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einem, isomeren, methyl, octahydro, indolizin, 19375300106, harms, xxvi, isochinolin, entstehenden, 19365250107, 525, xxv, hydrazo, verbindungen, 157, 19355190113, 147, kech, xxiv, 146, 19355190112, 140, möller, xxiii, stilbazol, dicarbonester, 19355160104, 516, friedrichsen, xxii, c4o3, ihre, eignung, ein, neues, prinzip, dihydro, 155, 19345130109, xxi, verlauf, methylalkoholischer, lösung, 19345130108, 129, hydrazobenzol, 182, 19345110114, 168, 511, xix, primärprodukte, bei, 128, 19345100106, 510, 1933, xviii, chinolizins, indolizins, pseudolupinins, 150, 19335050109, 505, xvii, isochinolins, 19324980103, xvi, pyrazole, 19324980102, 294, 19314900113, 277, xiv, 276, 19314900112, 267, xiii, cumaline, 266, 19314900111, furans, 19314900110, cyclopentadiens, cyclo, hexadiens, butadiens, 242, 19314900109, 236, pyrrol, seinen, homologen, 225, 19314860112, 211, camphenilons, santens, 210, 19314860111, viii, anthracens, forme, 19314860110, 191, 1930, harren, ernst, petersen, 154, 19304780109, 137, 478, gerhard, stein, partiell, hydrierte, naphtho, anthrachinone, wasserstoff, bzw, stellung, winckler, 2372, 19290620872, 2337, tetrahydro, phthalsäure, stellungnahme, farmer, warren, eigenschaften, konjugierter, 2090, 19290620830, maleinsäure, anhydrid, arylierte, triene, fulvene, 19290620829, 2081, iii, terpenen, camphern, heterocyclischen, systemen, herren, wolfgang, lübbert, erich, naujoks, karl, röhl, harro, segeberg, 19294700106, 470, 562, 19290620318, anlagerungen, kohlenwasserstoffen, 19284600106, 460, behr, arno, homogeneous, 3527306732, 14356007, a18_215, minami, atsushi, oikawa, hideaki, 506, 30482282, 27301662, 500, antibiotics, advances, alderases, fluegel, lucas, 2444, 33492939, 8008985, chemrev, 0c00825, 2413, 121, rev, generation, cycloisomerization, tethered, triynes, baire, niu, willoughby, woods, 7419, 212, 23060191, 3538845, nature11518, 2012natur, 208h, 208, ahrendt, borths, macmillan, organocatalytic, 4244, ja000092s, 2000jachs, 4243a, 4243, versatile, 335, 10891050, ar960062n, 325, ryu, 2003, triflimide, 6390, 12785777, ja035393r, 6388, 125, shibata, lee, triflic, 3809, 11942799, ja025848x, 3808, loh, application, design, 8967, ja00023a066, 8966c, 8966, chapman, bisaha, acyloxazolidinones, 1256, ja00212a037, 1238e, 1238, james, shaw, subrata, diaminobicyclo, octane, scaffold, salen, 21462988, ol2007378, 2488, lett, tiekink, eveline, 5283, 239089361, 2066, 241097, ejoc, 202101107, 5275, hansen, yoshisada, ryoji, filippov, dmitri, marel, gijsbert, codée, jeroen, 3573, 33538169, 7901664, joc, 0c02955, 3565, openings, oxide, brinkhuis, francine, 1174, 32012430, 7187256, asia, 202000009, 1167, asian, alkali, cations, 1981, 232337915, 33759502, 1871, a0090b38, 9ab8, 4c32, 9d9a, b3d5de4e5ed3, 1c00016, 20106, 34499069, 8457343, d1cp02456f, 2021pccp, 2320095v
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t 40 41 42 43 it is found that lewis acids accelerate the diels alder reaction by reducing the destabilizing steric pauli repulsion between the interacting diene and dienophile and not by lowering the energy of the dienophile s lumo and consequently enhancing the normal electron demand orbital interaction the lewis acid binds via a donor acceptor interaction to the dienophile and via that mechanism polarizes occupied orbital density away from the reactive c c double bond of the dienophile towards the lewis acid this reduced occupied orbital density on c c double bond of the dienophile will in turn engage in a less repulsive closed shell closed shell orbital interaction with the incoming diene reducing the destabilizing steric pauli repulsion and hence lowers the diels alder reaction barrier in addition the lewis acid catalyst also increases the asynchronicity of the diels alder reaction making the occupied π orbital located on the c c double bond of the dienophile asymmetric as a result this enhanced asynchronicity leads to an extra reduction of the destabilizing steric pauli repulsion as well as a diminishing pressure on the reactants to deform in other words it reduced the destabilizing activation strain also known as distortion energy 44 this working catalytic mechanism is known as pauli lowering catalysis 45 which is operative in a variety of organic reactions 46 47 48 the original rationale behind lewis acid catalyzed diels alder reactions is incorrect 40 49 50 51 because besides lowering the energy of the dienophile s lumo the lewis acid also lowers the energy of the homo of the dienophile and hence increases the inverse electron demand lumo homo orbital energy gap thus indeed lewis acid catalysts strengthen the normal electron demand orbital interaction by lowering the lumo of the dienophile but they simultaneously weaken the inverse electron demand orbital interaction by also lowering the energy of the dienophile s homo these two counteracting phenomena effectively cancel each other resulting in nearly unchanged orbital interactions when compared to the corresponding uncatalyzed diels alder reactions and making this not the active mechanism behind lewis acid catalyzed diels alder reactions asymmetric diels alder edit many methods have been developed for influencing the stereoselectivity of the diels alder reaction such as the use of chiral auxiliaries catalysis by chiral lewis acids 52 and small organic molecule catalysts 6 evans oxazolidinones 53 oxazaborolidines 54 55 56 bis oxazoline copper chelates 57 imidazoline catalysis 58 and many other methodologies exist for effecting diastereo and enantioselective diels alder reactions hexadehydro diels alder edit in the hexadehydro diels alder reaction alkynes and diynes are used instead of alkenes and dienes forming an unstable benzyne intermediate which can then be trapped to form an aromatic product this reaction allows the formation of heavily functionalized aromatic rings in a single step 59 60 applications and natural occurrence edit asymmetric diels alder reaction is one step in the biosynthesis of the statin lovastatin 61 the retro diels alder reaction is used in the industrial production of cyclopentadiene cyclopentadiene is a precursor to various norbornenes which are common monomers the diels alder reaction is also employed in the production of vitamin b6 typical route for production of ethylidene norbornene from cyclopentadiene through vinyl norbornene 62 history edit the reaction discovered by diels and alder in 1928 the diels alder reaction was the culmination of several intertwined research threads some near misses and ultimately the insightful recognition of a general principle by otto diels and kurt alder their seminal work detailed in a series of 28 articles published in the justus liebigs annalen der chemie and berichte der deutschen chemischen gesellschaft from 1928 to 1937 established the reaction s wide applicability and its importance in constructing six membered rings the first 19 articles were authored by diels and alder while the later articles were authored by diels and various other coauthors 63 64 however the history of the reaction extends further back revealing a fascinating narrative of discoveries missed and opportunities overlooked 65 several chemists working independently in the late 19th and early 20th centuries encountered reactions that in retrospect involved the diels alder process but remained unrecognized as such 65 theodor zincke performed a series of experiments between 1892 and 1912 involving tetrachlorocyclopentadienone a highly reactive diene analogue 66 67 68 69 in 1910 sergey lebedev systematically investigated thermal polymerization of three conjugated dienes butadiene isoprene and dimethylbutadiene a process now recognized as a diels alder self reaction providing a detailed analysis of the dimerization products and recognizing the importance of the conjugated system in the process 70 five years earlier carl harries studied the degradation of natural rubber leading him to propose a cyclic structure for the polymer 71 hermann staudinger s work with ketenes published in 1912 covered both 2 2 cycloadditions where one molecule of a ketene reacted with an unsaturated compound to form a four membered ring and importantly 4 2 cycloadditions in the latter case two molecules of ketene combined with one molecule of an unsaturated compound such as a quinone to yield a six membered ring 72 while not a classic diels alder reaction in the typical sense of a conjugated diene and a separate dienophile staudinger s observation of this 4 2 process forming a six membered ring foreshadowed the later work of diels and alder however his focus remained primarily on the more common 2 2 ketene cycloaddition hans von euler chelpin and k o josephson investigating isoprene and butadiene reactions in 1920 both observed products consistent with diels alder cycloadditions but didn t go on to research it further 73 perhaps the most striking near miss came from walter albrecht in early 1900s working in johannes thiele s laboratory albrecht investigated the reaction of cyclopentadiene with para benzoquinone his 1902 doctoral dissertation clearly describes the formation of the diels alder adduct even providing incorrect structural assignments 74 however influenced by thiele s focus on conjugation and partial valence albrecht in his 1906 publication 75 interpreted the reaction as a 1 4 addition followed by a 1 2 addition completely overlooking the cycloaddition aspect while these observations hinted at the possibility of a broader class of cycloaddition reactions they remained isolated incidents their significance not fully appreciated at the time with none of the researchers even trying to generalize their findings 65 it fell to diels and alder to synthesize these disparate threads into a coherent whole unlike the earlier researchers they recognized the generality and predictability of the diene and dienophile combining to form a cyclic structure through their systematic investigations exploring various combinations of dienes and dienophiles they firmly established the diene synthesis as a powerful new synthetic method their meticulous work not only demonstrated the reaction s scope and versatility but also laid the groundwork for future theoretical developments including the woodward hoffmann rules which would provide a deeper understanding of pericyclic reactions including the diels alder applications in total synthesis edit the diels alder reaction was one step in an early preparation of the steroids cortisone and cholesterol 76 the reaction involved the addition of butadiene to a quinone diels alder in the total synthesis of cortisone by r b woodward diels alder reactions were used in the original synthesis of prostaglandins f2α and e2 77 the diels alder reaction establishes the relative stereochemistry of three contiguous stereocenters on the prostaglandin cyclopentane core activation by lewis acidic cupric tetrafluoroborate was required a diels alder reaction was used in the synthesis of disodium prephenate 78 a biosynthetic precursor of the amino acids phenylalanine and tyrosine a synthesis of reserpine uses a diels alder reaction to set the cis decalin framework of the d and e rings 79 in another synthesis of reserpine the cis fused d and e rings was formed by a diels alder reaction intramolecular diels alder of the pyranone below with subsequent extrusion of carbon dioxide via a retro 4 2 afforded the bicyclic lactam epoxidation from the less hindered α face followed by epoxide opening at the less hindered c18 afforded the desired stereochemistry at these positions while the cis fusion was achieved with hydrogenation again proceeding primarily from the less hindered face 80 a pyranone was similarly used as the dienophile in the total synthesis of taxol 81 the intermolecular reaction of the hydroxy pyrone and α β unsaturated ester shown below suffered from poor yield and regioselectivity however when directed by phenylboronic acid 82 the desired adduct could be obtained in 61 yield after cleavage of the boronate with neopentyl glycol the stereospecificity of the diels alder reaction in this instance allowed for the definition of four stereocenters that were carried on to the final product a diels alder reaction is a key step in the synthesis of furaquinocin c 83 tabersonine was prepared by a diels alder reaction to establish cis relative stereochemistry of the alkaloid core conversion of the cis aldehyde to its corresponding alkene by wittig olefination and subsequent ring closing metathesis with a schrock catalyst gave the second ring of the alkaloid core the diene in this instance is notable as an example of a 1 amino 3 siloxybutadiene otherwise known as a rawal diene 84 sterpurene can be prepared by asymmetric d a reaction 85 that featured a remarkable intramolecular diels alder reaction of an allene the 2 3 sigmatropic rearrangement of the thiophenyl group to give the sulfoxide as below proceeded enantiospecifically due to the predefined stereochemistry of the propargylic alcohol in this way the single allene isomer formed could direct the diels alder reaction to occur on only one face of the generated diene the tetracyclic core of the antibiotic tetracycline was prepared with a diels alder reaction thermally initiated conrotatory opening of the benzocyclobutene generated the o quinodimethane which reacted intermolecularly to give the tetracycline skeleton the dienophile s free hydroxyl group is integral to the success of the reaction as hydroxyl protected variants did not react under several different reaction conditions 86 takemura et al synthesized cantharidin in 1980 by diels alder reaction utilizing high pressure 87 synthetic applications of the diels alder reaction have been reviewed extensively 88 89 90 91 92 see also edit bradsher cycloaddition wagner jauregg reaction aza diels alder reaction references edit kloetzel m c 1948 the diels alder reaction with maleic anhydride organic reactions vol 4 pp 1 59 doi 10 1002 0471264180 or004 01 isbn 978 0471264187 cite book isbn date incompatibility help holmes h l 1948 the diels alder reaction ethylenic and acetylenic dienophiles organic reactions vol 4 pp 60 173 doi 10 1002 0471264180 or004 02 isbn 978 0471264187 cite book isbn date incompatibility help 1 2 nicolaou k c snyder s a montagnon t vassilikogiannakis g 2002 the diels alder reaction in total synthesis angewandte chemie international edition 41 10 1668 1698 doi 10 1002 1521 3773 20020517 41 10 1668 aid anie1668 3 0 co 2 z pmid 19750686 atilla tasdelen mehmet 2011 diels alder click reactions recent applications in polymer and material science polymer chemistry 2 10 2133 2145 doi 10 1039 c1py00041a zweifel g s nantz m h 2007 modern organic synthesis an introduction w h freeman and co isbn 978 0 7167 7266 8 1 2 3 4 5 6 7 carey part b pp 474 526 1 2 dewar m j olivella s stewart j j 1986 mechanism of the diels alder reaction reactions of butadiene with ethylene and cyanoethylenes journal of the american chemical society 108 19 5771 5779 bibcode 1986jachs 108 5771d doi 10 1021 ja00279a018 pmid 22175326 1 2 3 carey part a pp 836 50 kiselev vladimir d konovalov alexander i 1989 factors that determine the reactivity of reactants in normal and catalysed diels alder reactions russian chemical reviews 58 turpion 230 239 doi 10 1070 rc1989v058n03abeh003437 translated from uspekhi khimii vol 58 pp 383 416 1989 1 2 carey part a p 839 gajewski j j peterson k b kagel j r 1987 transition state structure variation in the diels alder reaction from secondary deuterium kinetic isotope effects the reaction of a nearly symmetrical diene and dienophile is nearly synchronous journal of the american chemical society 109 18 5545 5546 bibcode 1987jachs 109 5545g doi 10 1021 ja00252a052 houk k n lin y t brown f k 1986 evidence for the concerted mechanism of the diels alder reaction of butadiene with ethylene journal of the american chemical society 108 3 554 556 bibcode 1986jachs 108 554h doi 10 1021 ja00263a059 pmid 22175504 goldstein e beno b houk k n 1996 density functional theory prediction of the relative energies and isotope effects for the concerted and stepwise mechanisms of the diels alder reaction of butadiene and ethylene journal of the american chemical society 118 25 6036 6043 bibcode 1996jachs 118 6036g doi 10 1021 ja9601494 breslow r guo t 1988 diels alder reactions in nonaqueous polar solvents kinetic effects of chaotropic and antichaotropic agents and of β cyclodextrin journal of the american chemical society 110 17 5613 5617 bibcode 1988jachs 110 5613b doi 10 1021 ja00225a003 1 2 rideout d c breslow r 1980 hydrophobic acceleration of diels alder reactions journal of the american chemical society 102 26 7816 7817 bibcode 1980jachs 102 7816r doi 10 1021 ja00546a048 breslow r rizzo c j 1991 chaotropic salt effects in a hydrophobically accelerated diels alder reaction journal of the american chemical society 113 11 4340 4341 bibcode 1991jachs 113 4340b doi 10 1021 ja00011a052 blokzijl wilfried engberts jan b f n 1992 initial state and transition state effects on diels alder reactions in water and mixed aqueous solvents journal of the american chemical society 114 13 5440 5442 bibcode 1992jachs 114 5440b doi 10 1021 ja00039a074 ashby e c chao l c neumann h m 1973 organometallic reaction mechanisms xii mechanism of methylmagnesium bromide addition to benzonitrile journal of the american chemical society 95 15 4896 4904 bibcode 1973jachs 95 4896a doi 10 1021 ja00796a022 fleming i 1990 frontier orbital and organic chemical reactions chichester uk wiley isbn 978 0471018193 kirmse w mönch d 1991 umlagerungen von 1 4 4 und 2 2 5 trimethylbicyclo 3 2 1 oct 6 yl kationen chemische berichte 124 1 237 240 doi 10 1002 cber 19911240136 bérubé g deslongchamps p 1987 stéréosélection acyclique 1 5 synthèse de la chaîne latérale optiquement active de la vi...
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