Sequential Intramolecular Diels–Alder Reaction of 3‐Heteroaryl‐2‐propenylamides of Ethenetricarboxylate

Competing Domino Knoevenagel-Cyclization Sequences with N-Arylcinnamylamines

Domino Knoevenagel-cyclization reactions of N-arylcinnamylamines were carried out with active methylene reagents, which took place with five competing cyclization mechanisms: intramolecular hetero Diels-Alder reaction, stepwise polar [2 + 2] cycloaddition, styryl or aza-Diels-Alder reactions followed by rearomatization, and [1,5]-hydride shift-6-endo cyclization. In the stepwise aza-Diels-Alder reaction, the N-vinylpyridinium moiety acted as an azadiene, producing a condensed heterocycle with tetrahydroquinolizinium and tetrahydroquiniline subunits. Antiproliferative activity with low micromolar IC50 values was identified for some of the novel scaffolds.

IntraMolecular Diels–Alder Reactions of Vinylarenes and Alkynyl Arenes (the IMDAV Reaction)

This comprehensive review summarizes the published literature data concerning the intramolecular Diels–Alder reactions of vinylarenes (the IMDAV reaction) and alkynyl arenes from 1970 to 2019, and covers mainly intramolecular [4+2] cycloaddition reactions of vinyl- or acetylene-substituted furans, thiophenes, pyrroles, indoles, imidazoles, benzenes, and naphthalenes, in which the unsaturated substituent is linked directly to an arene moiety. The selected area of the Diels–Alder reaction differs from other forms of [4+2] cycloadditions due to the uniqueness of the diene fragment, which, along with an exocyclic multiple bond, includes the double bond of an aromatic or heteroaromatic nucleus in its system. Thus, during the formation of the [4+2] cycloaddition intermediate, the aromaticity of furan, thiophene and even benzene rings is broken, leading, as a rule, to the formation of heterocyclic structures rarely accessible by other methods, in contrast to the majority of intermolecular Diels–Alder reactions, with the highest degree of chemo-, regio-, and diastereoselectivity. Therefore, the IMDAV approach is often used for the synthesis of naturally occurring and bioactive molecules, which are also discussed in this review alongside other applications of this reaction. Whenever possible, we have tried to avoid examples of radical, photochemical, oxidative, precious-metal-complex-catalyzed cyclizations and other types of formal [4+2] cycloadditions, focusing on thermal Diels–Alder reactions in the first step, according to the classical mechanism. The second stage of the process, aromatization, is unique for many initial substrates, and hence considerable attention in this overview is given to the detailed description of the reaction mechanisms.

1 Introduction

2 IMDAV Reactions of Vinylfurans

2.1 Alkenes as Internal Dienophiles

2.2 Alkynes and Allenes as Internal Dienophiles

3 IMDAV Reactions of Vinylthiophenes

3.1 Alkenes as Internal Dienophiles

3.2 Alkynes as Internal Dienophiles

4 IMDAV Reactions of Vinylbenzothiophenes

5 IMDAV Reactions of Vinylpyrroles

6 IMDAV Reactions of Vinylindoles

6.1 Alkenes as Internal Dienophiles

6.2 Alkynes as Internal Dienophiles

7 IMDAV Reactions of Styrenes and Vinylnaphthalenes

7.1 Alkenes as Internal Dienophiles

7.2 Alkynes as Internal Dienophiles

7.3 Alkynes as Internal Dienophiles in Aryl Acetylenes (the Intramolecular Dehydro Diels–Alder Reaction)

8 IMDAV Reactions of Vinylimidazoles, Vinylisoxazoles and Vinylpyridines

9 Conclusion

10 Abbreviations

Synthesis of functionalized tetrahydropyrans via cascade cycloaddition involving silyloxyallyl cation intermediates

An expedient synthesis of highly substituted tetrahydrobenzofuran via an unsymmetrical silyloxyallyl cation is reported.