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Cycloaddition molecules Review Review IndolizinesIndolizines andand TheirTheir Hetero/BenzoHetero/Benzo DerivativesDerivatives inin ReactionsReactions ofof [8+2][8+2] Cycloaddition EugeneEugene V.V. BabaevBabaev1,2,3, 1,2,3,** andand Ivan Ivan A. A.Shadrin Shadrin 1 1 11 ChemistryChemistry Department, Department, Moscow Moscow State State University, University, Leninskie Leninskie gory, Gory, 1 1 str.3, Str. 3, 119899 119899 Moscow, Moscow, Russia; Russia; [email protected]@gmail.com 22 HigherHigher School School of of Economics, Economics, National National Research Research University, University, 7 7 Vavilova Vavilova Str., 117312 Moscow, RussiaRussia 33 N.N. D. D. Zelinsky Zelinsky Institute Institute of of Or Organicganic Chemistry, Chemistry, Russian Russian Academy Academy of of Sciences, Sciences, 47 Leninsky Ave., 119991119991 Moscow, Moscow, Russia Russia ** Correspondence:Correspondence: [email protected]; [email protected]; Tel.: Tel.: +7-985-997-94-75 +7-985-997-94-75 Abstract: PeculiaritiesPeculiarities ofof [8+2][8+2] cycloaddition of acetylenesacetylenes toto indolizinesindolizines areare reviewed.reviewed. EspeciallyEspecially mentioned are are indolizines indolizines with with leaving leaving groups groups at positions at positions 3 and 3 and 5. Cycloaddition 5. Cycloaddition to aza- to and aza- benzo and benzoderivatives derivatives are reviewed, are reviewed, as well as 1,10-cyclizations well as 1,10-cyclizations and processe ands processes leading to leading cyclazines to cyclazines where in- wheredolizines indolizines are intermediates. are intermediates. Mechanistic Mechanistic features features(adducts (adducts and cycloadducts) and cycloadducts) and theoretical and theoretical aspects aspects(one- or (one- two-steps or two-steps mechanism) mechanism) are reviewed. are reviewed. Keywords: indolizine; azaindolizines; benzoindolizines;benzoindolizines; cyclazine; [8+2] cycloaddition; mechanism; 1,10-cyclizations;1,10-cyclizations; catalystscatalysts 1. Introduction Citation: Babaev, E.V.; Shadrin, I.A. Indolizine (A, SchemeScheme1 1)) isis the the simplest simplest heteroaromatic heteroaromatic molecule molecule containing containing both both a a Indolizines and Their Hetero/Benzo π-excessive-excessive pyrrolepyrrole andand aa ππ-deficient-deficient pyridinepyridine ringring withwith onlyonly oneone bridgeheadbridgehead nitrogen, Derivatives in Reactions of [8+2] the wholewhole systemsystem beingbeing isomericisomeric withwith indoleindole andand possesspossess pharmaceutical,pharmaceutical, agrochemicalagrochemical Cycloaddition. Molecules 2021, 26, Citation: Babaev, E.V.; Shadrin, I.A. andand fluorescentfluorescent properties [1]. [1]. Although Although indolizine indolizine is iscertainly certainly aromatic, aromatic, significant significant alter- al- 2050. https://doi.org/10.3390/ Indolizines and their Hetero/Benzo ternationsnations of the of the bond bond lengths lengths around around the thering ring system system were were detected detected by X-ray, by X-ray, NMR NMR and and UV molecules26072050 Derivatives in Reactions of [8+2] UVspectroscopy spectroscopy and and even even mass mass spectrometry spectrometry in in various various substituted indolizines.indolizines. ThisThis Cycloaddition. Molecules 2021, 26, x. promptsprompts somesome tetraene-like tetraene-like character character of of the the compound, compound, in particularin particular its abilityits ability to enter to enter into Academic Editor: Ionel Mangalagiu https://doi.org/10.3390/xxxxx cycloadditioninto cycloaddition reactions. reactions. Received: 17 March 2021 Academic Editor: Ionel Mangalagiu R R' Accepted: 30 March 2021 Received: 17 March 2021 Published: 3 April 2021 Accepted: 30 March 2021 N Published: 3 April 2021 8 H Publisher’s Note: MDPI stays neutral 1 H R' C1 withPublisher’s regard toNote: jurisdictional MDPI stays claims neu- in R N publishedtral with mapsregard and to institutional jurisdictional affil- N iations. 3 claims in published maps and institu- 5 A tional affiliations. N R R' H B H Copyright: © 2021 by the authors. R R' C2 R R' LicenseeCopyright: MDPI, © 2021 Basel, by the Switzerland. authors. ThisSubmitted article for is anposs openible accessopen access article SchemeScheme 1.1. IndolizinesIndolizines ((A)A) inin [8+2][8+2] cycloadditioncycloaddition reactionreaction formingforming cycl[3.2.2]azinescycl[3.2.2]azines ( B(B).). The The reaction reac- tion may proceed with alkynes or alkenes via dihydro- (C1) or tetrahydro- (C2) cyclazines. distributedpublication under under the the terms terms and con- and may proceed with alkynes or alkenes via dihydro- (C1) or tetrahydro- (C2) cyclazines. conditionsditions of the of theCreative Creative Commons Commons At- Attributiontribution (CC (CC BY) BY) license license (http://crea- (https:// IndolizineIndolizine isis usuallyusually regarded regarded as as the theπ -excessiveπ-excessive heterocycle heterocycle with with the the highest highest electron elec- creativecommons.org/licenses/by/tivecommons.org/licenses/by/4.0/). populationtron population of the of carbonthe carbon atom atom C-3, C-3, and and the the major major part part of of the the chemistry chemistry of of indolizinesindolizines 4.0/). is simple electrophilic addition and substitution at this position. Cycloaddition of various Molecules 2021, 26, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molecules Molecules 2021, 26, 2050. https://doi.org/10.3390/molecules26072050 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 2050 2 of 34 Molecules 2021, 26, x FOR PEER REVIEW 2 of 34 Molecules 2021, 26, x FOR PEER REVIEW 2 of 34 is simple electrophilic addition and substitution at this position. Cycloaddition of var- iousdienophiles dienophiles (alkenes (alkenes and andacetylenes) acetylenes) to toindolizines indolizines leading leading to to derivatives derivatives of thethe cycl[3.2.2]azinecycl[3.2.2]azinedienophiles (alkenes ((BB,, SchemeScheme and1 )1)acetylenes) is is well-known. well-known. to indolizines TheThe mechanismmechanism leading ofof thesetothese derivatives reactionsreactions isofis fre- fre-the quentlyquentlycycl[3.2.2]azine regardedregarded (B asas, Scheme aa rarerare example example1) is well-known. ofof [8+2][8+2] cycloaddition,cycloaddition, The mechanism wherewhere of these thethe tetraene tetraenereactions carboncarbon is fre- frameworkframeworkquently regarded ofof thethe indolizineindolizine as a rare bicyclebicycleexample playsplays of [8+2thethe role]role cycloaddition, ofof anan 88 ππ-electron-electron where fragment.fragment. the tetraene InIn general,general, carbon thisthisframework processprocess may ofmay the be be indolizine either either one-step one-step bicycle (concerted) (concertedplays the orrole) or involve ofinvolve an 8 zwitterionic π zwitterionic-electron fragment. (and (and even even In biradical) general, biradi- intermediates,cal)this intermediates, process may and be thereand either there is yet one-step is no yet experimental no (concerted experimental evidence) or evidenceinvolve for zwitterionic the for naturethe nature of (and the of process.theeven process. biradi- cal) Cyclazineintermediates,Cyclazine ((BB)) isisand anan there interestinginteresting is yet no 1212 πexperimeπ-electronic-electronicntal systemsystem evidence thatthat for breaksbreaks the nature thethe canonscanons of the process. ofof aro-aro- maticity.maticity.Cyclazine According (B) is to to an X-ray X-ray interesting data data the the 12 structuresπ structures-electronic B1B1 andsystemand B2B2 are thatare not breaks not correct correct the (Scheme canons (Scheme 2),of and2aro-), andthematicity. structure the structure According rather rather has to X-ray hasa peripheral a data peripheral the delocalization structures delocalization B1 andof aromatic ofB2 aromatic are not 10 correct π 10-electronπ-electron (Scheme system system 2), andB3. B3Therefore,the. Therefore, structure cyclazine rather cyclazine hasresembles resemblesa peripheral the thefamous delocalization famous spinning spinning of toy aromatic toy B4B4 wherewhere 10 πthe-electron the handle handle system (which (which B3 is. isnotTherefore, not rotated) rotated) cyclazine corresponds corresponds resembles to nitrogen to nitrogen the famouslone lone pair. spinning pair. Hence, Hence, thetoy structure B4 the where structure has the a hassymmetryhandle a symmetry (which plane, is plane,andnot rotated)this and influences this corresponds influences the number to the nitrogen number of positional lone of positional pair. isomers, Hence, isomers, saythe structurethe say number the has number of a symmetryaza-of and aza- benzo- plane, and benzo-derivativesderivativesand this influences possible possible thefor numbercyclazine for cyclazine of (Scheme positional (Scheme 3). isomers,3). say the number of aza- and benzo- derivatives possible for cyclazine (Scheme 3). SchemeScheme 2.2. DifferentDifferent imagesimages ofof thethe structurestructure ofof cyclazine.cyclazine. Scheme 2. Different images of the structure of cyclazine. N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N Scheme 3. Possible benzo- aza- and benzoazacyclazines discussed in this review. SchemeScheme 3. 3.Possible Possible benzo- benzo- aza- aza- and and benzoazacyclazines benzoazacyclazines discussed discussed in in this this review. review. Cyclazins and their hetero/benzo derivatives are important from a practical view- point.CyclazinsCyclazins They are and fluorescentand their their hetero/benzo hetero/benzo
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