The Stetter reaction is a reaction used in organic chemistry to form carbon-carbon bonds through a 1,4-addition reaction utilizing a nucleophiliccatalyst.[1] While the related 1,2-addition reaction, the benzoin condensation, was known since the 1830s, the Stetter reaction was not reported until 1973 by Dr. Hermann Stetter.[2] The reaction provides synthetically useful 1,4-dicarbonyl compounds and related derivatives from aldehydes and Michael acceptors. Unlike 1,3-dicarbonyls, which are easily accessed through the Claisen condensation, or 1,5-dicarbonyls, which are commonly made using a Michael reaction, 1,4-dicarbonyls are challenging substrates to synthesize, yet are valuable starting materials for several organic transformations, including the Paal–Knorr synthesis of furans and pyrroles. Traditionally utilized catalysts for the Stetter reaction are thiazolium salts and cyanide anion, but more recent work toward the asymmetric Stetter reaction has found triazolium salts to be effective. The Stetter reaction is an example of umpolung chemistry, as the inherent polarity of the aldehyde is reversed by the addition of the catalyst to the aldehyde, rendering the carbon center nucleophilic rather than electrophilic.
As the Stetter reaction is an example of umpolung chemistry, the aldehyde is converted from an electrophile to a nucleophile under the reaction conditions.[3] This is accomplished by activation from some catalyst - either cyanide (CN−) or thiazolium salt.[1] For the use of either catalyst, the mechanism is very similar; the only difference is that with thiazolium salts, the catalyst must be deprotonated first to form the active catalytic species. The active catalyst can be described as the combination of two contributing resonance forms - an ylide or a carbene, both of which portray the nucleophilic character at carbon. The thiazolium ylide or CN− can then add into the aldehyde substrate, forming a cyanohydrin in the case of CN− or the Breslow intermediate in the case of thiazolium salt. The Breslow intermediate was proposed by Ronald Breslow in 1958 and is a common intermediate for all thiamine-catalyzed reactions, whether in vitro or in vivo.[4]
Scheme 2. Formation of the Breslow intermediate
Once the "nucleophilic aldehyde" synthon is formed, whether as a cyanohydrin or stabilized by a thiazolium ylide, the reaction can proceed down two pathways. The faster pathway is self-condensation with another molecule of aldehyde to give benzoin products. However, benzoin condensation is completely reversible, and therefore does not interfere with product formation in the Stetter reaction. In fact, benzoins can be used instead of aldehydes as substrates to achieve the same overall Stetter transformation, because benzoins can be restored to their aldehyde precursors under the reaction conditions.[1] The desired pathway toward the Stetter product is the 1,4-addition of the nucleophilic aldehyde to a Michael-type acceptor. After 1,4-addition, the reaction is irreversible and ultimately, the 1,4-dicarbonyl is formed when the catalyst is kicked out to regenerate CN− or the thiazolium ylide.
Scheme 3. Mechanism of the Stetter reaction
Scope
The Stetter reaction produces classically difficult to access 1,4-dicarbonyl compounds and related derivatives. The traditional Stetter reaction is quite versatile, working on a wide variety of substrates.[1] Aromatic aldehydes, heteroaromatic aldehydes, and benzoins can all be used as acyl anion precursors with thiazolium salt and cyanide catalysts. However, aliphatic aldehydes can only be utilized if a thiazolium salt is used as a catalyst, as they undergo aldol condensation side reaction when a cyanide catalyst is used. In addition, α,β-unsaturated esters, ketones, nitriles, nitros, and aldehydes are all appropriate Michael acceptors with either catalyst. However, the general scope of asymmetric Stetter reactions is more limited. Intramolecular asymmetric Stetter reactions enjoy a range of acceptable Michael acceptors and acyl anion precursors in essentially any combination.[5] Intramolecular asymmetric Stetter reactions can utilize aromatic, heteroaromatic and aliphatic aldehydes with a tethered α,β-unsaturated ester, ketone, thioester, malonate, nitrile or Weinreb amide. It has been shown that α,β-unsaturated nitros and aldehydes are not suitable Michael acceptors and have markedly decreased enantiomeric excess in such reactions.[5] Another limitation encountered with intramolecular asymmetric Stetter reactions is that only substrates that result in the formation of a six-membered ring show synthetically useful enantiomeric excess; substrates which form five and seven-membered rings either do not react or show low stereoinduction.[5] On the other hand, intermolecular asymmetric reactions are quite confined to specifically matched combinations of acyl anion precursor and Michael acceptor, such as an aliphatic aldehyde with a nitroalkene.[6] In addition, these substrates tend to be rather activated, as the intermolecular asymmetric Stetter reaction is still in the early stages of development.
Scheme 4. Scope of Stetter Reaction
Variations
Several variations of the Stetter reaction have been developed since its discovery in 1973. In 2001, Murry et al reported a Stetter reaction of aromatic aldehydes onto acylimine derivatives to give α-amido ketone products.[7] The acylimine acceptors were generated in situ from α-tosylamide substrates, which underwent elimination in the presence of base. Good to excellent yields (75-90%) were observed. Mechanistic investigations showed that the corresponding benzoins were not adequate substrates, contrary to traditional Stetter reactions.[1] From this, the authors conclude the Stetter reaction of acylimines is under kinetic control, rather than thermodynamic control.
Scheme 5. Stetter reaction with acylimines as acceptors
Another variation of the Stetter reaction involves the use of 1,2-dicarbonyls as precursors to the acyl anion intermediate. In 2005, Scheidt and coworkers reported the use of sodium pyruvate, which loses CO2 to form the Breslow intermediate.[8] Similarly, in 2011 Bortolini and coworkers demonstrated the use of α-diketones to generate an acyl anion.[9] Under the conditions they developed, 2,3-butadienone is cleaved after addition to the thiazolium catalyst to release ethyl acetate and generate the Breslow intermediate necessary for the Stetter reaction to proceed.
Scheme 6. Stetter reaction with acyclic α-diketones
In addition, they showed the atom economy and utility of using a cyclic α-diketone to generate the Stetter product with a tethered ethyl ester. The reaction precedes through the same mechanism as the acyclic version, but the ester generated by attack of ethanol remains tethered to the product. However, the conditions only allow for the generation of ethyl esters, due to the necessity of ethanol as solvent. Substitution of ethanol with tert-butanol resulted in no product. The authors speculate this is due to the difference in acidity between the two alcoholic solvents.
Scheme 7. Stetter reaction with cyclic α-diketones
In 2004, Scheidt and coworkers introduced acyl silanes as competent substrates in the Stetter reaction, a variation they termed the "sila-Stetter reaction."[10] Under their reaction conditions, the thiazolium catalyst induces a [1,2] Brook rearrangement, which is followed by desilylation by an isopropanol additive to give the common Breslow intermediate of the traditional Stetter reaction. The desilylation step was found to be necessary, and the reaction does not proceed without an alcoholic additive. Acyl silanes are less electrophilic than the corresponding aldehydes, preventing typical benzoin-type byproducts often observed in the Stetter reaction.[11]
Scheme 8. Sila-Stetter reaction
Asymmetric Stetter Reaction
The first asymmetric variant of the Stetter reaction was reported in 1996 by Enders et al, employing a chiral triazolium catalyst 1.[12] Subsequently, several other catalysts were reported for asymmetric Stetter reactions, including 2,[13]3,[14] and 4.[15]
Scheme 9. First Asymmetric Stetter Reactions
The success of the Rovis group's catalyst 2 led them to further explore this family of catalysts and expand their use for asymmetric Stetter reactions. In 2004, they reported the enantioselective formation of quaternary centers from aromatic aldehydes in an intramolecular Stetter reaction with a slightly modified catalyst.[16] Further work extended the scope of this reaction to include aliphatic aldehydes as well.[17] Subsequently, it was shown that the olefin geometry of the Michael acceptor dictates diastereoselectivity in these reactions, whereby the catalyst dictates the enantioselectivity of the initial carbon bond formation and allylic strain minimization dictates the diastereoselective intramolecular protonation.[18]
Scheme 10. Formation of quaternary centers
The inherent difficulties of controlling enantioselectivity in intermolecular reactions made the development of an intermolecular asymmetric Stetter reaction a challenge. While limited enantiomeric excess had been reported by Enders in the early 1990s for the reaction of n-butanal with chalcone,[19] conditions for a synthetically useful asymmetric intermolecular Stetter reaction were not reported until 2008 when both the groups of Enders and Rovis published such reactions. The Enders group utilized a triazolium-based catalyst to effect the coupling of aromatic aldehydes with chalcone derivatives with moderate yields.[20] The concurrent publication from the Rovis group also employed a triazolium-based catalyst and reported the Stetter reaction between glyoxamides and alkylidenemalonates in good to excellent yields.[21]
Scheme 12. First Asymmetric Intermolecular Stetter Reactions
Rovis and coworkers subsequently went on to explore the asymmetric intermolecular Stetter reaction of heterocyclic aldehydes and nitroalkenes.[22] During optimization of this reaction, it was found that a catalyst with a fluorinated backbone greatly enhanced enantioselectivity in the reaction. It was proposed that the fluorinated backbone helps to lock the conformation of the catalyst in a way the increases enantioselectivity. Further computational studies on this system verified that the stereoelectronic attraction between the developing partial negative charge on the nitroalkene in the transition state and the partial positive charge of the C-F dipole is responsible for the increase in enantiomeric excess observed with the use of the catalyst with backbone fluorination.[23] While this is a marked advance in the area of intermolecular asymmetric Stetter reactions, the substrate scope is limited and the catalyst is optimized for the specific substrates being utilized.
Scheme 13. Asymmetric Intermolecular Stetter Reaction with Nitroalkenes
Another contribution to the development of asymmetric intermolecular Stetter reactions came from Glorius and coworkers in 2011.[6] They demonstrated the synthesis of α-amino acids enantioselectively by utilizing N-acylamido acrylate as the conjugate acceptor. Significantly, the reaction can be run on a 5mmol scale without loss of yield or enantioselectivity.
Scheme 14. Enantioselective synthesis of amino acids with intermolecular Stetter reaction
Applications
The Stetter reaction is an effective tool in organic synthesis. The products of the Stetter reaction, 1,4-dicarbonyls, are valuable moieties for the synthesis of complex molecules. For example, Trost and coworkers employed a Stetter reaction as one step in their synthesis of rac-hirsutic acid C.[24] The intramolecular coupling of an aliphatic aldehyde with a tethered α,β-unsaturated ester led to the desired tricyclic 1,4-dicarbonyl in 67% yield. This intermediate was converted into rac-hirsutic acid C in seven more steps.
Scheme 15. Total synthesis of hirsutic acid c utilizing an intramolecular Stetter reaction
The Stetter reaction is commonly used in sequence with the Paal-Knorr synthesis of furans and pyrroles, which a 1,4-dicarbonyl undergoes condensation with itself or in the presence of an amine under high temperature, acidic conditions. In 2001, Tius and coworkers reported the asymmetric total synthesis of roseophilin utilizing an intermolecular Stetter reaction to couple an aliphatic aldehyde with a cyclic enone.[25] After ring-closing metathesis and alkene reduction, the 1,4-dicarbonyl product was converted to a pyrrole via the Paal-Knorr synthesis and further elaborated to the natural product.
Scheme 16. Total synthesis of roseophilin utilizing an intermolecular Stetter reaction and Paal-Knorr pyrrole synthesis
In 2004, a one-pot coupling-isomerization-Stetter-Paal Knorr sequence was reported.[26] This procedure first utilizes palladium cross-coupling chemistry to couple aryl halides with propargylic alcohols to give α,β-unsaturated ketones, which can then undergo a Stetter reaction with an aldehyde. Once the 1,4-dicarbonyl compound is formed, heating in the presence of acid will give the furan, while heating in the presence of ammonium chloride and acid will give the pyrrole. The entire sequence is performed in one-pot with no work-up or purification between steps.
Ma and coworkers developed an alternative method for accessing furans utilizing the Stetter reaction.[27] In their report, 3-aminofurans are synthesized under Stetter conditions for coupling aromatic aldehydes with dimethyl acetylenedicarboxylate (DMAD), whereby the thiazolium ylide is hydrolyzed by the aromatization of the furan product. As the thiazolium is destroyed under these conditions, it is not catalytic and must be used in stoichiometric quantities.
Scheme 18. Synthesis of 3-aminofurans using Stetter chemistry
They further elaborated on this work by developing a method in which 2-aminofurans are synthesized by cyclization onto a nitrile.[28] In this method, the thiazolium ylide is employed catalytically and the free amine product is generated.
Scheme 19. Synthesis of 2-aminofurans using Stetter chemistry
Elias James Corey is an American organic chemist. In 1990, he won the Nobel Prize in Chemistry "for his development of the theory and methodology of organic synthesis", specifically retrosynthetic analysis. Regarded by many as one of the greatest living chemists, he has developed numerous synthetic reagents, methodologies and total syntheses and has advanced the science of organic synthesis considerably.
In organic chemistry, the ene reaction is a chemical reaction between an alkene with an allylic hydrogen and a compound containing a multiple bond, in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift. The product is a substituted alkene with the double bond shifted to the allylic position.
The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles. Mechanistic investigation and synthetic application were established in the 1960s, primarily through the work of Rolf Huisgen. Hence, the reaction is sometimes referred to as the Huisgen cycloaddition. 1,3-dipolar cycloaddition is an important route to the regio- and stereoselective synthesis of five-membered heterocycles and their ring-opened acyclic derivatives. The dipolarophile is typically an alkene or alkyne, but can be other pi systems. When the dipolarophile is an alkyne, aromatic rings are generally produced.
The Bamford–Stevens reaction is a chemical reaction whereby treatment of tosylhydrazones with strong base gives alkenes. It is named for the British chemist William Randall Bamford and the Scottish chemist Thomas Stevens Stevens (1900–2000). The usage of aprotic solvents gives predominantly Z-alkenes, while protic solvent gives a mixture of E- and Z-alkenes. As an alkene-generating transformation, the Bamford–Stevens reaction has broad utility in synthetic methodology and complex molecule synthesis.
The benzoin addition is an addition reaction involving two aldehydes. The reaction generally occurs between aromatic aldehydes or glyoxals, and results in formation of an acyloin. In the classic example, benzaldehyde is converted to benzoin.
The Henry reaction is a classic carbon–carbon bond formation reaction in organic chemistry. Discovered in 1895 by the Belgian chemist Louis Henry (1834–1913), it is the combination of a nitroalkane and an aldehyde or ketone in the presence of a base to form β-nitro alcohols. This type of reaction is also referred to as a nitroaldol reaction. It is nearly analogous to the aldol reaction that had been discovered 23 years prior that couples two carbonyl compounds to form β-hydroxy carbonyl compounds known as "aldols". The Henry reaction is a useful technique in the area of organic chemistry due to the synthetic utility of its corresponding products, as they can be easily converted to other useful synthetic intermediates. These conversions include subsequent dehydration to yield nitroalkenes, oxidation of the secondary alcohol to yield α-nitro ketones, or reduction of the nitro group to yield β-amino alcohols.
A cascade reaction, also known as a domino reaction or tandem reaction, is a chemical process that comprises at least two consecutive reactions such that each subsequent reaction occurs only in virtue of the chemical functionality formed in the previous step. In cascade reactions, isolation of intermediates is not required, as each reaction composing the sequence occurs spontaneously. In the strictest definition of the term, the reaction conditions do not change among the consecutive steps of a cascade and no new reagents are added after the initial step. By contrast, one-pot procedures similarly allow at least two reactions to be carried out consecutively without any isolation of intermediates, but do not preclude the addition of new reagents or the change of conditions after the first reaction. Thus, any cascade reaction is also a one-pot procedure, while the reverse does not hold true. Although often composed solely of intramolecular transformations, cascade reactions can also occur intermolecularly, in which case they also fall under the category of multicomponent reactions.
Nucleophilic conjugate addition is a type of organic reaction. Ordinary nucleophilic additions or 1,2-nucleophilic additions deal mostly with additions to carbonyl compounds. Simple alkene compounds do not show 1,2 reactivity due to lack of polarity, unless the alkene is activated with special substituents. With α,β-unsaturated carbonyl compounds such as cyclohexenone it can be deduced from resonance structures that the β position is an electrophilic site which can react with a nucleophile. The negative charge in these structures is stored as an alkoxide anion. Such a nucleophilic addition is called a nucleophilic conjugate addition or 1,4-nucleophilic addition. The most important active alkenes are the aforementioned conjugated carbonyls and acrylonitriles.
The Johnson–Corey–Chaykovsky reaction is a chemical reaction used in organic chemistry for the synthesis of epoxides, aziridines, and cyclopropanes. It was discovered in 1961 by A. William Johnson and developed significantly by E. J. Corey and Michael Chaykovsky. The reaction involves addition of a sulfur ylide to a ketone, aldehyde, imine, or enone to produce the corresponding 3-membered ring. The reaction is diastereoselective favoring trans substitution in the product regardless of the initial stereochemistry. The synthesis of epoxides via this method serves as an important retrosynthetic alternative to the traditional epoxidation reactions of olefins.
The Petasis reaction is the multi-component reaction of an amine, a carbonyl, and a vinyl- or aryl-boronic acid to form substituted amines.
The Rubottom oxidation is a useful, high-yielding chemical reaction between silyl enol ethers and peroxyacids to give the corresponding α-hydroxy carbonyl product. The mechanism of the reaction was proposed in its original disclosure by A.G. Brook with further evidence later supplied by George M. Rubottom. After a Prilezhaev-type oxidation of the silyl enol ether with the peroxyacid to form the siloxy oxirane intermediate, acid-catalyzed ring-opening yields an oxocarbenium ion. This intermediate then participates in a 1,4-silyl migration to give an α-siloxy carbonyl derivative that can be readily converted to the α-hydroxy carbonyl compound in the presence of acid, base, or a fluoride source.
In organic chemistry, organocatalysis is a form of catalysis in which the rate of a chemical reaction is increased by an organic catalyst. This "organocatalyst" consists of carbon, hydrogen, sulfur and other nonmetal elements found in organic compounds. Because of their similarity in composition and description, they are often mistaken as a misnomer for enzymes due to their comparable effects on reaction rates and forms of catalysis involved.
In organic chemistry, hydroamination is the addition of an N−H bond of an amine across a carbon-carbon multiple bond of an alkene, alkyne, diene, or allene. In the ideal case, hydroamination is atom economical and green. Amines are common in fine-chemical, pharmaceutical, and agricultural industries. Hydroamination can be used intramolecularly to create heterocycles or intermolecularly with a separate amine and unsaturated compound. The development of catalysts for hydroamination remains an active area, especially for alkenes. Although practical hydroamination reactions can be effected for dienes and electrophilic alkenes, the term hydroamination often implies reactions metal-catalyzed processes.
The Baylis–Hillman reaction is a carbon-carbon bond forming reaction between the α-position of an activated alkene and a carbon electrophile such as an aldehyde. Employing a nucleophilic catalyst, such as a tertiary amine and phosphine, this reaction provides a densely functionalized product. It is named for Anthony B. Baylis and Melville E. D. Hillman, two of the chemists who developed this reaction while working at Celanese. This reaction is also known as the Morita–Baylis–Hillman reaction or MBH reaction, as K. Morita had published earlier work on it.
Hydrogen-bond catalysis is a type of organocatalysis that relies on use of hydrogen bonding interactions to accelerate and control organic reactions. In biological systems, hydrogen bonding plays a key role in many enzymatic reactions, both in orienting the substrate molecules and lowering barriers to reaction. However, chemists have only recently attempted to harness the power of using hydrogen bonds to perform catalysis, and the field is relatively undeveloped compared to research in Lewis acid catalysis.
Synergistic catalysis is a specialized approach to catalysis whereby at least two different catalysts act on two different substrates simultaneously to allow reaction between the two activated materials. While a catalyst works to lower the energy of reaction overall, a reaction using synergistic catalysts work together to increase the energy level of HOMO of one of the molecules and lower the LUMO of another. While this concept has come to be important in developing synthetic pathways, this strategy is commonly found in biological systems as well.
Proline organocatalysis is the use of proline as an organocatalyst in organic chemistry. This theme is often considered the starting point for the area of organocatalysis, even though early discoveries went unappreciated. Modifications, such as MacMillan’s catalyst and Jorgensen's catalysts, proceed with excellent stereocontrol.
Photoredox catalysis is a branch of photochemistry that uses single-electron transfer. Photoredox catalysts are generally drawn from three classes of materials: transition-metal complexes, organic dyes, and semiconductors. While organic photoredox catalysts were dominant throughout the 1990s and early 2000s, soluble transition-metal complexes are more commonly used today.
Shiina esterification is an organic chemical reaction that synthesizes carboxylic esters from nearly equal amounts of carboxylic acids and alcohols by using aromatic carboxylic acid anhydrides as dehydration condensation agents. In 1994, Prof. Isamu Shiina reported an acidic coupling method using Lewis acid, and, in 2002, a basic esterification using nucleophilic catalyst.
Alkene carboamination is the simultaneous formation of C–N and C–C bonds across an alkene. This method represents a powerful strategy to build molecular complexity with up to two stereocenters in a single operation. Generally, there are four categories of reaction modes for alkene carboamination. The first class is cyclization reactions, which will form a N-heterocycle as a result. The second class has been well established in the last decade. Alkene substrates with a tethered nitrogen nucleophile have been used in these transformations to promote intramolecular aminocyclization. While intermolecular carboamination is extremely hard, people have developed a strategy to combine the nitrogen and carbon part, which is known as the third class. The most general carboamination, which takes three individual parts and couples them together is still underdeveloped.
1 2 3 de Alaniz, J. R.; Kerr, M. S.; Moore, J. L.; Rovis, T. J. Org. Chem.2008, 73, 2033.
1 2 Jousseaume, T.; Wurz, N. E.; Glorius, F. Angew. Chem. Int. Ed.2011, 50, 1410.
↑ Murry, J. A.; Frantz, D. E.; Soheili, A.; Tillyer, R.; Grabowski, E. J. J.; Reider, P. J. J. Am. Chem. Soc.2001, 123, 9696.
↑ Myers, M. C.; Bharadwaj, A. R.; Milgram, B. C.; Scheidt, K. A. J. Am. Chem. Soc.2005, 127, 14675.
↑ Bortolini, O.; Fantin, G.; Fogagnolo, M.; Giovannini, P. P.; Massi, A.; Pacifico, S. Org. Biomol. Chem.2011, 9, 8437.
↑ Mattson, A. E.; Bharadwaj, A. R.; Scheidt, K. A. J. Am. Chem. Soc.2004, 126, 2314.
↑ Mattson, A. E.; Bharadwaj, A. R.; Zuhl, A. M.; Scheidt, K. A. "Thiazolium-Catalyzed Additions of Acylsilanes: A General Strategy for Acyl Anion Addition Reactions." J. Org. Chem.2006, 71, 5715. doi:10.1021/jo060699c
↑ Enders, D.; Breuer K.; Runsink, J.; Teles, J. H. Helv. Chim. Acta1996, 79, 1899.
↑ Kerr, M. S.; de Alaniz, J. R.; Rovis, T. J. Am. Chem. Soc.2002, 124, 10298.
↑ Pesch, J.; Harms, K.; Bach, T. Eur. J. Org. Chem.2004, 2025.
↑ Mennen, S. M.; Blank, J. T.; Tran-Dubé, M. B.; Imbriglio, J. E.; Miller, S. J. Chem. Commun.2005, 195.
↑ Kerr, M. S.; Rovis, T. J. Am. Chem. Soc.2004, 126, 8876.
↑ Moore, J. L.; Kerr, M. S.; Rovis, T. Tetrahedron2006, 62, 11477.
↑ de Alaniz, J. R.; Rovis, T. J. Am. Chem. Soc.2005, 127, 6284.
↑ Enders, D. Enzymemimetic C-C and C-N Bond Formations. In Stereoselective Synthesis; Ottow, E., Schoellkopf, K., Schulz, B.-G., eds.; Springer-Verlag: Berlin-Heidelberg, 1994; pp 63-90.
↑ Enders, D.; Han, J.; Henseler, A. Chem. Commun.2008, 3989.
↑ Liu, Q.; Perreault, S.; Rovis, T. J. Am. Chem. Soc.2008, 130, 14066.
↑ DiRocco, D. A.; Oberg, K. M.; Dalton, D. M.; Rovis, T. J. Am. Chem. Soc.2009, 131, 10872.
↑ Um, J. M.; DiRocco, D. A.; Noey, E. L.; Rovis, T.; Houk, K. N. J. Am. Chem. Soc.2011, 133, 11249.
↑ Trost, B.M.; Shuey, C. D.; DiNinno, F., Jr.; McElvain, S. S. J. Am. Chem. Soc.1979, 101, 1284.
↑ Harrington, P. E.; Tius, M. A. J. Am. Chem. Soc.2001, 123, 8509.
↑ Braun, R. U.; Müller, T. J. J. Synthesis2004, 14, 2391.
↑ Liu, P.; Lei, M.; Ma, L.; Hu, L. Synlett2011, 8, 1133.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
