Diels-Alder dienophiles

Fluoroalkyl acetylenes are powerful enophiles and Diels-Alder dienophiles but also can give good yields of cyclobutenes in their cycloadditions [115,116,117] (equations 48 and 49)... 

The Diels-Alder cycloadclition reaction occurs most rapidly if the alkene component, or dienophile ("diene lover"), has an electron-withdrawing substituent group. Thus, ethylene itself reacts sluggishly, but propenal, ethyl propenoate, maleic anhydride, benzoquinone, propenenitrile, and similar compounds are highly reactive. Note also that alkynes, such as methyl propynoate, can act as Diels-Alder dienophiles. 

Which of the following alkenes would you expect to be good Diels-Alder dienophiles ... 

Keywords Facial selection. Orbital phase, Secondary orbital interaction. Orbital unsymmetrization. Ketones, Olefins, Diels-Alder dienophiles, Diels-Alder dienes, Michael acceptor. Amine nitrogen atom... 

From the same zirconocene intermediates, Huang and co-workers have prepared vinyl sulfides [24,35] and sulfones [26] through use of the appropriate quenching agents (Scheme 4.6). Treatment of vinyl zirconocenes with an equivalent of a disulfide in THF at 60 °C affords, after work-up and purification, (E)-vinyl sulfides in good isolated yields. Vinyl sulfones, which as a class are generally useful as Michael acceptors and Diels—Alder dienophiles, are obtained in about two hours upon treatment of (fc)-vinyl zirconocenes with various sulfonyl chlorides in THF at 40°C. 

When the epoxy-isobutenyl ester of crotonic acid is treated with the Cp2ZrCl2/AgC104 Lewis acid, the epoxide is opened by intramolecular assistance of the ester carbonyl group, giving a dioxolenium cation (Scheme 8.49). This species is a highly electrophilic Diels—Alder dienophile that reacts with a range of dienes. THF deactivates cationic zirconocene species and is therefore not tolerated as a solvent [89]. 

Usually, one would expect that an acrylate ester would be prepared by the acylation of an alcohol with acryloyl chloride. Jonathan M.J. Williams of the University of Bath reports (Tetrahedron Lett. 44 5523,2003) that this acylation can also be effected with the mild combination of Ph,P and maleic anhydride. The acrylate esters so prepared are interesting as polymerization precursors, and as Diels-Alder dienophiles. The allylic acrylates invite tandem conjugate addition / Ireland Claisen rearrangement. 

The synthesis started with levoglucosenone 4, available by the pyrolysis of cellulose, e.g. old newspapers. Bromination-dehydrobromination gave the enantiomerically-pure Diels-Alder dienophile 5, which was combined with isoprene to give predominantly the crystalline adduct 1. Hydrolysis and acetylation led to 6, which was carried on to the geometrically-defined allylic alcohol 7 via reduction with Zn-Cu couple. Overman rearrangement of 7 proceeded with high facial control, to give 8. 

The N-acetyl derivative of 4-oxazolin-2-one (395) has been shown to function as a Diels-Alder dienophile (77TL3115, 77LA2027). Reaction of (395) with 2,3-dimethyl-l,3-butadiene took place at 160 °C to furnish the cycloadduct (396) in 97% yield. On heating with methanolic potassium hydroxide, the adduct was converted to the c/s-/3-amino alcohol (397) in high yield (Scheme 87). This oxazolinone thus provides a new entry to the /3-amino alcohol structural unit found in many natural substances. 

Diels-Alder dienophiles Bis(trimethylsilyl)acetylene, 309 Cyclopropenone 1,3-propanediyl ketal,... 

CONTENTS Preface, Mark Lautens. Photocyclization and Photocycloaddition Reactions of 4- and 2-Pyrones, Frederick G. West. Intramolecular [4+3] Cycloaddition Reactions, Michael Harmata. Lewis Acid Catalyzed [2+2] Cycloaddition Reactions of Vinyl Sulfides and Their Analogues Catalytic Asymmetric [2+2] Cycloaddition Reactions, Koichi Narasaka and Yujiro Hayashi. Vinylboranes as Diels-Alder Dienophiles, Daniel A. Singleton. Preparation and Exo-Selective [4+2] Cycloaddition Reactions of Cobaloxime-Substituted 1,3-Dienes, P... 

A slightly different course of reaction is available to a,a -dibromo-0-xylene. In the presence of highly reactive nickel powder, the synthetic equivalent of 0-xylylene is generated, which undergoes cycloaddition with normal Diels-Alder dienophiles in a nonstereospecific manner (equation 140)266. The exact nature of 40 has not been deduced. 

Benzoquinone is a good Diels-Alder dienophile. Predict the products of its reaction with (a) buta-1,3-diene (b) eyelopenta-1,3-diene... 

DIELS-ALDER DIENOPHILES Bis(2,2,2-trichloroethyl) azodicarboxylate. 1-Bromo-2-chlorocyclopropene. Chromium carbene complexes. (R)-Ethyl p-tolysulfinyl-methylenepropionate.l (S)-a-Hydroxy-p,p-dimethylpropyl vinyl iketone. (IR)-m-3-Hydroxy isobomyl ndopentyl ether. Oxa-zolidones, chiral. Phdnylselenenyl ben-zenesulfonate. 

Compounds containing triple bonds are also Diels-Alder dienophiles. With this in mind, draw the products of each reaction. 

The usefulness of the cation radical Diels-Alder reaction for natural product synthesis has been illustrated in the synthesis of / -selinene (Scheme 41) [81], which utilizes phenyl vinyl sulfide as a Diels-Alder dienophile and the photosensitized electron transfer method as the preferable method for generating the corresponding cation radical. 

Nitroso compounds of various structural types have been widely utilized as Diels-Alder dienophiles, and the subject has been comprehensively reviewed several times (77CSR1 82T3087). It has been known for many years that a-chloronitroso compounds react with 1,3-dienes to yield unstable adducts of... 

Vinyl sulfides and the corresponding sulfoxides are versatile reagents which react as Michael acceptors, Diels-Alder dienophiles, annelating agents and carbonyl equivalents. They are also interesting substrates for extended enolate chemistry. 

Vinyl sulfoxides are useful Diels-Alder dienophiles. Thermolysis of the adducts gives rise to alkenes via sulfoxide elimination and so over the two steps the vinyl sulfoxides are the synthetic equivalents of alkynes, e.g. phenyl vinyl sulfoxide reacts with anthracene and 1,4-diphenylbuta-1,3-diene with concomitant sulfoxide elimination under the Diels-Alder conditions to give adducts (70) and (71 Scheme 15). 3-Phenylsulfinyl-a,3-unsaturated-carbonyl compounds have also been used as Diels-Alder dienophiles in chemistry developed for a synthesis of disodium prephenate and epiprephenate. With the electron rich diene (72), vinyl sulfoxides (73) and (75) gave rise to products (74 equation 33) and (76 equation 34) after hydrolysis. For the prephenate synthesis the phenylsulfinylmethylenelactone (77) was used, giving cyclohexadienone (78 Scheme 16) after sulfoxide elimination and hydrolysis. ... 

Paredes, E., Biolatto, B., Kneeteman, M., Mancini, P. M. Nitronaphthalenes as Diels-Alder dienophiles. Tetrahedron Lett. 2000,41, 8079-8082. 

Diels-Alder dienophile. The adduct of the reagent with 2,3-dimethylbutadiene was obtained in unrecorded yield by refluxing the components in benzene for 10... 

Diels-Alder dienophile. It is somewhat less reactive in this respect than diethyl azodiformate. 

---