) condensation with primary amines, (ii) dehydration and (iii) DDQ oxidation. 

Pyrano-[4,2-b]-pyrrol-5-ones (40) and pyrano-[4,3-b]-pyrrol-6-ones (41) (Figure 2.4) are stable cyclic analogs of pyrrole 2,3-quinodimethane and undergo Diels Alder reaction [40, 41] with various dienophiles to afford indole derivatives after loss of carbon dioxide. 

Indole-2,3-quinodimethanes [44] 44 are bicyclic outer-ring dienes that are widely used to prepare a variety of heterocyclic polycyclic compounds. These dienes, generated by extrusion of CO2 from lactones, are then trapped by dienophiles. Some examples of Diels Alder reactions of the dienes 44 are reported in Scheme 2.19. 

Indole is a weak dienophile in normal Diels-Alder reactions and must be activated by electron-withdrawing substituents at C-2 and C-3. High... 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

Table 4.13 Photoinduced Diels-Alder reactions of indoles 64 and dienes 61-63 with sensitizers 59 and 65...

Pindur U., Lemster T. Recent Advances in the Synthesis of Carbazoles and Anellated Indoles With Antitumor Activity DNA-Binding Ligands and Protein Kinase C Inhibitors Recent Res. Dev. Org. Bioorg. Chem. 1997 1 33-54 Keywords Diels-Alder reactions of a 4,7-dioxo-indole with carbodienophiles... 

Merour J. Y., Piroelle S., Joseph B. Synthesis and Reactivity of lH-Indol-3(2H)-One and Related Compounds Trends Heterocycl. Chem. 1997 J 115-126 Keywords inverse electron-demand Diels-Alder reaction, indolone... 

The inverse electron demand Diels-Alder reaction has also been used to provide expedient access to unnatural 6-carboline alkaloids from 1,2,4-triazines, prepared by microwave-assisted MCR [92]. One-pot reaction of an acyl hydrazide-tethered indole 73, 1,2-diketone and ammonium acetate in acetic acid provided triazines 74 (see Sect. 3.2, Scheme 22), bearing an electron-rich dienophilic indole moiety (Scheme 31). By carrying out the... 

The inverse electron demand Diels-Alder reaction of 3-substituted indoles with 1,2,4-triazines and 1,2,4,5-tetrazines proceeds in excellent yields both inter- and intramolecularly. The cycloaddition of tryptophan 124 with a tethered 1,2,4-triazine produced a diastereomerically pure cycloadduct 125 <96TL5061>. 

Waldmann used (R) and (5>aminoacid methyl esters and chiral amines as chiral auxiliaries in analogous aza-Diels-Alder reactions with cyclodienes.111 The diastereoselectivity of these reactions ranged from moderate to excellent and the open-chain dienes reacted similarly. Recently, the aza-Diels-Alder reaction was used by Waldmann in the asymmetric synthesis of highly functionalized tetracyclic indole derivatives (Eq. 12.45), which is useful for the synthesis of yohimbine- and reserpine-type alkaloids.112... 

It has been known that aromatic heterocycles such as furan, thiophene, and pyrrole undergo Diels-Alder reactions despite their aromaticity and hence expected inertness. Furans have been especially used efficiently as dienes due to their electron-rich properties. Thiophenes and pyrroles are less reactive as dienes than furans. But pyrroles with A-elecIron-withdrawing substituents are efficient dienes. There exists a limited number of examples of five-membered, aromatic heterocycles acting as dienophiles in Diels-Alder reactions. Some nitro heteroaromatics serve as dienophiles in the Diels-Alder reactions. Heating a mixture of l-(phenylsulfonyl)-3-nitropyrrole and isoprene at 175 °C followed by oxidation results in the formation of indoles (see Eq. 8.22).35a A-Tosyl-3-nitroindole undergoes high-yielding Diels-Alder reactions with... 

In contrast to the failure of Diels-Alder reactions, dipolar cycloadditions of indoles are much more successful, and the Boger group has reported a fascinating [4+2]/l,3-dipolar cycloaddition cascade involving indole as the dipolarophile in their impressive synthesis of vindoline (Scheme 4.8) [26]. After the initial... 

Diels-Alder reaction of the 1,3,4-oxadiazole with the pendant olefin and loss of N2, the C2-C3 7t bond participates in a subsequent 1,3-dipolar cycloaddition with the carbonyl ylide to generate complex polycycles such as 45 as single diastereomers with up to six new stereocenters. That the cascade reaction is initiated by a Diels-Alder reaction with the alkene rather than with the indole is supported by the lack of reaction even under forcing conditions with substrate 46, in which a Diels-Alder reaction with the indole C2-C3 n bond would be required [26a]. 

In 2002, Leadbeater and Torenius reported the base-catalyzed Michael addition of methyl acrylate to imidazole using ionic liquid-doped toluene as a reaction medium (Scheme 6.133 a) [190], A 75% product yield was obtained after 5 min of microwave irradiation at 200 °C employing equimolar amounts of Michael acceptor/donor and triethylamine base. As for the Diels-Alder reaction studied by the same group (see Scheme 6.91), l-(2-propyl)-3-methylimidazolium hexafluorophosphate (pmimPF6) was the ionic liquid utilized (see Table 4.3). Related microwave-promoted Michael additions studied by Jennings and coworkers involving indoles as heterocyclic amines are shown in Schemes 6.133 b [230] and 6.133 c [268], Here, either lithium bis(trimethylsilyl)amide (LiHMDS) or potassium tert-butoxide (KOtBu) was em-... 

Some other ring expansions involving the intramolecular amino Claisen rearrangement of vinylarylaziridine [ 123], the Diels-Alder reaction of indoles with acetylene derivative [124-127] and the dibromocarbene insertion into quinoline enol ethers [ 128] have been used to prepare 1-benzazepines. On the other hand, treatment of 3-chloro-3-phenyl-l,2,3,4,5,6-hexahydro-l-benz-azocin-2-ones with piperidine causes a ring contraction to give 2-phenyl-2-(l-piperidinylcarbonyl)-2,3,4,5-tetrahydro-l//-l-bcnzazepines in an excellent yield [23]. 

In carbocyclic chemistry, rather firm dividing lines usually exist between aromatic, non-aromatic, and anti-aromatic compounds, while in heterocyclic chemistry enormous variations in the extent of aromatic character are displayed.52 Furthermore, there is an enormous number of potential heterocycles as compared to carbocycles, as will be detailed in section 3 of this review. The degree of aromaticity has classically been judged qualitatively in connection with the diene character of heterocycles manifested in Diels— Alder reactions or polymerizations. In this regard for instance, furan (42) is less aromatic than benzene (43), as is isoindole (44) compared to indole (45) (Scheme 18). Therefore, a quantitative aromaticity scale would be useful. 

An intramolecular Diels-Alder reaction of allenic dienamide 181 provided the tet-rahydroindole ring system 182, which was oxidized with DDQ or Mn02 to give indole derivatives [147]. 

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