Arielle Cruz
Ring contraction refers to the isomerization process of converting large rings into smaller bicyclic systems through electrocyclic reactions. It involves the loss of one or more ring members from the heterocycle, followed by the formation of a new ring or the extrusion of atoms to form a new substituent or side chain. This process is often used to make smaller, more strained rings from larger rings. Ring contraction reactions can be coupled with ring expansion reactions to synthesize the cores of complex molecules. The Favorskii rearrangement is a classic example of an anionic ring contraction, which proceeds through a carbanion that attacks an endocyclic carbon and expels a halide to form a bicyclic molecule with smaller rings. The use of strong acids, such as H2SO4, plays a crucial role in ring contractions by facilitating the formation of carbocations and influencing the reactivity of alcohols.
| Characteristics | Values |
|---|---|
| Definition | Ring contraction refers to the isomerization process of large rings into smaller bicyclic systems through electrocyclic reactions |
| Process | It involves the loss of one or more ring members from the heterocycle, followed by the formation of a new ring or the extrusion of atoms from the ring to form a new substituent or side chain |
| Ring Contraction Reactions | The Favorskii rearrangement is a classic anionic ring contraction |
| Ring Expansion Reactions | Examples include the Beckmann rearrangement and the Baeyer-Villiger oxidation |
| Migration | The group to which the endocyclic bond migrates can also be selectively added to the ring based on the functionality already present, for example, 1,2 addition into a cyclic ketone |
| Ring Expansion and Contraction | Contraction reactions of one ring can be coupled with an expansion of another to give an unequal bicycle from equally sized fused rings |
| Ring Contraction in Alcohols | The hydroxyl group of alcohols is normally a poor leaving group, but when treated with a strong acid, it can become a good leaving group |
| Treatment of 2-methylenecyclobutanol with acid leads to the formation of 1-methylcyclopropanecarbaldehyde |
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What You'll Learn
The hydroxyl group of alcohols is a poor leaving group
Ring contraction refers to the isomerization process of large rings into smaller bicyclic systems through electrocyclic reactions. It involves the loss of one or more ring members from the heterocycle, followed by the formation of a new ring or the extrusion of atoms from the ring to form a new substituent or side chain. An example of a ring contraction reaction is the conversion of cyclohexanone to the methyl ester of cyclopentanecarboxylic acid through the Favorskii rearrangement, a classic anionic ring contraction.
Ring contractions are useful for making smaller, more strained rings from larger rings. This is because it is difficult to make a fully elaborated small ring, but it is easier to start with a larger ring and excise an atom or work with a more accessible larger scaffold. Ring contractions can also be coupled with ring expansions to give an unequal bicycle from equally sized fused rings. These cationic rearrangements have been used to synthesize the cores of complex molecules.
The hydroxyl group of alcohols can undergo elimination reactions to form alkenes. For example, treating a primary alcohol like 1-butanol with a strong acid like H2SO4 will result in the elimination of water and the formation of an alkene. This occurs through an E1 mechanism, which involves the protonation of alcohol, the loss of H2O to form a carbocation, and then the attack of a nucleophile on the carbocation.
The hydroxyl group of alcohols can also be replaced with a nucleophile through the Mitsunobu reaction. Additionally, the hydroxyl group can be converted into a sulfonate ester, such as a mesylate or tosylate, which are better leaving groups than halides.
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Ring contractions can be coupled with ring expansions
Ring contractions refer to the isomerization process of large rings into smaller bicyclic systems through electrocyclic reactions. It involves the loss of one or more ring members from the heterocycle, followed by the formation of a new ring or the extrusion of atoms from the ring to form a new substituent or side chain. These reactions are common for transition metal complexes. Ring contractions are useful for making smaller, more strained rings from larger rings. The difficulty associated with making a fully elaborated small ring can be circumvented by starting with a larger ring from which an atom can be removed.
Ring expansions, on the other hand, allow access to larger systems that are difficult to synthesize through a single cyclization due to the slow rate of formation. They are valuable in synthesizing complex molecules. Ring expansions are classified by the mechanism of expansion and the atom(s) added. Carbon insertions, for instance, introduce an additional carbon atom into the ring.
Contraction reactions of one ring can be coupled with an expansion of another to give an unequal bicycle from equally sized fused rings. This is achieved through cationic rearrangements, which proceed through the loss of a leaving group and the migration of an endocyclic bond to the carbocation. Pinacol-type rearrangements are often used for this purpose, aided by an electron-donating group. These rearrangements are similar to those observed in carbon expansions, which occur through migration/insertion pathways.
The Favorskii rearrangement is a classic example of an anionic ring contraction. It involves a carbanion attacking an endocyclic carbon and expelling a leaving group (usually a halide) to form a bicyclic molecule with smaller rings. This bicycle is then opened by a nucleophilic attack on the ketone, resulting in the contracted product. Another example of a ring contraction reaction is the Arndt-Eistert reaction, where an α-diazoketone releases N2, forming a highly reactive sextet carbon center adjacent to the carbonyl. This species then undergoes a Wolff rearrangement to give an ester in the presence of alcohols.
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Ring contractions can be achieved using oxidants
Ring contractions refer to the isomerization process of large rings into smaller bicyclic systems through electrocyclic reactions. They are commonly used in organic chemistry, where they are useful for making smaller, more strained rings from larger rings. This is because it is often difficult to make a fully elaborated small ring, whereas a larger ring can be made first, and then an atom can be excised.
Another example of a ring contraction achieved using an oxidant is the photochemically induced ring contraction of dibenzo-fused dithiepins. Irradiation of 2-phenyldibenzo [d,f] [1,3] dithiepin 1-oxide and 9-phenyl-4,8,9-trithiadibenzo [cd,ij] azulene 8-oxide using a high-pressure mercury lamp gave disulfides.
Ring contractions can also be achieved using oxidants in the synthesis of indoles. For example, photolysis of 3-diazo-3,4-dihydro-4-quinolones in alcohols gives indole-3-carboxylate esters.
Ring contractions via 1,2-rearrangements have seen wide application in natural product synthesis. For example, the enantioselective synthesis of (−)-citrinadin A featured a substrate-controlled, oxidative semi-pinacol rearrangement of indole to oxindole using William's approach.
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Ring contractions can be used to make smaller, more strained rings
Ring contractions are useful for making smaller, more strained rings from larger rings. The process involves the isomerization of large rings into smaller bicyclic systems through electrocyclic reactions. This is done by losing one or more ring members from the heterocycle, followed by the formation of a new ring or the extrusion of atoms from the ring to form a new substituent or side chain. The Favorskii rearrangement is a classic example of an anionic ring contraction, where a carbanion attacks an endocyclic carbon and expels a leaving group (a halide), resulting in a bicyclic molecule with smaller rings.
The impetus for making these smaller, more strained rings stems from the challenges associated with creating a fully elaborated small ring. It is often more feasible to start with an elaborated larger ring, from which an atom can be removed, or the original larger scaffold may be more readily accessible.
These smaller, more strained rings have bond angles that deviate from the optimal tetrahedral (109.5°) and trigonal planar (120°) angles required by their respective sp3 and sp2 bonds. As a result, the bonds have higher energy and adopt more p-character to reduce their energy. Additionally, the ring structures of these small rings offer limited conformational flexibility, leading to higher ring strain energy due to van der Waals repulsion.
An example of a highly strained small ring is cyclopropane, which has carbon-carbon bond angles of 60°, far from the idealized angle of 109.5°. This deviation from the ideal bond angle contributes to the ring strain in cyclopropane. Cyclopropane also exhibits high torsional strain due to the eclipsed conformation of its substituents.
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Ring contractions can be used to synthesise complex molecules
Ring contractions are a set of reactions that can lead to the contraction of an existing ring. This often makes it possible to access structures that would be difficult, if not impossible, to synthesise with single cyclisation reactions. Ring contractions are useful for making smaller, more strained rings from larger rings. This is because it is difficult to make a fully elaborated small ring, whereas a larger ring with a similar structure is often more accessible.
Ring contraction reactions can be used to synthesise complex molecules. For example, the contraction reactions of one ring can be coupled with an expansion of another to give an unequal bicycle from equally sized fused rings. These cationic rearrangements have been used to synthesise the cores of complex molecules.
The Favorskii rearrangement is a classic example of an anionic ring contraction. It proceeds through a carbanion that attacks an endocyclic carbon and expels a leaving group (a halide), forming a bicyclic molecule with rings smaller than the original. The bicycle is then opened by a nucleophilic attack on the ketone to give the contracted product. This reaction has been used to convert cyclohexanone to the methyl ester of cyclopentanecarboxylic acid.
Ring contractions in the synthesis of indoles are usually based on quinoline derivatives. Hydrolysis of 4-acyl-3,4-dihydro-2-quinolones gives 2-substituted indole-3-acetic acids by a reaction which can be recognised as involving the hydrolytic formation of an o-aminobenzyl ketone. The ring contraction reaction of 1,3,4-thiadiazines can also be employed for the synthesis of annelated pyrazoles.
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Frequently asked questions
Ring contraction is the isomerization process of large rings into smaller bicyclic systems through electrocyclic reactions. It involves the loss of one or more ring members from the heterocycle, followed by the formation of a new ring or the extrusion of atoms from the ring to form a new substituent or side chain.
The Favorskii rearrangement is a classic example of an anionic ring contraction. It proceeds through a carbanion that attacks an endocyclic carbon and expels a leaving group (a halide) to form a bicyclic molecule with smaller rings. Another example is the ring contraction of 1-benzosuberone to methyl 1,2,3,4-tetrahydronaphthalene-1-carboxylate using 1H-1-hydroxy-5-methyl-1,2,3-benziodoxathiole as an oxidant.
Alcohols can be used as a solvent during ring contractions. The hydroxyl group of alcohols is typically a poor leaving group, but when treated with a strong acid like H2SO4, it can be converted into a better leaving group, leading to the formation of a carbocation. This process is essential for the dehydration reaction of alcohols with acids to form alkenes.
