Involving the Generation of Two Bonds 5.1. Introduction 5.2. Formation of the NÀC 2 and C 3 ÀC 4 Bonds 5.2.1. DielsÀAlder Related Reactions 5.2.2. Tandem Michael AdditionÀCyclization Sequence 5.2.3. Miscellaneous Reactions 5.3. Formation of the NÀC 2 and C 4 ÀC 4a Bonds 5.3.1. Michael Addition Initiated Reactions 5.3.2. Miscellaneous Reactions 5.4. Formation of the NÀC 2 and C 2 ÀC 3 Bonds 5.5. Formation of the C 2 ÀC 3 and C 3 ÀC 4 Bonds 5.6. Formation of the C 2 ÀC 3 and C 4 ÀC 4a Bonds 5.7. Formation of the C 3 ÀC 4 and C 4 ÀC 4a Bonds 5.8. Formation of the C 8a ÀN and C 4 ÀC 4a Bonds 5.9. Formation of the NÀC 2 and C 8a ÀN Bonds 6. Synthesis of 1,2,3,4-Tetrahydroquinolines Involving the Generation of Three or More Bonds 6.1. Formation of the NÀC 2 , C 2 ÀC 3 , and C 4 ÀC 4a Bonds: The Povarov and Related Reactions

The CAN-catalyzed three-component between reaction between primary amines, beta-dicarbonyl compounds and naphthoquinones or 2-bromonaphthoquinones afforded, respectively, 5-hydroxybenzo[g]indoles and benzo[f]indole-4,9-diones, the former of which were transformed into tetracyclic azepino[1,2-a]benzo[g]indole systems through a gamma-alkylation/ring-closing metathesis sequence.

A catalytic, atom-economical, domino 5-endo-dig cyclization-intramolecular olefin insertion sequence was developed under mild conditions. Aryl alkynoic acids bearing a tethered enone partner afforded the indeno[1,2-b]furan-2-ones, the core skeleton present in a number of biologically significant molecules including the natural product solanacol, under ligand-free, palladium-catalyzed reaction conditions in high yields. The competitive β-hydride elimination in the final step leading to the conjugated analogs was avoided by the addition of lithium bromide. A plausible mechanism for this domino sequence is proposed involving intramolecular carboxypalladation and olefin insertion steps.

The cerium(IV) ammonium nitrate (CAN)-catalyzed sequential multicomponent reaction between tryptamine, α,β-unsaturated aldehydes, and β-dicarbonyl compounds affords highly substituted indolo[2,3-a]quinolizines in a single synthetic operation. Two rings are generated through the creation of two C-C and two C-N bonds by a domino process comprising initial β-enaminone formation, followed by individual Michael addition, 6-exo-trig cyclization, iminium formation, and Pictet-Spengler steps. Furthermore, the reaction is diastereoselective and affords exclusively compounds with a trans relationship between the H-2 and H-12b protons. The use of amines bearing a less nucleophilic side chain aromatic ring (5-bromotryptamine, 3,4-dimethoxyphenylethylamine) prevents the Pictet-Spengler final step and leads to N-indolylethyl or N-phenylethyl-1,4-dihydropyridines, which are cyclized to the corresponding indolo[2,3-a]quinolizines or benzo[a]quinolizines in the presence of HCl in methanol/water. Treatment of the fused quinolizine derivatives with sodium triacetoxyborohydride led to the corresponding indolo[2,3-a]quinolizidines or benzo[a]quinolizidines, possessing four stereogenic centers, as mixtures of two diastereomers.

The cerium(IV) ammonium nitrate (CAN)-catalyzed, three-component reaction between primary amines, β-dicarbonyl compounds, and α,β-unsaturated aldehydes in ethanol heated to reflux, constitutes a general, one-pot synthesis of 1,4-dihydropyridines. Their reduction with sodium triacetoxyborohydride furnished piperidine derivatives bearing up to five substituents with full diastereoselectivity in a hitherto inaccessible stereochemical arrangement. The reaction proceeded with no significant loss of enantiomeric purity under mild reduction conditions that are compatible with several functional groups that are normally sensitive to reduction. Octahydroquinolin-5-one derivatives, which were prepared by a modified version of the initial multicomponent reaction, were not suitable substrates for the sodium triacetoxyborohydride mediated reduction, but they were transformed into the corresponding decahydroquinolines, including a precursor of the amphibian alkaloid pumiliotoxin C, by catalytic hydrogenation under a variety of conditions.

Starting from aryl alkynoic acids bearing an enone tether, conditions for a 5‐endo‐dig cyclization‐intramolecular olefin insertion cascade are developed.

A β-Enaminone-Initiated Multicomponent Domino Reaction for the Synthesis of Indoloquinolizines and Benzoquinolizines from Acyclic Precursors. -(SURYAVANSHI, P. A.; SRIDHARAN, V.; MENENDEZ*, J. C.; Chem. -Eur. J. 19 (2013) 39, 13207-13215, http://dx.

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