Diels–Alder Reactions of 2‐Vinylindoles with Open‐Chain C=C Dienophiles

Abstract

The synthesis of diastereoisomeric 3,4‐disubstituted 1,2,3,4‐tetrahydrocarbazoles by Diels–Alder cycloaddition reactions between [(E)‐2‐vinyl]indole‐1‐carboxylic acid ethyl esters and open‐chain C=C dienophiles is reported and discussed. The reactions proceed with high regioselectivity whereas the diastereoselectivity ranges from moderate to excellent depending on the substitution pattern on both the dienes and dienophiles and on the catalyst employed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Catalytic Functionalization of Indoles in a New Dimension

“…In general, anellation reactions under conventional heating are slow (days);197 thus, acid catalysis is routinely adopted to speed up the reaction. Rossi and co‐workers recently used this approach in the synthesis of functionalized tetrahydrocarbazoles 198. Catalytic amounts of Mg(ClO 4 ) 2 , Sc(OTf) 3 , or Cu(OTf) 2 efficiently assisted the [4+2] cycloadditions between a number of 2‐vinylindoles and electron‐poor alkenes 199…”

Cycloaddition versus Alkylation Reactions of 2‐Vinylindoles with α,β‐Unsaturated Carbonyl Compounds Under Gold Catalysis

“…In all of the experiments, endo ‐tetrahydrocarbazole (±)‐ 3a was the main reaction product, and the Michael‐type adduct was neither isolated nor observed by 1 H NMR analysis of the crude reaction mixture. Our previously reported results with Mg(ClO 4 ) 2 11a are included in Table 1, Entry 1, and the results of the evaluation of scandium triflate (OTf) and boron trifluoride–diethyl ether, which were selected as the best catalysts for the DA reactions of 2‐vinylindoles with cyclic C=C dienophiles,11b are listed in Table 1, Entries 2 and 3. In our model reaction, excellent yields were obtained with both catalysts, but better diastereoselectivity resulted when the reaction was performed in toluene at –20 °C with boron trifluoride–diethyl ether.…”

“…(±)‐( trans )‐Ethyl 3‐Acetyl‐2‐( p ‐tolyl)‐3,4‐dihydro‐1 H ‐carbazole‐9(2 H )‐carboxylate (3′a): 11a White solid; m.p. 165.9–166.2 °C.…”

“…(±)‐( trans )‐Ethyl 3‐Formyl‐4‐phenyl‐2‐( p ‐tolyl)‐3,4‐dihydro‐1 H ‐carbazole‐9(2 H )‐carboxylate (3′j): 11a Yellow oil. IR (neat): $\tilde {\nu}$ = 2906, 2895, 2786, 1732 cm –1 .…”

Katalytische Funktionalisierung von Indolen in einer neuen Dimension

“…Weil Anellierungen bei bloßem Erhitzen im Allgemeinen nur langsam ablaufen (Reaktionszeiten im Bereich von Tagen),197 werden sie unter Säurekatalyse ausgeführt, um die Reaktionsgeschwindigkeit zu erhöhen. Rossi und Mitarbeiter nutzten dies kürzlich bei der Synthese von funktionalisierten Tetrahydrocarbazolen,198 indem sie katalytische Mengen von Mg(ClO 4 ) 2 , Sc(OTf) 3 oder Cu(OTf) 2 für die [4+2]‐Cycloaddition zwischen verschiedenen 2‐Vinylindolen und elektronenarmen Alkenen einsetzten 199…”