Aromatic ring current

Electric current observed in aromatic compounds

This article is about chemistry and physics of aromatic compounds. For the meteorological effect, see Ring current.

A diagram of an aromatic ring current. B0 is the applied magnetic field, the red arrow indicating its direction. The orange ring shows the direction of the ring current, and the purple rings show the direction of the induced magnetic field.

An aromatic ring current is an effect observed in aromatic molecules such as benzene and naphthalene. If a magnetic field is directed perpendicular to the plane of the aromatic system, a ring current is induced in the delocalized π electrons of the aromatic ring.[1] This is a direct consequence of Ampère's law; since the electrons involved are free to circulate, rather than being localized in bonds as they would be in most non-aromatic molecules, they respond much more strongly to the magnetic field.

The ring current creates its own magnetic field. Outside the ring, this field is in the same direction as the externally applied magnetic field; inside the ring, the field counteracts the externally applied field. As a result, the net magnetic field outside the ring is greater than the externally applied field alone, and is less inside the ring.

## Relevance to NMR spectroscopy

[edit]

Aromatic ring currents are relevant to NMR spectroscopy, as they dramatically influence the chemical shifts of 1H nuclei ("protons") in aromatic molecules.[2] The effect helps distinguish these nuclear environments and is therefore of great use in molecular structure determination. In benzene, the ring protons experience deshielding because the induced magnetic field has the same direction outside the ring as the external field and their chemical shift is 7.3 parts per million (ppm) compared to 5.6 for the vinylic proton in cyclohexene. In contrast any proton inside the aromatic ring experiences shielding because both fields are in opposite direction. This effect can be observed in cyclooctadecanonaene ([18]annulene) with 6 inner protons at −3 ppm.

The situation is reversed in antiaromatic compounds. In the dianion of [18]annulene the inner protons are strongly deshielded at 20.8 ppm and 29.5 ppm with the outer protons significantly shielded (with respect to the reference) at −1.1 ppm. Hence a diamagnetic ring current or diatropic ring current is associated with aromaticity whereas a paratropic ring current signals antiaromaticity.

A similar effect is observed in three-dimensional fullerenes; in this case it is called a sphere current.[3]

Magnetically induced probability current density vectors in benzene (C6H6) calculated explicitly using quantum chemical methods. B0 is set perpendicular to the molecular plane, in the left subfigure only vectors in the molecular plane are shown, in the right subfigure only vectors 1 a.u. (~52 pm) above the molecular plane are shown. Only vectors with a modulus between 0.01 and 0.1 nA/T are displayed. Contrasting the schematic picture, which gives in analogy to the laws of classical electrodynamics only diatropic contributions, the full quantum mechanical picture also yield paratropic contributions, as counter-clockwise vortices in this diagram. These are located in benzene mainly in the molecular plane, inside the C6 ring.

## Relative aromaticity

[edit]

Numerous attempts have been made to quantify aromaticity with respect to the observed ring current.[4] One method is called diamagnetic susceptibility exaltation Λ defined as the difference between the measured magnetic susceptibility of a compound and a calculated value based on group additivity tables. Large negative values are aromatic, for example, benzene (Λ = −13.4). Values close to zero are non-aromatic, for example, borazine (Λ = −1.7) and cyclohexane (Λ = 1.1). Large positive values are antiaromatic, for example, cyclobutadiene (Λ = +18).

Another measurable quantity is the chemical shift of lithium ions Li+ in complexes of lithium with aromatic structures because lithium tends to bond as a π-coordinate complex to the face of the aromatic rings. Thus the lithium atom in cyclopentadienyl lithium (CpLi) has a chemical shift of −8.6 ppm (aromatic) and its Cp2Li− complex a shift of −13.1.

Both methods suffer from the disadvantage that values depend on ring size.

Selected NICS values[5] Chemical | ppm

|

Pyrrole | −15.1
Thiophene | −13.6
Furan | −12.3
Naphthalene | −9.9
Benzene | −9.7
Tropylium | −7.6
Cyclopentadiene | −3.2
Cyclohexane | −2.2
Pentalene | 18.1
Heptalene | 22.7
Cyclobutadiene | 27.6

### Nucleus-independent chemical shift

[edit]

The nucleus-independent chemical shift (NICS) is a computational method that calculates the absolute magnetic shielding at the center of a ring. The values are reported with a reversed sign to make them compatible with the chemical shift conventions of NMR spectroscopy.[5] In this method, negative NICS values indicate aromaticity, and positive values indicate antiaromaticity.[6][7] There are a variety of methods to calculate NICS values, however, the most robust method for calculating NICS values involves scanning the molecule in a NICSzz scan. In this process, the NICS value is calculated above the rings, and this allows for evaluating the aromatics of each ring, which is especially useful for polycyclic compounds.[8]

### Harmonic oscillator model of aromaticity

[edit]

Yet another method called the harmonic oscillator model of aromaticity (HOMA)[9] is defined as a normalized sum of squared deviations of bond lengths from the optimal value, which is assumed to be realized for a fully aromatic system.[10] An aromatic compound has HOMA value 1, whereas a non-aromatic compound has value 0. For all-carbon systems, the HOMA value is defined as

A = 1 − V n ∑ i n ( d opt − d i ) 2 , {\displaystyle \mathrm {A} =1-{\frac {V}{n}}\sum _{i}^{n}(d_{\text{opt}}-d_{i})^{2},}

where V = 257.7 Å−2 is the normalization value, n is the number of carbon–carbon bonds, and d are bond lengths (dopt = 1.388 Å is the optimal value, and di are the observed or computed values).

## References

[edit]

1. ^ The induced magnetic field in cyclic molecules. Merino, G.; Heine, T.; Seifert, G. Chem. Eur. J.; 2004; 10; 4367-4371. doi:10.1002/chem.200400457
2. ^ Aromaticity and Ring Currents. Gomes, J. A. N. F.; Mallion, R. B. Chem. Rev.; (Review); 2001; 101(5); 1349-1384. doi:10.1021/cr990323h
3. ^ Sphere Currents of Buckminsterfullerene, Mikael P. Johansson, Jonas Jusélius, and Dage Sundholm, Angew. Chem. Int. Ed., Vol. 44, No. 12, pp. 1843-1846, 2005 doi:10.1002/anie.200462348 PMID 15706578
4. ^ What is aromaticity? Paul von Ragué Schleyer and Haijun Jiao Pure Appl. Chem., Vol. 68, No. 2, pp. 209-218, 1996 Link
5. ^ a b Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe Paul von Ragué Schleyer, Christoph Maerker, Alk Dransfeld, Haijun Jiao, and Nicolaas J. R. van Eikema Hommes J. Am. Chem. Soc.; 1996; 118(26) pp 6317-6318; (Communication) doi:10.1021/ja960582d
6. ^ Nucleus-Independent Chemical Shifts (NICS) as an Aromaticity Criterion Zhongfang Chen, Chaitanya S. Wannere, Clémence Corminboeuf, Ralph Puchta, and Paul von Ragué Schleyer Chem.Rev.; 2005; 105(10) pp 3842-3888; (Review) doi:10.1021/cr030088+
7. ^ 25 years of NICS – much more than nothing! Ralph Puchta, Slađana Đorđević, Slavko Radenković, Haijun Jiao, and Nico van Eikema Hommes J. Serb. Chem. Soc.; 2022; 87(12) pp 1439-1446; (Communication) doi:10.2298/JSC211203057P
8. ^ Stanger, Amnon (2020-06-08). "NICS – Past and Present". European Journal of Organic Chemistry. 2020 (21): 3120–3127\. doi:10.1002/ejoc.201901829. ISSN 1434-193X.
9. ^ Definition of aromaticity basing on the harmonic oscillator model, Tetrahedron Letters, volume 13, issue 36, 1972, pages 3839–3842. J. Kruszewski and T. M. Krygowski. doi:10.1016/S0040-4039(01)94175-9.
10. ^ How far is the π-electron delocalization of the phenanthrene moiety modified in the aza-analogues and their N-oxides? Beata T. Stępień, Tadeusz M. Krygowski, a Michał K. Cyrański, Jacek Młochowski, Pierluigi Orioli, and Francesco Abbate Arkivoc, 2003.

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Topics in organic reactions

* Addition reaction
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* A value
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* Baird's rule
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List of organic reactions

Carbon-carbon
bond forming
reactions|

* Acetoacetic ester synthesis
* Acyloin condensation
* Aldol condensation
* Aldol reaction
* Alkane metathesis
* Alkyne metathesis
* Alkyne trimerisation
* Alkynylation
* Allan–Robinson reaction
* Arndt–Eistert reaction
* Auwers synthesis
* Aza-Baylis–Hillman reaction
* Barbier reaction
* Barton–Kellogg reaction
* Baylis–Hillman reaction
* Benary reaction
* Bergman cyclization
* Biginelli reaction
* Bingel reaction
* Blaise ketone synthesis
* Blaise reaction
* Blanc chloromethylation
* Bodroux–Chichibabin aldehyde synthesis
* Bouveault aldehyde synthesis
* Bucherer–Bergs reaction
* Buchner ring expansion
* Cadiot–Chodkiewicz coupling
* Carbonyl allylation
* Carbonyl olefin metathesis
* Castro–Stephens coupling
* Chan rearrangement
* Chan–Lam coupling
* Claisen condensation
* Claisen rearrangement
* Claisen-Schmidt condensation
* Combes quinoline synthesis
* Corey–Fuchs reaction
* Corey–House synthesis
* Coupling reaction
* Cross-coupling reaction
* Cross dehydrogenative coupling
* Cross-coupling partner
* Dakin–West reaction
* Darzens reaction
* Diels–Alder reaction
* Doebner reaction
* Wulff–Dötz reaction
* Ene reaction
* Enyne metathesis
* Ethenolysis
* Favorskii reaction
* Ferrier carbocyclization
* Friedel–Crafts reaction
* Fujimoto–Belleau reaction
* Fujiwara–Moritani reaction
* Fukuyama coupling
* Gabriel–Colman rearrangement
* Gattermann reaction
* Glaser coupling
* Grignard reaction
* Grignard reagent
* Hammick reaction
* Heck reaction
* Henry reaction
* Heterogeneous metal catalyzed cross-coupling
* High dilution principle
* Hiyama coupling
* Homologation reaction
* Horner–Wadsworth–Emmons reaction
* Hydrocyanation
* Hydrovinylation
* Hydroxymethylation
* Ivanov reaction
* Johnson–Corey–Chaykovsky reaction
* Julia olefination
* Julia–Kocienski olefination
* Kauffmann olefination
* Knoevenagel condensation
* Knorr pyrrole synthesis
* Kolbe–Schmitt reaction
* Kowalski ester homologation
* Kulinkovich reaction
* Kumada coupling
* Liebeskind–Srogl coupling
* Malonic ester synthesis
* Mannich reaction
* McMurry reaction
* Meerwein arylation
* Methylenation
* Michael reaction
* Minisci reaction
* Mizoroki-Heck vs. Reductive Heck
* Nef isocyanide reaction
* Nef synthesis
* Negishi coupling
* Nierenstein reaction
* Nitro-Mannich reaction
* Nozaki–Hiyama–Kishi reaction
* Olefin conversion technology
* Olefin metathesis
* Palladium–NHC complex
* Passerini reaction
* Peterson olefination
* Pfitzinger reaction
* Piancatelli rearrangement
* Pinacol coupling reaction
* Prins reaction
* Quelet reaction
* Ramberg–Bäcklund reaction
* Rauhut–Currier reaction
* Reformatsky reaction
* Reimer–Tiemann reaction
* Rieche formylation
* Ring-closing metathesis
* Robinson annulation
* Sakurai reaction
* Seyferth–Gilbert homologation
* Shapiro reaction
* Sonogashira coupling
* Stetter reaction
* Stille reaction
* Stollé synthesis
* Stork enamine alkylation
* Suzuki reaction
* Takai olefination
* Thermal rearrangement of aromatic hydrocarbons
* Thorpe reaction
* Ugi reaction
* Ullmann reaction
* Wagner-Jauregg reaction
* Weinreb ketone synthesis
* Wittig reaction
* Wurtz reaction
* Wurtz–Fittig reaction
* Zincke–Suhl reaction

| Homologation reactions|

* Arndt–Eistert reaction
* Hooker reaction
* Kiliani–Fischer synthesis
* Kowalski ester homologation
* Methoxymethylenetriphenylphosphorane
* Seyferth–Gilbert homologation
* Wittig reaction

|

Olefination reactions|

* Bamford–Stevens reaction
* Barton–Kellogg reaction
* Boord olefin synthesis
* Chugaev elimination
* Cope reaction
* Corey–Winter olefin synthesis
* Dehydrohalogenation
* Elimination reaction
* Grieco elimination
* Hofmann elimination
* Horner–Wadsworth–Emmons reaction
* Hydrazone iodination
* Julia olefination
* Julia–Kocienski olefination
* Kauffmann olefination
* McMurry reaction
* Peterson olefination
* Ramberg–Bäcklund reaction
* Shapiro reaction
* Takai olefination
* Wittig reaction

Carbon-heteroatom
bond forming
reactions|

* Azo coupling
* Bartoli indole synthesis
* Boudouard reaction
* Cadogan–Sundberg indole synthesis
* Diazonium compound
* Esterification
* Grignard reagent
* Haloform reaction
* Hegedus indole synthesis
* Hurd–Mori 1,2,3-thiadiazole synthesis
* Kharasch–Sosnovsky reaction
* Knorr pyrrole synthesis
* Leimgruber–Batcho indole synthesis
* Mukaiyama hydration
* Nenitzescu indole synthesis
* Oxymercuration reaction
* Reed reaction
* Schotten–Baumann reaction
* Ullmann condensation
* Williamson ether synthesis
* Yamaguchi esterification

Degradation
reactions|

* Barbier–Wieland degradation
* Bergmann degradation
* Edman degradation
* Emde degradation
* Gallagher–Hollander degradation
* Hofmann rearrangement
* Hooker reaction
* Isosaccharinic acid
* Marker degradation
* Ruff degradation
* Strecker degradation
* Von Braun amide degradation
* Weerman degradation
* Wohl degradation

Organic redox
reactions|

* Acyloin condensation
* Adkins–Peterson reaction
* Akabori amino-acid reaction
* Alcohol oxidation
* Algar–Flynn–Oyamada reaction
* Amide reduction
* Andrussow process
* Angeli–Rimini reaction
* Aromatization
* Autoxidation
* Baeyer–Villiger oxidation
* Barton–McCombie deoxygenation
* Bechamp reduction
* Benkeser reaction
* Bergmann degradation
* Birch reduction
* Bohn–Schmidt reaction
* Bosch reaction
* Bouveault–Blanc reduction
* Boyland–Sims oxidation
* Cannizzaro reaction
* Carbonyl reduction
* Clemmensen reduction
* Collins oxidation
* Corey–Itsuno reduction
* Corey–Kim oxidation
* Corey–Winter olefin synthesis
* Criegee oxidation
* Dakin oxidation
* Davis oxidation
* Deoxygenation
* Dess–Martin oxidation
* DNA oxidation
* Elbs persulfate oxidation
* Emde degradation
* Eschweiler–Clarke reaction
* Étard reaction
* Fischer–Tropsch process
* Fleming–Tamao oxidation
* Fukuyama reduction
* Ganem oxidation
* Glycol cleavage
* Griesbaum coozonolysis
* Grundmann aldehyde synthesis
* Haloform reaction
* Hydrogenation
* Hydrogenolysis
* Hydroxylation
* Jones oxidation
* Kiliani–Fischer synthesis
* Kolbe electrolysis
* Kornblum oxidation
* Kornblum–DeLaMare rearrangement
* Leuckart reaction
* Ley oxidation
* Lindgren oxidation
* Lipid peroxidation
* Lombardo methylenation
* Luche reduction
* Markó–Lam deoxygenation
* McFadyen–Stevens reaction
* Meerwein–Ponndorf–Verley reduction
* Methionine sulfoxide
* Miyaura borylation
* Mozingo reduction
* Noyori asymmetric hydrogenation
* Omega oxidation
* Oppenauer oxidation
* Oxygen rebound mechanism
* Ozonolysis
* Parikh–Doering oxidation
* Pinnick oxidation
* Prévost reaction
* Reduction of nitro compounds
* Reductive amination
* Riley oxidation
* Rosenmund reduction
* Rubottom oxidation
* Sabatier reaction
* Sarett oxidation
* Selenoxide elimination
* Shapiro reaction
* Sharpless asymmetric dihydroxylation
* Epoxidation of allylic alcohols
* Sharpless epoxidation
* Sharpless oxyamination
* Stahl oxidation
* Staudinger reaction
* Stephen aldehyde synthesis
* Swern oxidation
* Transfer hydrogenation
* Wacker process
* Wharton reaction
* Whiting reaction
* Wohl–Aue reaction
* Wolff–Kishner reduction
* Wolffenstein–Böters reaction
* Zinin reaction

Rearrangement
reactions|

* 1,2-rearrangement
* 1,2-Wittig rearrangement
* 2,3-sigmatropic rearrangement
* 2,3-Wittig rearrangement
* Achmatowicz reaction
* Alkyne zipper reaction
* Allen–Millar–Trippett rearrangement
* Allylic rearrangement
* Alpha-ketol rearrangement
* Amadori rearrangement
* Arndt–Eistert reaction
* Aza-Cope rearrangement
* Baker–Venkataraman rearrangement
* Bamberger rearrangement
* Banert cascade
* Beckmann rearrangement
* Benzilic acid rearrangement
* Bergman cyclization
* Bergmann degradation
* Boekelheide reaction
* Brook rearrangement
* Buchner ring expansion
* Carroll rearrangement
* Chan rearrangement
* Claisen rearrangement
* Cope rearrangement
* Corey–Fuchs reaction
* Cornforth rearrangement
* Criegee rearrangement
* Curtius rearrangement
* Demjanov rearrangement
* Di-π-methane rearrangement
* Dimroth rearrangement
* Divinylcyclopropane-cycloheptadiene rearrangement
* Dowd–Beckwith ring-expansion reaction
* Electrocyclic reaction
* Ene reaction
* Enyne metathesis
* Favorskii reaction
* Favorskii rearrangement
* Ferrier carbocyclization
* Ferrier rearrangement
* Fischer–Hepp rearrangement
* Fries rearrangement
* Fritsch–Buttenberg–Wiechell rearrangement
* Gabriel–Colman rearrangement
* Group transfer reaction
* Halogen dance rearrangement
* Hayashi rearrangement
* Hofmann rearrangement
* Hofmann–Martius rearrangement
* Ireland–Claisen rearrangement
* Jacobsen rearrangement
* Kornblum–DeLaMare rearrangement
* Kowalski ester homologation
* Lobry de Bruyn–Van Ekenstein transformation
* Lossen rearrangement
* McFadyen–Stevens reaction
* McLafferty rearrangement
* Meyer–Schuster rearrangement
* Mislow–Evans rearrangement
* Mumm rearrangement
* Myers allene synthesis
* Nazarov cyclization reaction
* Neber rearrangement
* Newman–Kwart rearrangement
* Overman rearrangement
* Oxy-Cope rearrangement
* Pericyclic reaction
* Piancatelli rearrangement
* Pinacol rearrangement
* Pummerer rearrangement
* Ramberg–Bäcklund reaction
* Ring expansion and contraction
* Ring-closing metathesis
* Rupe reaction
* Schmidt reaction
* Semipinacol rearrangement
* Seyferth–Gilbert homologation
* Sigmatropic reaction
* Skattebøl rearrangement
* Smiles rearrangement
* Sommelet–Hauser rearrangement
* Stevens rearrangement
* Stieglitz rearrangement
* Thermal rearrangement of aromatic hydrocarbons
* Tiffeneau–Demjanov rearrangement
* Vinylcyclopropane rearrangement
* Wagner–Meerwein rearrangement
* Wallach rearrangement
* Weerman degradation
* Westphalen–Lettré rearrangement
* Willgerodt rearrangement
* Wolff rearrangement

Ring forming
reactions|

* 1,3-Dipolar cycloaddition
* Annulation
* Azide-alkyne Huisgen cycloaddition
* Baeyer–Emmerling indole synthesis
* Bartoli indole synthesis
* Bergman cyclization
* Biginelli reaction
* Bischler–Möhlau indole synthesis
* Bischler–Napieralski reaction
* Blum–Ittah aziridine synthesis
* Bobbitt reaction
* Bohlmann–Rahtz pyridine synthesis
* Borsche–Drechsel cyclization
* Bucherer carbazole synthesis
* Bucherer–Bergs reaction
* Cadogan–Sundberg indole synthesis
* Camps quinoline synthesis
* Chichibabin pyridine synthesis
* Cook–Heilbron thiazole synthesis
* Cycloaddition
* Darzens reaction
* Davis–Beirut reaction
* De Kimpe aziridine synthesis
* Debus–Radziszewski imidazole synthesis
* Dieckmann condensation
* Diels–Alder reaction
* Feist–Benary synthesis
* Ferrario–Ackermann reaction
* Fiesselmann thiophene synthesis
* Fischer indole synthesis
* Fischer oxazole synthesis
* Friedländer synthesis
* Gewald reaction
* Graham reaction
* Hantzsch pyridine synthesis
* Hegedus indole synthesis
* Hemetsberger indole synthesis
* Hofmann–Löffler reaction
* Hurd–Mori 1,2,3-thiadiazole synthesis
* Iodolactonization
* Isay reaction
* Jacobsen epoxidation
* Johnson–Corey–Chaykovsky reaction
* Knorr pyrrole synthesis
* Knorr quinoline synthesis
* Kröhnke pyridine synthesis
* Kulinkovich reaction
* Larock indole synthesis
* Madelung synthesis
* Nazarov cyclization reaction
* Nenitzescu indole synthesis
* Niementowski quinazoline synthesis
* Niementowski quinoline synthesis
* Paal–Knorr synthesis
* Paternò–Büchi reaction
* Pechmann condensation
* Petrenko-Kritschenko piperidone synthesis
* Pictet–Spengler reaction
* Pomeranz–Fritsch reaction
* Prilezhaev reaction
* Pschorr cyclization
* Reissert indole synthesis
* Ring-closing metathesis
* Robinson annulation
* Sharpless epoxidation
* Simmons–Smith reaction
* Skraup reaction
* Urech hydantoin synthesis
* Van Leusen reaction
* Wenker synthesis

| Cycloaddition|

* 1,3-Dipolar cycloaddition
* 4+4 Photocycloaddition
* (4+3) cycloaddition
* 6+4 Cycloaddition
* Alkyne trimerisation
* Aza-Diels–Alder reaction
* Azide-alkyne Huisgen cycloaddition
* Bradsher cycloaddition
* Cheletropic reaction
* Conia-ene reaction
* Cyclopropanation
* Diazoalkane 1,3-dipolar cycloaddition
* Diels–Alder reaction
* Enone–alkene cycloadditions
* Hexadehydro Diels–Alder reaction
* Intramolecular Diels–Alder cycloaddition
* Inverse electron-demand Diels–Alder reaction
* Ketene cycloaddition
* McCormack reaction
* Metal-centered cycloaddition reactions
* Nitrone-olefin (3+2) cycloaddition
* Oxo-Diels–Alder reaction
* Ozonolysis
* Pauson–Khand reaction
* Povarov reaction
* Prato reaction
* Retro-Diels–Alder reaction
* Staudinger synthesis
* Trimethylenemethane cycloaddition
* Vinylcyclopropane (5+2) cycloaddition
* Wagner-Jauregg reaction

|

Heterocycle forming reactions|

* Algar–Flynn–Oyamada reaction
* Allan–Robinson reaction
* Auwers synthesis
* Bamberger triazine synthesis
* Banert cascade
* Barton–Zard reaction
* Bernthsen acridine synthesis
* Bischler–Napieralski reaction
* Bobbitt reaction
* Boger pyridine synthesis
* Borsche–Drechsel cyclization
* Bucherer carbazole synthesis
* Bucherer–Bergs reaction
* Chichibabin pyridine synthesis
* Cook–Heilbron thiazole synthesis
* Diazoalkane 1,3-dipolar cycloaddition
* Einhorn–Brunner reaction
* Erlenmeyer–Plöchl azlactone and amino-acid synthesis
* Feist–Benary synthesis
* Fischer oxazole synthesis
* Gabriel–Colman rearrangement
* Gewald reaction
* Hantzsch ester
* Hantzsch pyridine synthesis
* Herz reaction
* Knorr pyrrole synthesis
* Kröhnke pyridine synthesis
* Lectka enantioselective beta-lactam synthesis
* Lehmstedt–Tanasescu reaction
* Niementowski quinazoline synthesis
* Nitrone-olefin (3+2) cycloaddition
* Paal–Knorr synthesis
* Pellizzari reaction
* Pictet–Spengler reaction
* Pomeranz–Fritsch reaction
* Prilezhaev reaction
* Robinson–Gabriel synthesis
* Stollé synthesis
* Urech hydantoin synthesis
* Wenker synthesis
* Wohl–Aue reaction

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