Dipolarophiles fumaronitrile

Pyrido[l,2-c]quinazolin-8-ones (215) were obtained from the cycloadducts (214) of anhydro 3-hydroxythiazolo[3,2-c]quinazolin-4-ium hydroxides (213) and alkynic (dimethyl acetylenedicarboxylate and ethyl propio-late) and alkenic dipolarophiles (fumaronitrile and methyl vinyl ketone) by extrusion of S or H2S (85JOC1666). A better yield could sometimes be... 

Recently, dipolarophile 1)13 (fumaronitrile) (777) has been used in the synthesis of indolizine lactone (677). Both, intermolecular and intramolecular cycloadditions were studied. Intermolecular 1,3-cycloaddition of nitrone (671) to D13 led to the formation of isoxazolidine (672). Subsequent deprotection and esterification of the obtained alcohol (673) with (674) gave isoxazolidine (675) in 65% yield. Ester (675), when refluxed in xylene for 10 min, after elimination of fumaronitrile by cyclo-reversion, underwent spontaneously intramolecular cycloaddition to give the tricyclic cycloadduct (676) in 84% yield (Scheme 2.291). 

However, use of a less reactive reagent where [R = R =(CH2)4, (CH2)s, (CH2)20(CH2)2] led to the isolation of products 61 and 62, with a reduction in the yields of the desired cycloadducts. The product 62 arises from Michael addition of the liberated methanethiol to A-methylmaleimide. The protocol was further extended to olefinic dipolarophiles with dimethyl fiimurate, dimethyl maleate, fumaronitrile, and 2-chloroacrylonitrile leading to the corresponding adducts, although these dipolarophiles proved somewhat less reactive with reduced yields being observed. Where applicable, the alkene configuration was reflected in the relative stereochemistry of the cycloadducts (Fig. 3.5). 

Olefinic dipolarophiles, such as dimethyl fumarate, fumaronitrile, ethyl vinyl ether, A-phenylmaleimide, norbornene, and norbornadiene, react with... 

Cyclocondensation of anhydro-1 -hydroxythiazolo[3,2-a]quinazolinium hydroxides (334) with dipolarophiles (335) afforded 5-methyl-5,6-dihydro-1 //-pyrido[l, 2-n]quinazoline-l, 6-diones (337) (85JOC1666). Sometimes better yields were achieved when mesoionic compounds (334) were prepared in situ from S-(quinazolin-2-yl)thioglycolic acids (333, R = Ph) with acetic anhydride or DCC in the presence of the dipolarophiles (335). Cycloadducts 336 (R = H, Ph, R1 = R2 = CN) could be characterized in the case of fumaronitrile by IR and 1H NMR spectroscopy. 

When acrylonitrile or ethyl acrylate was used as the dipolarophile, the azomethine adducts (134) and (135) were formed no thiocarbonyl ylide addition products were isolable in refluxing toluene or xylene, although the isoindoles (136a) and (136b) derived from them were isolated. In contrast to the reactions with fumaronitrile or AT-phenylmaleimide, the azomethine adducts (134) and (135) were still present at higher reaction temperatures — almost 50% in toluene and 4-5% in xylene. Under the same reaction conditions other electron-deficient dipolarophiles like dimethyl fumarate, norbornene, dimethyl maleate, phenyl isocyanate, phenyl isothiocyanate, benzoyl isothiocyanate, p-tosyl isocyanate and diphenylcyclopropenone failed to undergo cycloaddition to thienopyrrole (13), presumably due to steric interactions (77HC(30)317). 

This above result was only obtained with maleimide as the dipolarophile. With dimethyl fumarate and fumaronitrile, A2-pyrrolines are obtained, probably because of the acidity of the hydrogen atoms a- to ester and nitrile functionalities. Reaction with alkynes produces pyrrole derivatives in good to excellent yields and with aromatic aldehydes leads regioselectively to oxazolidines in moderate yields.263... 

Pipecolic acid, a cyclic aminoacid, when reacted with isatin, suffers decarboxylation, furnishing an azomethine ylide, which reacts with dipolarophiles, such as fumaronitrile, to yield a spiro derivative285 (Scheme 65). 

Cyanopyrazole heterocycles were easily prepared via thermolysis of tetrazolo[l,5-3]pyridazines, tetrazolo[l,5- ]pyr-imidines, or tetrazolo[l,5- ]pyridines <2000TL2699>. Tetrazolylacroleins 809 underwent ring transformation on reaction with fumaronitrile 810 to afford pyrazolyl derivatives 811 (Equation 181) <2003T7485>. Mechanistically, the tetrazolylacrolein is believed to react with the dipolarophile via a 1,3-dipolar cyclization route to give a... 

The second method was based on a cycloaddition reaction of olefinic and acetylenic dipolarophiles to xanthinium-(N7) ylides. Xanthinium-(N7) ylides 22 were generated from 7-substituted 1,3,9-trimethylxanthinium tosylates 21 by deprotonation with triethylamine in dry acetonitrile followed by reaction with dipolarophiles 23 and 25 (dimethyl fumarate, fumaronitrile, methyl acrylate, acrylonitrile and the like). Reaction with dimethyl fumarate afforded the endo-(24a) and exo-(24b) adducts in a 3 2 ratio. The stereochemistry of these compounds was established by H-NMR and X-ray analysis (84H2199). With derivatives of acrylic acid 25, only endo adducts (26)... 

The formation of 1,3-oxathiolane with chloral and 1,3-dithio-lanes with cycloaliphatic thioketones occurs regioselectively to yield the sterically less hindered products. On the other hand, aromatic thioketones, e.g., thiobenzophenone or 9/ fluorene-9-thione, intercept 8 to give comparable amounts of both regio-isomeric adducts. Usually, stereoisomeric dipolarophiles such as fumaronitrile and maleonitrile as well as dimethyl fumarate and maleate form 18 in a stereoselective manner. However, in the case of extremely electron-poor dipolarophiles, e.g., dimethyl 1,2-dicyanofumarate or ( )-l,2-bis(trifluoromethyl)ethylene-l,2-dicarbonitrile, non-stereospecific formations of the corresponding tetrahydrothiophenes are described. i- s This result is interpreted in terms of a stepwise reaction mechanism with a zwitterion as the key intermediate. Alternatively, this intermediate can cyclize to form seven-membered ketenimines of type 20. With R = Cp3, this product can be isolated in a crystalline form, whereas in the case of R = CN, stable lactam 21 is obtained only after addition of water (eq 10). 

Dipolarophiles which contain an electron-deficient substituent undergo smooth cycloaddition reactions with nitrile ylides. The relative reactivity of the nitrile ylide toward a series of dipolarophiles is determined primarily by the extent of stabilization afforded the transition state by interaction of the dipole highest-occupied (HO) and dipolarophile lowest-unoccupied (LU) orbitals. Substituents which lower the dipolarophile LU energy accelerate the 1,3-dipolar cycloaddition reaction. For example, fumaronitrile undergoes cycloaddition at a rate which is 189,000 times faster than methyl crotonate. Ordinary olefins react so sluggishly that their bimolecular rate constants cannot be measured. 

Arnold first discovered that PET reactions of cis- and rrans-2,3-diphenyloxiranes (1 and 2) with electron-accepting sensitizers such as DCN, 1,4-dicyanobenzene, dimethyl terephthalate, and methyl 4-cyanoben-zoate produce the isomeric tetrahydrofuran derivatives 7 and 8 (Scheme 2). The proposed mechanism involves the CC bond cleavage of the oxirane radical cations to give ylide radical cations 3 and 4, followed by back electron transfer (BET) to generate carbonyl ylides 5 and 6. The resulting carbonyl ylides are trapped with dipolarophiles, such as acrylonitrile, maleonitrile, and fumaronitrile, to produce the observed products. In the absence of dipolarophiles, C)S-/ir s-isomerization (1/2 = 0.19-0.28) was observed, which is consistent with the formation of carbonyl ylides 5 and 6, as well as their radical cations 3 and 4. This method was successfully apphed to the synthesis of various tetrahydrofurans (Scheme 3) and dihydrofurans (Scheme 4). ... 

---