Intramolecular [4 + 2] cycloaddition reaction of six- and seven-membered cyclic N-allyl-C-arylethynyl iminium salts

Intramolecular cyclization reactions of arylpropargyl amides of electron-deficient α,β-alkenyl carboxylates and related compounds

Intramolecular cyclization reactions of arylpropargyl amides of electron-deficient α,β-alkenyl carboxylates such as fumarates and ethenetricarboxylates were investigated. The reaction of the fumaramides with a base, Et₃N or DBU in xylenes at 140 °C under air gave benz[f]isoindoline derivatives in 21-63% yields. The benz[f]isoindolines may be produced via the formation of an allenic intermediate, intramolecular Diels-Alder reaction, proton transfer, and dehydrogenation by oxygen. The suitable bases and the product yields depend on the substituents on the benzene ring. On the other hand, the reaction of the amides of fumarate and ethenetricarboxylate by heating in DMSO gave aroyl-substituted pyrrolidine derivatives as major products, probably via addition of water under metal-free conditions. Furthermore, cyclization reactions of H and Me substituted alkyne derivatives were investigated for comparison. The selective formation of various types of products, such as ethyl 2-(1-benzyl-4-formyl-2-oxopyrrolidin-3-yl)acetate and diethyl 2-(1-benzyl-2-oxo-4-vinylidenepyrrolidin-3-yl)malonate, was found, depending upon the alkyne substituents and the reaction conditions. The reaction mechanisms have been discussed using density functional theory (DFT) calculations.

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

Iminium-functionalized 1,2,3-triazoles by [3+2] cycloaddition reactions of internal acetylenic iminium triflates with organoazides

Propyne iminium triflates 1–6 react as dipolarophiles in thermal [3 + 2]-cycloadditions with sufficiently electron rich organoazides to form 1,4,5-trisubstituted 1,2,3-triazoles with iminium functionalization. The reactions require rather strong thermal activation, but can be accelerated by microwave irradiation. The regioselectivity of the cycloaddition at the internal acetylenic bond of 3-cyclopropylpropyne and 3-arylpropyne iminium ions (1–3 and 4, respectively) is very high, but is lowered in the presence of sterically demanding substituents at the opposite end of the iminium-substituted C,C triple bond. The iminium-functionalized triazoles can easily be transformed into neutral compounds; herein reported is the formation of triazolyl ketones 10 by hydrolysis and of tertiary triazolyldimethyl amines 12 by LiAlH4 reduction. When the reduction is performed with sodium boranate or sodium cyanoboranate, amine–borane complexes 15 and 16 are obtained.

Synthesis of a stable iminium salt and propellane derivatives

The treatment of morphinan 1 with NaH and MsCl provided very stable iminium salt 7 possessing propellane skeleton. One of the synthesized iminium salts 7, isobutyl derivative 7b, was crystallized and its structure was determined by X-ray crystallography. The natural bond orbital analysis suggested that the stability of the iminium should result from the stereoelectronic effect (hyperconjugation) attributed to their own structures.