Base-catalyzed Dehydration Research Articles

Reduction of PVA Aerogel Flammability by Incorporation of an Alkaline Catalyst.

Sodium hydroxide was used as a base catalyst to reduce the flammability of poly(vinyl alcohol) (PVA) aerogels. The base-modified aerogels exhibited significantly enhanced compressive moduli, likely resulting in decreased gallery spacing and increased numbers of “struts” in their structures. The onset of decomposition temperature decreased for the PVA aerogels in the presence of the base, which appears to hinder the polymer pyrolysis process, leading instead to the facile formation of dense char. Cone calorimetry testing showed a dramatic decrease in heat release when the base was added. The results indicate that an unexpected base-catalyzed dehydration occurs at fire temperatures, which is the opposite of the chemistry normally observed under typical synthesis conditions.

Differences in Mechanism and Rate of Zeolite-Catalyzed Cyclohexanol Dehydration in Apolar and Aqueous Phase

The rate of acid–base-catalyzed dehydration of alcohols strongly depends on the solvent and the environment of the acid sites. We find that Bronsted acidic sites in large-pore zeolites, but not in ...

Renewable linear alpha-olefins by base-catalyzed dehydration of biologically-derived fatty alcohols

The two-step production of linear alpha olefins through the coupling of fermentation and catalytic dehydration was demonstrated experimentally and evaluated with a techno-economic analysis.

Concerted or Stepwise: How Much Do Free-Energy Landscapes Tell Us about the Mechanisms of Elimination Reactions?

The base-catalyzed dehydration of benzene cis-1,2-dihydrodiols is driven by formation of an aromatic product as well as intermediates potentially stabilized by hyperaromaticity. Experiments exhibit surprising shifts in isotope effects, indicating an unusual mechanistic balance on the E2-E1cB continuum. In this study, both 1- and 2-dimensional free energy surfaces are generated for these compounds with various substituents, using density functional theory and a mixed implicit/explicit solvation model. The computational data help unravel hidden intermediates along the reaction coordinate and provide a novel conceptual framework for distinguishing between competing pathways in this and any other system with borderline reaction mechanisms.

4-Hydroxy-4-methyl-5-tosylhexahydropyrimidin-2-imines: synthesis and different dehydration pathways

Synthesis of 4-hydroxy-4-methyl-5-tosylhexahydropyrimidin-2-imines by reaction of N-cyano- N′-(1-tosylalk-1-yl)guanidines with enolates of tosylacetone was developed. Base-catalyzed dehydration of the obtained pyrimidines gave the expected 6-methyl-5-tosyl-1,2,3,4-tetrahydropyrimidin-2-imines. However, their acid-catalyzed dehydration led to mixtures of the latter compounds and the products of tosyl group migration, 4-tosylmethyl derivatives. This reaction was strongly influenced by the nature of the solvent. Plausible explanations of the obtained data were given.

Structural Insight into Substrate Binding and Catalysis of a Novel 2-Keto-3- deoxy- d-arabinonate Dehydratase Illustrates Common Mechanistic Features of the FAH Superfamily

The archaeon Sulfolobus solfataricus converts d-arabinose to 2-oxoglutarate by an enzyme set consisting of two dehydrogenases and two dehydratases. The third step of the pathway is catalyzed by a novel 2-keto-3- deoxy- d-arabinonate dehydratase (KdaD). In this study, the crystal structure of the enzyme has been solved to 2.1 Å resolution. The enzyme forms an oval-shaped ring of four subunits, each consisting of an N-terminal domain with a four-stranded β-sheet flanked by two α-helices, and a C-terminal catalytic domain with a fumarylacetoacetate hydrolase (FAH) fold. Crystal structures of complexes of the enzyme with magnesium or calcium ions and either a substrate analog 2-oxobutyrate, or the aldehyde enzyme product 2,5-dioxopentanoate revealed that the divalent metal ion in the active site is coordinated octahedrally by three conserved carboxylate residues, a water molecule, and both the carboxylate and the oxo groups of the substrate molecule. An enzymatic mechanism for base-catalyzed dehydration is proposed on the basis of the binding mode of the substrate to the metal ion, which suggests that the enzyme enhances the acidity of the protons α to the carbonyl group, facilitating their abstraction by glutamate 114. A comprehensive structural comparison of members of the FAH superfamily is presented and their evolution is discussed, providing a basis for functional investigations of this largely unexplored protein superfamily.

An Organic-Based pH Oscillator

In a recent paper, we suggested that the acid- or base-catalyzed dehydration of a hydrated carbonyl compound provides a suitable foundation for an organic-based pH oscillator. Here we present the first experimental example of such an oscillator in a flow reactor, utilizing the base-catalyzed dehydration of methylene glycol as a source of positive feedback (OH- autocatalysis) coupled with the base-catalyzed hydrolysis of gluconolactone for negative feedback (H+ production). The large amplitude oscillations (between pH 7 and 10) are reproduced in a kinetic model of the reaction. Such experiments present new possibilities in the design of pH oscillators.

Stabilizing and solubilizing effects of sulfobutyl ether beta-cyclodextrin on prostaglandin E1 analogue.

Parent cyclodextrins are known to accelerate the degradations such as dehydration and isomerization of E-type prostaglandins in neutral and alkaline solutions. The objective of this study was to attempt the stabilization and solubilization of E1-type prostaglandin analogue in aqueous solution by biocompatible cyclodextrin derivatives. The interaction of an E1-type prostaglandin, methyl 7-[(1R,2R,3R)-3-hydroxy-2-[(E)-(3S)-3-hydroxy-4-(m-methoxymethylphenyl)1-butenyl]-5-oxocyclopentyl]-5-thiaheptanoate (MEester) with cyclodextrins (CyDs) was studied by spectroscopies and the solubility method. The degradation of MEester was monitored by high-performance liquid chromatography. 1H-nuclear magnetic resonance spectroscopic studies indicated that MEester forms 1:1 inclusion complexes with alpha-, beta-, and gamma-CyDs in solutions, where alpha-CyD interacts with the a-side chain containing methyl ester moiety of the drug, whereas beta- and gamma-CyDs preferentially include around the five-membered ring and both side chains of the drug. Parent alpha-CyD and hydrophilic derivatives, such as 2-hydoxypropyl-alpha- and -beta-CyDs, sulfobutyl ether beta-CyD (SBE-beta-CyD) and maltosyl beta-CyD showed higher solubilizing abilities against MEester over parent beta- and gamma-CyDs. SBE-beta-CyD and 2,6-dimethyl-beta-CyD (DM-beta-CyD) significantly decelerated the degradation of MEester, particularly the base-catalyzed dehydration, in neutral and alkaline solutions, whereas other CyDs accelerated the degradation. The acid-catalyzed degradation of MEester (pH < 3) was decelerated by the addition of CyDs, especially alpha-CyD. SBE-beta-CyD with low hemolytic activity and low toxicity is useful as a pharmaceutical carrier for the preparation of injectable MEester, because of its higher stabilizing and solubilizing effects on MEester. Furthermore, SBE-beta-CyD can be useful as a stabilizing agent for drugs, that are subject to base-catalyzed degradations, probably because of the electric repulsion between anionic charges of the sulfobutyl moiety and catalytic anionic species such as hydroxide ion.

Effects of Oligosaccharides Having the Glucuronic Acid Residue on Base-Catalyzed Prostaglandin E2 Dehydration and Isomerization.

Examination was made of the effects of 2-(trimethylsilyl)ethyl 4-Ο-methyl-β-D-glucopyranosyluronic acid-(1→6)-β-D-galactopyranoside (A), 2-(trimethylsilyl)ethyl 4-Ο-methyl-β-D-glucopyranosyluronic acid-(1→6)-β-D-galactopyranosyl-(1→6)-β-D-galactopyranoside (B) and N, N',N''-tri-{5-[4-Ο-methyl-β-D-glucopyranosyluronic acid-(1→6)-β-D-galactopyranosyloxy]pentylcarbonylaminoethyl}-1,3,5-benzenetriamide (C), each possessing the glucuronic acid residue, on drug degradation. Oligosaccharide mixed micelles containing the nonionic surfactant, heptaethyleneglycol dodecylether (HED), were studied so as to assess oligosaccharides A∼C for drug stabilization potential. The nonionic surfactant was required since oligosaccharides A∼C do not form micelles in single systems. Base-catalyzed dehydration and then isomerization of prostaglandin E2 (PGE2), PGE2→PGA2→PGB2, were conducted as model experiments. The rate of the degradation of PGE2 with base was determined based on concentrations of PGA2 and PGB2 using high-performance liquid chromatography. Mixed oligosaccharide-HED micelles inhibited the dehydration and isomerization of PGE2, possibly owing to suppression of the approach of OH- as catalysis toward PGE2 in mixed oligosaccharide-HED micelles by electrostatic repulsion between negatively charged micellar surfaces and OH-. The clusterized molecular structure of oligosaccharide C was the reason for the inhibition of both these processes. Oligosaccharide C may possibly be situated on the micellar surface and this would lead to greater steric shielding and the above electrostatic repulsion compared to A or B.

Mechanism of Hydroxyl Radical Addition to Imidazole and Subsequent Water Elimination

The addition reaction of the hydroxyl radical to imidazole and subsequent elimination of water to form the 1-dehydroimidazolyl radical is investigated using MP2 and B3LYP methods, including large basis sets and SCI-PCM modeling of solvent effects. It is found that the barrier to addition of the hydroxyl radical at the 5-position is energetically favored over addition to the 2- or 4-positions by 2−3 kcal/mol at the SCI-PCM/MP2/6-311G(2df,p)//MP2/6-31G(d,p) level, whereas the corresponding B3LYP calculations yield a barrier-free addition at the 5-position. The lower barrier and NBO analysis explain the experimentally observed specificity for the 5-hydroxylation of imidazole and histidine, albeit the 2-adduct is about 4 kcal/mol more stable than the 5-adduct. The NBO energetic analysis shows that the exoanomeric effect stabilizes the transition state at the 5-position about 0.3 kcal/mol more than that at the 2-position. Moreover, the π-interaction between the attacking nonbonding spin orbital of the hydroxyl radical and the π-cloud of imidazole is the least for the transition state at the 5-position, favoring the σC5-O bond formation. The 5-hydroxyimidazolyl radical undergoes a slow elimination of water (the added OH group and the hydrogen at the N1 position) to yield the 1-dehydroimidazolyl radical. The base-catalyzed dehydration profile was modeled in two steps at the B3LYP/6-311G(2df,p)//6-31G(d,p) level. The PES for the dehydration reaction seems rather flat. The first step is a barrier-free loss of the proton at N1 induced by the hydroxide ion to yield the 1-dehydro-5-hydroxyimidazolyl radical anion. In the second step, the hydroxide ion is regenerated from the intermediate to yield the final product with a barrier of 2.7 kcal/mol. The calculated hyperfine structures in the presence of the continuum solvent model for the 5-hydroxyimidazolyl and 1-dehydroimidazolyl radicals are in close agreement with the experimental ones recorded in aqueous solution.

19-Hydroxyprostaglandin B 2 as an internal standard for on-line extraction-high-performance liquid chromatography analysis of lipoxygenase products

Variable recovery of polar compounds is a potential pitfall in the on-line extraction-reversed-phase HPLC analysis of lipoxygenase products. Therefore, the addition of a polar internal standard to biological samples, which permits the assessment of the recovery of the polar components in individual samples, appears to be essential. 19-Hydroxyprostaglandin (PG) B 2 was found to be a suitable compound for this application. It was easily prepared from deproteinized human semen by base-catalyzed dehydration of 19-hydroxy-PGE 2, and purified in a single step by reversed-phase HPLC. Similarly to PGB 2, already used as internal in HPLC, 19-hydroxy-PGB 2 has excellent chemical stability and carries a strong uv chromophore that enables detection at 280 nm. Most importantly, it is eluted just prior to the polar arachidonic acid metabolites 20-hydroxy- and 20-carboxy-leukotriene B 4,12-oxo-dodecatrienoic acid and the lipoxines A and B. The measurement of the ratio of PGB 2 and 19-hydroxy-PGB 2 used in combination as internal standards in HPLC analysis of lipoxygenase products, provides a simple and reliable way to assess sample to sample recovery of the polar components when using on-line extraction procedures.

Polarographic reduction of aldehydes and ketones: Part XXV. Electroreduction of pyridinecarboxaldehydes in aqueous buffered solutions

The mechanisms of electroreduction of 2-, 3-, and 4-pyridinecarboxaldehydes have been studied in aqueous buffered solutions of pH between 2 and 12 by means of dc and differential pulse polarography, linear-sweep voltammetry and controlled-potential electrolysis. Spectroscopic measurements confirmed that these compounds in aqueous solutions exist in equilibria involving both the hydrated and unhydrated forms. The degree of hydration depends strongly on protonation both of the pyridine ring and of the side-chain. Dehydration reactions producing the aldehyde species occur prior to the electron (and possibly the proton) transfer. The loss of water can take place from a protonated geminal diol form or from a neutral diol, depending on pH. Alternatively, the geminal diol anion can eliminate hydroxide ion. A diprotonated pyridinecarboxaldehyde is the electroactive species at pH < 7, the monoprotonated and unprotonated forms are reduced at higher pH-values. For the 2- and 4-aldehyde base-catalyzed dehydration occurs at pH > 8 manifested by an increase of the total limiting current. In this reaction the rate-determining step is the dissociation of a proton from the geminal diol forming a geminal diol anion, which is followed by a rapid elimination of a hydroxide ion to yield the reducible aldehyde. Reaction schemes involving sequences of hydration-dehydration and acid-base reactions as well as electron transfers at mercury electrodes have been proposed for the three isomers. The predominating processes in individual pH-ranges have been pointed out. Electroreduction of the 3-aldehyde occurs up to pH 10 in a single two-electron step. The 2- and 4-aldehydes are reduced between pH 2 and 12 in two, one-electron steps. Electrodimerization occurs at the potential of the limiting current of the first one-electron wave for the 2- and 4-isomer, similarly as for the 3-isomer at pH > 10, where its reduction occurs in a single one-electron wave. Surface dimerization or reaction of the electrolysis products with the parent aldehyde was considered in interpretation of the reduction of the 4-pyridinecarboxaldehyde at pH > 7. Electropolymerization accompanies the reduction of 2-pyridinecarboxaldehyde in alkaline media.

Computer modeling of the recombination reaction of rhodopsin

Various mechanistic schemes for the recombination reaction of rhodopsin were designed and tested using computer modeling and simulation with data from kinetics experiments. The reaction schemes were mathematically modeled by systems of nonlinear first-order ordinary differential equations (ODEs) with unknown rate constants. Each model was fitted to the experimental data by using a modified simplex algorithm for parameter (rate constant) estimation and Gear's method for solving stiff systems of ODEs. The recombination reaction of rhodopin was best modeled by branched, multistep reaction schemes which included formation of noncovalent complexes, acid-base equilibria, and acid and base-catalyzed dehydration of a Schiff base intermediate. The biochemical bases for these models are discussed.

Competitive reduction of the carbonyl group and of the C-S bond in 3-thio-2-oxoalkanoic acids and their derivatives at the dropping mercury and mercury pool electrodes

3-Thio-2-oxoalkanoic acids I, III and IV (AH3) undergo dissociation and form the carboxylate (AH2), the thiolate-carboxylate dianion (AH) and for I even a trianion (A), when AH acts as a C-acid. Formation of a carbanion-enolate was confirmed by following the dissociation of thioether II. Generally, the hydration decreases in the sequence AH3>AH2>AH. Similarly for the thioether II, the conjugate acid is more strongly hydrated than the carboxylate anion. In ester V the conjugate acid is also more strongly hydrated than the thiolate anion. Hydration of ester V is comparable to the hydration of the acid form H3A of I. Introduction of an isopropyl group in α-position in IV decreases the hydration similar to the replacement of the SH group by a SC2H5 grouping. In acidic solutions polarographic reduction of acid form AH3 occurs after pre-protonation of the carbonyl group, either by electron transfer to the carbonyl group followed by elimination of the sulfide anion or by hydrogenolytic cleavage of the C−S bond. At pH 3–6 the monoanion AH2 is furthermore protonated on the carboxyl group and then reduced as AH3. At pH 6–11 the monoanion AH2 is reduced, either at the CO group or with cleavage of the C−S bond. At pH 8–11 this monoanion is formed by a rapid protonation of the dianion AH. For the thioether II where such acid-base equilibrium involving the thiol form is impossible, the half-wave of wave i2A potential at pH>6 is pH-independent. In more alkaline media, base-catalyzed dehydration takes place, occurring via geminal diol anion. Changes in the rate of dehydration resulting from structural changes considerably affect the pH-dependences of polarographic waves, particularly of wave i1 at pH<6. Differences between the behavior of 3-thio-2-oxobutanoic acid (III) (see Fig. 6) and 3-thio-4-methyl-2-oxopentanoic acid (IV) (see Fig. 7) similarly as for those between 3-thio-2-oxopropanoic acid (I) (see Fig. 1) and its 3-ethylthioether (II) (see Fig. 3) are most strking. Whereas at the DME the competition between C=O reduction and C−S cleavage is governed by differences in the rates of establishment of the acid-base equilibria, yields of preparative reductions obtained with a stirred mercury pool electrode are given by the position of these equilibria in the bulk of the solution. At the pool electrode AH3 is predominantly reduced on the CO group, monoanion AH2 shows comparable reduction of both the CO group and cleavage of the C−S bond, whereas the dianion AH is predominantly reduced at the C−S bond.

Polarographic reduction of aldehydes and ketones Part XXIII. electroreduction of 1-methylpyrtoiniumcarboxaldehydes

All three isomeric 1-methylpyridiniumcarboxaldehydes are present in aqueous solutions predominantly in the hydrated form (PyCH(OH) 2 ). At pH smaller than about 7, the protonated form of the free aldehyde is reduced in two one-electron steps following the sequence H + , e , H + , e . At pH greater than about 8 the free aldehydic form is reduced in two one-electron steps following the sequence, e , H + , e , H + . Up to pH about 10 the reducible species is formed by dehydration of PyCH(OH) 2 , at higher pH by elimination of hydroxide ion from PyCH(OH)O − . The increase in polarographic reduction current between pH about 8 and 10 is due to a base-catalyzed dehydration, involving a rate-determining formation of PyCH(OH)O − . The polarographic dissociation curve ( i 1 vs. pH) is in this case shifted to lower pH-values when compared with the conditional dissociation curve ([PyCH(OH) 2 ] vs. pH). Such behavior shows that the effective reduction of the conjugate base of an electroinactive acid (PyCH(OH) 2 ) occurs. The increase in polarographic reduction current at pH smaller than about 1 is due to an acid-catalyzed dehydration involving formation of PyCH(OH)(OH 2 + ). The rate of protonation governs the limiting current, since the increase in current with increasing acidity occurs in a region one to two acidity units higher than p K 1 values obtained spectrophotometrically, using acidity functions H C(OH) 2 . The free carbonyl species produced by dehydration is reduced following a sequence H + , e , H + , e .

Competitive reduction of the carbonyl group and of the C−S bond in 3-thio-2-oxoalkanoic acids and their derivatives at the dropping mercury and mercury pool electrodes

3-Thio-2-oxoalkanoic acids I, III and IV (AH 3) undergo dissociation and form the carboxylate (AH 2), the thiolate-carboxylate dianion (AH) and for I even a trianion (A), when AH acts as a C-acid. Formation of a carbanion-enolate was confirmed by following the dissociation of thioether II. Generally, the hydration decreases in the sequence AH 3>AH 2>AH. Similarly for the thioether II, the conjugate acid is more strongly hydrated than the carboxylate anion. In ester V the conjugate acid is also more strongly hydrated than the thiolate anion. Hydration of ester V is comparable to the hydration of the acid form H 3A of I. Introduction of an isopropyl group in α-position in IV decreases the hydration similar to the replacement of the SH group by a SC 2H 5 grouping. In acidic solutions polarographic reduction of acid form AH 3 occurs after pre-protonation of the carbonyl group, either by electron transfer to the carbonyl group followed by elimination of the sulfide anion or by hydrogenolytic cleavage of the C−S bond. At pH 3–6 the monoanion AH 2 is furthermore protonated on the carboxyl group and then reduced as AH 3. At pH 6–11 the monoanion AH 2 is reduced, either at the CO group or with cleavage of the C−S bond. At pH 8–11 this monoanion is formed by a rapid protonation of the dianion AH. For the thioether II where such acid-base equilibrium involving the thiol form is impossible, the half-wave of wave i 2A potential at pH>6 is pH-independent. In more alkaline media, base-catalyzed dehydration takes place, occurring via geminal diol anion. Changes in the rate of dehydration resulting from structural changes considerably affect the pH-dependences of polarographic waves, particularly of wave i 1 at pH<6. Differences between the behavior of 3-thio-2-oxobutanoic acid (III) (see Fig. 6) and 3-thio-4-methyl-2-oxopentanoic acid (IV) (see Fig. 7) similarly as for those between 3-thio-2-oxopropanoic acid (I) (see Fig. 1) and its 3-ethylthioether (II) (see Fig. 3) are most strking. Whereas at the DME the competition between C=O reduction and C−S cleavage is governed by differences in the rates of establishment of the acid-base equilibria, yields of preparative reductions obtained with a stirred mercury pool electrode are given by the position of these equilibria in the bulk of the solution. At the pool electrode AH 3 is predominantly reduced on the CO group, monoanion AH 2 shows comparable reduction of both the CO group and cleavage of the C−S bond, whereas the dianion AH is predominantly reduced at the C−S bond.

Polarographic reduction of aldehydes and ketones: Part XXIII. Electroreduction of 1-methylpyridiniumcarboxaldehydes

All three isomeric 1-methylpyridiniumcarboxaldehydes are present in aqueous solutions predominantly in the hydrated form (PyCH(OH) 2). At pH smaller than about 7, the protonated form of the free aldehyde is reduced in two one-electron steps following the sequence H +, e, H +, e. At pH greater than about 8 the free aldehydic form is reduced in two one-electron steps following the sequence, e, H +, e, H +. Up to pH about 10 the reducible species is formed by dehydration of PyCH(OH) 2, at higher pH by elimination of hydroxide ion from PyCH(OH)O −. The increase in polarographic reduction current between pH about 8 and 10 is due to a base-catalyzed dehydration, involving a rate-determining formation of PyCH(OH)O −. The polarographic dissociation curve ( i 1 vs. pH) is in this case shifted to lower pH-values when compared with the conditional dissociation curve ([PyCH(OH) 2] vs. pH). Such behavior shows that the effective reduction of the conjugate base of an electroinactive acid (PyCH(OH) 2) occurs. The increase in polarographic reduction current at pH smaller than about 1 is due to an acid-catalyzed dehydration involving formation of PyCH(OH)(OH 2 +). The rate of protonation governs the limiting current, since the increase in current with increasing acidity occurs in a region one to two acidity units higher than p K 1 values obtained spectrophotometrically, using acidity functions H C(OH) 2 . The free carbonyl species produced by dehydration is reduced following a sequence H +, e, H +, e.

Tetrabutylammonium hydroxide: a reagent for the base-catalyzed dehydration of vicinal dihydrodiols of aromatic hydrocarbons. Implications to ion-pair chromatography

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTetrabutylammonium hydroxide: a reagent for the base-catalyzed dehydration of vicinal dihydrodiols of aromatic hydrocarbons. Implications to ion-pair chromatographyDavid W. McCourt, Peter P. Roller, and Harry V. GelboinCite this: J. Org. Chem. 1981, 46, 21, 4157–4161Publication Date (Print):October 1, 1981Publication History Published online1 May 2002Published inissue 1 October 1981https://doi.org/10.1021/jo00334a010RIGHTS & PERMISSIONSArticle Views336Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (677 KB) Get e-Alerts Get e-Alerts

Acid- and base-catalyzed dehydration of prostaglandin E2 to prostaglandin A2 and general-base-catalyzed isomerization of prostaglandin A2 to prostaglandin B2

Acid- and base-catalyzed dehydration of prostaglandin E2 to prostaglandin A2 and general-base-catalyzed isomerization of prostaglandin A2 to prostaglandin B2

The formation of thiophenes by a base-catalyzed sulfoxide dehydration

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