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58-534: The Pictet–Spengler reaction is a chemical reaction in which a β-arylethylamine undergoes condensation with an aldehyde or ketone followed by ring closure. The reaction was first discovered in 1911 by Amé Pictet and Theodor Spengler (22 February 1886 – 18 August 1965). Traditionally, an acidic catalyst in protic solvent was employed with heating; however, the reaction has been shown to work in aprotic media in superior yields and sometimes without acid catalysis . The Pictet–Spengler reaction can be considered

116-482: A cell . The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions , radioactive decays and reactions between elementary particles , as described by quantum field theory . Chemical reactions such as combustion in fire, fermentation and the reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as

174-429: A double bond , where the cis vs trans relative positions of substituents give two non-superposable isomers. Many conformational isomers are diastereomers as well. In the case of diastereomerism occurring at a double bond, E-Z , or entgegen and zusammen (German), is used in notating nomenclature of alkenes . As stated previously, two diastereomers will not have identical chemical properties. This knowledge

232-467: A double displacement reaction , the anions and cations of two compounds switch places and form two entirely different compounds. These reactions are in the general form: AB + CD ⟶ AD + CB {\displaystyle {\ce {AB + CD->AD + CB}}} For example, when barium chloride (BaCl 2 ) and magnesium sulfate (MgSO 4 ) react, the SO 4 anion switches places with

290-434: A first-order reaction , which could be the disintegration of a substance A, is given by: Its integration yields: Here k is the first-order rate constant, having dimension 1/time, [A]( t ) is the concentration at a time t and [A] 0 is the initial concentration. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself can be described with

348-736: A single displacement reaction , a single uncombined element replaces another in a compound; in other words, one element trades places with another element in a compound These reactions come in the general form of: A + BC ⟶ AC + B {\displaystyle {\ce {A + BC->AC + B}}} One example of a single displacement reaction is when magnesium replaces hydrogen in water to make solid magnesium hydroxide and hydrogen gas: Mg + 2 H 2 O ⟶ Mg ( OH ) 2 ↓ + H 2 ↑ {\displaystyle {\ce {Mg + 2H2O->Mg(OH)2 (v) + H2 (^)}}} In

406-432: A tetrahydroisoquinoline . The Pictet–Spengler reaction has been applied to solid-phase combinatorial chemistry with great success. An analogous reaction with an aryl-β-ethanol is called oxa-Pictet–Spengler reaction . The reaction mechanism occurs by initial formation of an iminium ion ( 2 ) followed by electrophilic addition at the 3-position, in accordance with the expected nucleophilicity of indoles , to give

464-623: A "vital force" and distinguished from inorganic materials. This separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold , who, among many discoveries, established the mechanisms of substitution reactions . The general characteristics of chemical reactions are: Chemical equations are used to graphically illustrate chemical reactions. They consist of chemical or structural formulas of

522-526: A characteristic half-life . More than one time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows the Arrhenius equation : where E a is the activation energy and k B is the Boltzmann constant . One of the simplest models of reaction rate is the collision theory . More realistic models are tailored to a specific problem and include

580-500: A chemical reaction are called reactants or reagents . Chemical reactions are usually characterized by a chemical change , and they yield one or more products , which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions , and the information on the precise course of action is part of the reaction mechanism . Chemical reactions are described with chemical equations , which symbolically present

638-450: A compound reacts with others. Glucose and galactose , for instance, are diastereomers. Even though they share the same molar weight, glucose is more stable than galactose. This difference in stability causes galactose to be absorbed slightly faster than glucose in human body. Diastereoselectivity is the preference for the formation of one or more than one diastereomer over the other in an organic reaction . In general, stereoselectivity

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696-399: A few molecules, usually one or two, because of the low probability for several molecules to meet at a certain time. The most important elementary reactions are unimolecular and bimolecular reactions. Only one molecule is involved in a unimolecular reaction; it is transformed by isomerization or a dissociation into one or more other molecules. Such reactions require the addition of energy in

754-405: A fire-like element called "phlogiston", which was contained within combustible bodies and released during combustion . This proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as a reaction with oxygen from the air. Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a certain relationship with each other. Based on this idea and

812-717: A molecule. In general, the number of stereoisomers of a molecule can be determined by calculating 2 , where n  = the number of chiral centers in the molecule. This holds true except in cases where the molecule has meso forms. These meso compounds are molecules that contain stereocenters , but possess an internal plane of symmetry allowing it to be superposed on its mirror image. These equivalent configurations cannot be considered diastereomers. For n = 3, there are eight stereoisomers. Among them, there are four pairs of enantiomers: R,R,R and S,S,S; R,R,S and S,S,R; R,S,S and S,R,R; and R,S,R and S,R,S. There are many more pairs of diastereomers, because each of these configurations

870-438: A neutral radical . In the second case, both electrons of the chemical bond remain with one of the products, resulting in charged ions . Dissociation plays an important role in triggering chain reactions , such as hydrogen–oxygen or polymerization reactions. For bimolecular reactions, two molecules collide and react with each other. Their merger is called chemical synthesis or an addition reaction . Another possibility

928-546: A new chiral center at C-1 adopts either a cis or trans configuration towards the C-3 carboxyl group. The cis conduction is kinetically controlled, i.e. it is performed at lower temperatures. At higher temperatures the reaction becomes reversible and usually favours racemisation . 1,3- trans dominated products can be obtained with N b - benzylated tryptophans, which are accessible by reductive amination . The benzyl group can be removed hydrogenolytically afterwards. As

986-518: A rough rule, C NMR signals for C1 and C3 are downfield shifted in cis products relative to trans products (see steric compression effect ). Chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products are generated. Classically, chemical reactions encompass changes that only involve

1044-460: A special case of the Mannich reaction , which follows a similar reaction pathway. The driving force for this reaction is the electrophilicity of the iminium ion generated from the condensation of the aldehyde and amine under acid conditions. This explains the need for an acid catalyst in most cases, as the imine is not electrophilic enough for ring closure but the iminium ion is capable of undergoing

1102-542: Is a diastereomer with respect to every other configuration excluding its own enantiomer (for example, R,R,R is a diastereomer of R,R,S; R,S,R; and R,S,S). For n = 4, there are sixteen stereoisomers, or eight pairs of enantiomers. The four enantiomeric pairs of aldopentoses and the eight enantiomeric pairs of aldohexoses (subsets of the five- and six-carbon sugars) are examples of sets of compounds that differ in this way. Double bond isomers are always considered diastereomers, not enantiomers. Diastereomerism can also occur at

1160-444: Is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination of iron and sulfur to form iron(II) sulfide : 8 Fe + S 8 ⟶ 8 FeS {\displaystyle {\ce {8Fe + S8->8FeS}}} Another example is simple hydrogen gas combined with simple oxygen gas to produce a more complex substance, such as water. A decomposition reaction

1218-403: Is attributed to torsional and steric interactions in the stereocenter resulting from electrophiles approaching the stereocenter in reaction. When the single bond between the two centres is free to rotate, cis/trans descriptors become invalid. Two widely accepted prefixes used to distinguish diastereomers on sp³-hybridised bonds in an open-chain molecule are syn and anti . Masamune proposed

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1276-490: Is common to find the amino acid tryptophan and various aldoses used as the biological feedstock . Nucleophilic aromatic rings such as indole or pyrrole give products in high yields and mild conditions, while less nucleophilic aromatic rings such as a phenyl group give poorer yields or require higher temperatures and strong acid. The original Pictet–Spengler reaction was the reaction of phenethylamine and dimethoxymethane , catalysed by hydrochloric acid forming

1334-559: Is endothermic at low temperatures, becoming less so with increasing temperature. Δ H ° is zero at 1855  K , and the reaction becomes exothermic above that temperature. Changes in temperature can also reverse the direction tendency of a reaction. For example, the water gas shift reaction is favored by low temperatures, but its reverse is favored by high temperatures. The shift in reaction direction tendency occurs at 1100 K . Reactions can also be characterized by their internal energy change, which takes into account changes in

1392-462: Is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. A reaction may be classified as redox in which oxidation and reduction occur or non-redox in which there is no oxidation and reduction occurring. Most simple redox reactions may be classified as a combination, decomposition, or single displacement reaction. Different chemical reactions are used during chemical synthesis in order to obtain

1450-427: Is released. Typical examples of exothermic reactions are combustion , precipitation and crystallization , in which ordered solids are formed from disordered gaseous or liquid phases. In contrast, in endothermic reactions, heat is consumed from the environment. This can occur by increasing the entropy of the system, often through the formation of gaseous or dissolved reaction products, which have higher entropy. Since

1508-537: Is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in redox and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton. This type of reaction is also called metathesis . for example Most chemical reactions are reversible; that is, they can and do run in both directions. The forward and reverse reactions are competing with each other and differ in reaction rates . These rates depend on

1566-413: Is used in retro reactions. The elementary reaction is the smallest division into which a chemical reaction can be decomposed, it has no intermediate products. Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially. The actual sequence of the individual elementary reactions is known as reaction mechanism . An elementary reaction involves

1624-532: Is when a more complex substance breaks down into its more simple parts. It is thus the opposite of a synthesis reaction and can be written as AB ⟶ A + B {\displaystyle {\ce {AB->A + B}}} One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas: 2 H 2 O ⟶ 2 H 2 + O 2 {\displaystyle {\ce {2H2O->2H2 + O2}}} In

1682-534: The Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water, air and earth. In the Middle Ages , chemical transformations were studied by alchemists . They attempted, in particular, to convert lead into gold , for which purpose they used reactions of lead and lead-copper alloys with sulfur . The artificial production of chemical substances already

1740-1359: The Gibbs free energy of the reaction. They require input of energy to proceed in the forward direction. Examples include: In a combustion reaction, an element or compound reacts with an oxidant, usually oxygen , often producing energy in the form of heat or light . Combustion reactions frequently involve a hydrocarbon . For instance, the combustion of 1 mole (114 g) of octane in oxygen C 8 H 18 ( l ) + 25 2 O 2 ( g ) ⟶ 8 CO 2 + 9 H 2 O ( l ) {\displaystyle {\ce {C8H18(l) + 25/2 O2(g)->8CO2 + 9H2O(l)}}} releases 5500 kJ. A combustion reaction can also result from carbon , magnesium or sulfur reacting with oxygen. 2 Mg ( s ) + O 2 ⟶ 2 MgO ( s ) {\displaystyle {\ce {2Mg(s) + O2->2MgO(s)}}} S ( s ) + O 2 ( g ) ⟶ SO 2 ( g ) {\displaystyle {\ce {S(s) + O2(g)->SO2(g)}}} Diastereoisomer Diastereomers differ from enantiomers in that

1798-491: The Le Chatelier's principle . For example, an increase in pressure due to decreasing volume causes the reaction to shift to the side with fewer moles of gas. The reaction yield stabilizes at equilibrium but can be increased by removing the product from the reaction mixture or changed by increasing the temperature or pressure. A change in the concentrations of the reactants does not affect the equilibrium constant but does affect

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1856-454: The contact process in the 1880s, and the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont , Robert Boyle , and Isaac Newton tried to establish theories of experimentally observed chemical transformations. The phlogiston theory was proposed in 1667 by Johann Joachim Becher . It postulated the existence of

1914-471: The stoichiometry , the number of atoms of each species should be the same on both sides of the equation. This is achieved by scaling the number of involved molecules (A, B, C and D in a schematic example below) by the appropriate integers a, b, c and d . More elaborate reactions are represented by reaction schemes, which in addition to starting materials and products show important intermediates or transition states . Also, some relatively minor additions to

1972-558: The transition state theory , the calculation of the potential energy surface , the Marcus theory and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory . In a synthesis reaction, two or more simple substances combine to form a more complex substance. These reactions are in the general form: A + B ⟶ AB {\displaystyle {\ce {A + B->AB}}} Two or more reactants yielding one product

2030-481: The 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride . With the development of the lead chamber process in 1746 and the Leblanc process , allowing large-scale production of sulfuric acid and sodium carbonate , respectively, chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in

2088-555: The 2Cl anion, giving the compounds BaSO 4 and MgCl 2 . Another example of a double displacement reaction is the reaction of lead(II) nitrate with potassium iodide to form lead(II) iodide and potassium nitrate : Pb ( NO 3 ) 2 + 2 KI ⟶ PbI 2 ↓ + 2 KNO 3 {\displaystyle {\ce {Pb(NO3)2 + 2KI->PbI2(v) + 2KNO3}}} According to Le Chatelier's Principle , reactions may proceed in

2146-479: The Pictet–Spengler cyclization with strong acid, one can acylate the iminium ion forming the intermediate N -acyliminium ion. The N -acyliminium ion is a very powerful electrophile and most aromatic ring systems will cyclize under mild conditions with good yields. Tadalafil is synthesized via the N -acyliminium Pictet–Spengler reaction. This reaction can also be catalyzed by AuCl 3 and AgOTf . When

2204-527: The Pictet–Spengler reaction is performed with an aldehyde other than formaldehyde , a new chiral center is created. Several substrate- or auxiliary-controlled diastereoselective Pictet–Spengler reactions have been developed. Additionally, List et al. have published a chiral Brønsted acid that catalyzes asymmetric Pictet–Spengler reactions. Tryptophans: diastereocontrolled reaction The reaction of enantiopure tryptophan or its short-chain alkyl esters leads to 1,2,3,4-tetrahydro- β -carbolines in which

2262-408: The atomic theory of John Dalton , Joseph Proust had developed the law of definite proportions , which later resulted in the concepts of stoichiometry and chemical equations . Regarding the organic chemistry , it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically . According to the concept of vitalism , organic matter was endowed with

2320-499: The case of saccharides, when drawn in the Fischer projection the erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite sides. When drawn as a zig-zag chain, the erythro isomer has two identical substituents on different sides of the plane (anti). The names are derived from the diastereomeric four-carbon aldoses erythrose and threose . These prefixes are not recommended for use outside of

2378-481: The concentration and therefore change with the time of the reaction: the reverse rate gradually increases and becomes equal to the rate of the forward reaction, establishing the so-called chemical equilibrium. The time to reach equilibrium depends on parameters such as temperature, pressure, and the materials involved, and is determined by the minimum free energy . In equilibrium, the Gibbs free energy of reaction must be zero. The pressure dependence can be explained with

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2436-534: The descriptors which work even if the groups are not attached to adjacent carbon atoms. It also works regardless of CIP priorities. Syn describes groups on the same face while anti describes groups on opposite faces. The concept applies only to the Zigzag projection. The descriptors only describe relative stereochemistry rather than absolute stereochemistry. All isomers are same. Two older prefixes still commonly used to distinguish diastereomers are threo and erythro . In

2494-444: The desired product. In biochemistry , a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) form metabolic pathways . These reactions are often catalyzed by protein enzymes . Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperature and concentrations present within

2552-455: The entropy term in the free-energy change increases with temperature, many endothermic reactions preferably take place at high temperatures. On the contrary, many exothermic reactions such as crystallization occur preferably at lower temperatures. A change in temperature can sometimes reverse the sign of the enthalpy of a reaction, as for the carbon monoxide reduction of molybdenum dioxide : This reaction to form carbon dioxide and molybdenum

2610-405: The entropy, volume and chemical potentials . The latter depends, among other things, on the activities of the involved substances. The speed at which reactions take place is studied by reaction kinetics . The rate depends on various parameters, such as: Several theories allow calculating the reaction rates at the molecular level. This field is referred to as reaction dynamics. The rate v of

2668-406: The equilibrium position. Chemical reactions are determined by the laws of thermodynamics . Reactions can proceed by themselves if they are exergonic , that is if they release free energy. The associated free energy change of the reaction is composed of the changes of two different thermodynamic quantities, enthalpy and entropy : Reactions can be exothermic , where Δ H is negative and energy

2726-448: The form of heat or light. A typical example of a unimolecular reaction is the cis–trans isomerization , in which the cis-form of a compound converts to the trans-form or vice versa. In a typical dissociation reaction, a bond in a molecule splits ( ruptures ) resulting in two molecular fragments. The splitting can be homolytic or heterolytic . In the first case, the bond is divided so that each product retains an electron and becomes

2784-407: The forward or reverse direction until they end or reach equilibrium . Reactions that proceed in the forward direction (from left to right) to approach equilibrium are often called spontaneous reactions , that is, Δ G {\displaystyle \Delta G} is negative, which means that if they occur at constant temperature and pressure, they decrease the Gibbs free energy of

2842-544: The latter are pairs of stereoisomers that differ in all stereocenters and are therefore mirror images of one another. Enantiomers of a compound with more than one stereocenter are also diastereomers of the other stereoisomers of that compound that are not their mirror image (that is, excluding the opposing enantiomer). Diastereomers have different physical properties (unlike most aspects of enantiomers) and often different chemical reactivity . Diastereomers differ not only in physical properties but also in chemical reactivity — how

2900-454: The positions of electrons in the forming and breaking of chemical bonds between atoms , with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation . Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur. The substance (or substances) initially involved in

2958-404: The reactants on the left and those of the products on the right. They are separated by an arrow (→) which indicates the direction and type of the reaction; the arrow is read as the word "yields". The tip of the arrow points in the direction in which the reaction proceeds. A double arrow (⇌) pointing in opposite directions is used for equilibrium reactions . Equations should be balanced according to

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3016-457: The reaction can be indicated above the reaction arrow; examples of such additions are water, heat, illumination, a catalyst , etc. Similarly, some minor products can be placed below the arrow, often with a minus sign. Retrosynthetic analysis can be applied to design a complex synthesis reaction. Here the analysis starts from the products, for example by splitting selected chemical bonds, to arrive at plausible initial reagents. A special arrow (⇒)

3074-538: The reaction. The Pictet–Spengler reaction is widespread in both industry and biosynthesis. It has remained an important reaction in the fields of alkaloid and organic synthesis since its inception, where it has been employed in the development of many beta-carbolines . Natural Pictet–Spengler reaction typically employ an enzyme , such as strictosidine synthase . Pictet–Spengler products can be isolated from many products initially derived from nature, including foodstuffs such as soy sauce and ketchup . In such cases it

3132-477: The reaction. They require less energy to proceed in the forward direction. Reactions are usually written as forward reactions in the direction in which they are spontaneous. Examples: Reactions that proceed in the backward direction to approach equilibrium are often called non-spontaneous reactions , that is, Δ G {\displaystyle \Delta G} is positive, which means that if they occur at constant temperature and pressure, they increase

3190-480: The realm of saccharides because their definitions can lead to conflicting interpretations. Another threo compound is threonine , one of the essential amino acids. The erythro diastereomer of it is allothreonine . If a molecule contains two asymmetric centers, there are up to four possible configurations, and they cannot all be non-superposable mirror images of each other. The possibilities for different isomers continue to multiply as more stereocenters are added to

3248-446: The spirocycle 3 . After migration of the best migrating group , deprotonation gives the product ( 5 ). Replacing an indole with a 3,4-dimethoxyphenyl group give the reaction named the Pictet–Spengler tetrahydroisoquinoline synthesis. Reaction conditions are generally harsher than the indole variant, and require refluxing conditions with strong acids like hydrochloric acid , trifluoroacetic acid or superacids . Instead of catalyzing

3306-464: The starting materials, end products, and sometimes intermediate products and reaction conditions. Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Some reactions produce heat and are called exothermic reactions , while others may require heat to enable the reaction to occur, which are called endothermic reactions . Typically, reaction rates increase with increasing temperature because there

3364-456: Was a central goal for medieval alchemists. Examples include the synthesis of ammonium chloride from organic substances as described in the works (c. 850–950) attributed to Jābir ibn Ḥayyān , or the production of mineral acids such as sulfuric and nitric acids by later alchemists, starting from c. 1300. The production of mineral acids involved the heating of sulfate and nitrate minerals such as copper sulfate , alum and saltpeter . In

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