Simple Diels-Alder Additions

Simple Diels-Alder Additions.—The use of halogenovinylene carbonates (280) and 

The ester (285) was heated with maleic anhydride but gave (287), perhaps via (286) in which the furan is the dienophile Fulvenes add to 2,3-dialkylorthoquinones 

Mild hydrogenation of (293) gives (295) whereas zinc powder converts (293) into 

Adler and K. Holmberg, Acta Chem. Scand., 1974, B28, 549 ibid., p. 465. 

Holmberg, H. Kirrudd, and G. Westin, Acta Chem. Scand., 1974, B28, 913. E. Adler, K. Holmberg, and L.-O. Ryrfors, Acta Chem. Scand.. 1974, B28, 888. K. Holmberg, Acta Chem. Scand., 1974, B28, 857. 

Simple Diels-Alder Additions.—The [4 + 2] and reverse-[4 + 2] cycloaddition reactions of species containing C=X bonds (X = P, As, Sb, Bi, Si, Ge, etc.) have been discussed and the evidence reviewed.The general chemical reactivities of cyclo-propenones and triafulvenes, including their behaviour in Diels-Alder reactions, have been broadly summarized. Kagi and Johnson have reported extensive studies on the Diels-Alder reactions between cyclopropene and various chloro-cyclopentadienes in which endo-adducts are obtained, selective dechlorination, hydrolysis and other reactions of these adducts, and H n.m.r. spectral correlations. 

The synthetic scope of the Diels-Alder reaction continues to be widened as a result of the studies of Corey s group. A brief note this year describes the use of 3-hydroxy-2-pyrone (195) as a vinylketene equivalent for the synthesis of dihydrophenols and cyclohexenones. These compounds are the result of the loss of CO2 from the initially formed adducts in a reverse [4 + 2]reaction, but in the reaction of (195) with maleic anhydride the adduct (196) is stable. 

Two groups have recorded MO rationalizations for the widely disparate Diels-Alder reactivities of (197) and (198) whereas (197) reacts sluggishly with tetracyano- 

The Diels-Alder dimer of 2,3-dichlorocyclopentadienone formed at 25 °C has been shown to have the endo-configuration as in (199), the result of regiospeciflc and stereoselective cycloaddition. At 170°C cheletropic loss of CO from (199) affords the trienone (200) which itself dimerizes with subsequent loss of HCl to yield the stereoisomeric aromatic derivatives (201) and (202). The attempted dehydrobromination of (203) with l,5-diazabicyclo[5,4,0]undec-5-ene (DBU) gave 

An improved procedure for the preparation of exo-dicyclopentadiene (207) from the cndo-isomer has been reported. Cycloaddition of norboradiene to anthracene affords the cxo-adduct (208), the bromination of which gives (209) Dehydrobromination of (209) yields (210 X = Br) which on reductive debromination is converted into the cndo-adduct (210 X = H). The dihydro-derivatives of (208) and (210 X = H) were also synthesized. 

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Bis-l,2,4-triazole-3,5-diones such as 110 have also been used in Diels-Alder reactions, and give bispyridazines.171 The pyridazine derivative 111 is formed in quantitative yield from PTAD and 2,7-dimethyl-2,3,5,6-octatetraene,172 and the azadiene, 4-aza-l,3,5-triphenylpenta-2,4-diene, also reacts readily with PTAD to give 112.173 There are many other examples of Diels-Alder additions of ADC compounds to simple acyclic dienes which proceed entirely as expected the above selection has been limited to reactions of synthetic potential and with novel features. 

A new multistep synthesis of ( )-reserpine (109) has been published by Wender et al. (258). The key building block of the synthesis is cw-hexahydroiso-quinoline derivative 510, prepared by the extension of the previously elaborated (259) Diels-Alder addition-Cope rearrangement sequence. Further manipulation of 510 gave 2,3-secoreserpinediol derivative 512, which already possesses the required stereochemistry in ring E. Oxidative cyclization of 512 yielded 3-isoreserpinediol (513), which was transformed by the use of simple reaction... 

Diels-Alder addition of simple benzene derivatives is difficult and occurs only with very reactive dienophiles. Formation of an adduct between benzene and dicyanoacetylene in the presence of A1C13 has been reported, for example.51 52 53... 

Wiseman and coworkers have succeeded in preparing tricyclo[5.3.0.0 ]decane (393), a CioHig hydrocarbon which unlike adamantane is chiral. Their elegantly simple approach entails Diels-Alder addition of cyclobutene to l,4-dihalocyclohexa-l,3-dienes, catalytic hydrogenation of the adduct, reaction with aluminium triiodide, and ultimately di- -butyltin dihydride reduction. 

At low temperature and in the presence of catalyst, simple conjugated dienes add SO2 reversibly via hetero-Diels-Alder addition and generate 3,6-dihydro-l,2-oxathiine 2-oxides 8 (sultines) <1992JA9210, 1993TL6269,... 

The thermal polymerization of 1,3-cyclopentadiene by way of the Diels-Alder addition is not an important polymerization, but it does provide a simple concrete example of how a monomer and a polymer are related ... 

The high simple diastereoselectivities seen in Figures 15.29 and 15.30 are due to the same preferred orientation of the ester group in the transition states. The stereostructure of the cycloadduct shows unequivocally that the ester group points underneath the diene plane in each of the transition states of both cycloadditions and not away from that plane. Figure 15.31 exemplifies this situation for two transition states of simple Diels-Alder reactions of 1,3-butadiene A shows a perspective drawing of the transition state of the acrylic acid ester addition, and B provides a side view of the addition of ethene, which will serve as an aid in the following discussion. Both structures were determined by computational chemistry. 

Fig. 15.32. Sequence of a Lewis-acid-catalyzed inter-molecular and a Lewis-acid-catalyzed intramolecular Diels-Alder addition. The first Diels-Alder addition occurs with simple and the second with induced diastereoselec-tivity.

Several authors have recently demonstrated a simple, high-yield route to dihydrofullerenes via Diels-Alder addition of electron-rich dienes to Cgo [87,95-98]. This is a very versatile route which allows easy variation of functional groups and considerable control of the solubility characteristics of the adducts. For example, using 2-silyloxybutadiene derivatives, the cyclohexanone adduct shown below can be made after hydrolysis of the primary adduct. The corresponding alcohol is then made by reduction of the ketone, and is easily converted to amino acid derivatives which allow solubility in water [87]. 

Use as a Chiral Auxiliary. (5)-Ethyl lactate has been used as a chiral auxiliary in a variety of simple Diels-Alder reactions. As the fumaric acid diester, the de employing cyclopentadiene can almost be completely reversed by addition of Titanium(IV) Chloride (eq 8). In general, superior de values are achieved using (R)-Pantolactone in this context, and also for base-mediated addition to ketenes. ... 

Diels-Alder Reactions. (i )-Pantolactone is one of the most effective chiral auxiliaries for preparative scale Diels-Alder additions of simple enoate esters in the presence of Lewis acids (eq 1). ... 

EtAlClz- or Me2AlCl-promoted Diels-Alder addition of butadiene or isoprene to (363a) also proceeded readily at -78 or -94 C to give, after recrystallization, -100% pure (S)-cyclohexenes (370) (Scheme 90, Table 22, entries 8-10). Reductive cleavage of the cycloadducts (370) with LiAlHa refurnished the sultam (362) (89-95% after crystallization) and gave the pure alcohols (372) on simple bulb-to-bulb distillation. Alternatively, saponification of adduct (370a) with LiOH afford acid (373a) (a potential precursor for a synthesis of (-)-shikimic acid, 347) without epimerization. 

In the present context, the term electron rich alkenes refers primarily to enol ethers, enol sulfides, and A-vinylamides or A-vinylamines. Such alkenes are typically much more readily ionizable than are simple alkenes. The conversion of these substrates to the corresponding (highly electron deficient) cation radicals represents a sharp Umpolung. The Diels-Alder additions of tra j -anethole, phenyl vinyl ether, phenyl vinyl sulfide, 1,3-dioxole, and A-methylindole to 1,3-cyclohexadiene have been reported (Scheme 22) [49, 52]. 

Acetylenic monoesters are much less reactive and add only slowly to simple furans, so McCulloch and Innes have exploited the outstanding catalytic effect of aluminium chloride upon Diels-Alder additions. Not only... 

There are no reports of thermal electrocyclic reactions involving simple pyridines. 2-Pyridones, however, participate as 4n components in Diels-Alder additions, especially under high pressure. ... 

Development of synthetic routes to the anthracycline antibiotics is important because of the cytotoxic properties of many of these compounds. A short route " to possible intermediates proceeds via double Diels-Alder addition to (16). The problem of the availability of (16) has been overcome " by efficient palladium-catalysed carbomethoxylation of the furan-maleic anhydride Diels-Alder adduct to give (122) in 92% yield. Elaboration of Diels-Alder adducts of (16) to anthracyclines requires rupture of the ether bridge. Acid-catalysed ringopening " in simple 7-oxabicyclo[2.2.1]heptanes has been studied. 

Addition reactions of vinyl sulphones (Diels-Alder addition, [2 -i- 2]-photocycloaddition, and conventional functionalization processes, e.g. IN 3 2-azido-l-iodoalkyl sulphones °°) and substitution reactions of / -halogeno-vinyl sulphones have been reported. A point of interest in the cycloaddition study is the effect of ( )-(Z) equilibration in hindering the participation of simple vinyl sulphones in photocycloaddition reactions. Elimination of SO2 has been observed in the photolysis of 3-oxo-alk-l-enyl sulphones in benzene, giving radicals which attack the solvent. ... 

Synthesis using Simple Olefins.—1,3-DipoIar reagents and the classical Diels-Alder addition of dienes both usually require activated olefins, and few enophiles are known which readily cycloadd to isolated double bonds. Such an avid enophile is the A -alkyl-iV-vinylnitrosium ion (447), generated in situ by silver(i)-induced ionization of a-chloronitrones (448). Cycloaddition... 

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