Acrylate in Diels-Alder reaction

It has been established that alkoxy alkenylcarbene complexes participate as dienophiles in Diels-Alder reactions not only with higher rates but also with better regio- and stereoselectivities than the corresponding esters [95]. This is clearly illustrated in Scheme 51 for the reactions of an unsubstituted vinyl complex with isoprene. This complex reacts to completion at 25 °C in 3 h whereas the cycloaddition reaction of methyl acrylate with isoprene requires 7 months at the same temperature. The rate enhancement observed for this complex is comparable to that for the corresponding aluminium chloride-catalysed reactions of methyl acrylate and isoprene (Scheme 51). 

Enantioselective Diels-Alder reactions proceed smoothly in the presence of a chiral Sc catalyst, prepared in situ from Sc(OTf)3, R)- I )-l,l -bi-2-napluhol [(R)-BINOL], and a tertiary amine in dichloromethane.58 The catalyst is also effective in Diels-Alder reactions of an acrylic acid derivative with dienes (Scheme 14). 

Hetero substituted 2-cyclopropylideneacetates are ring-strain activated acrylates, highly reactive dienophiles in Diels-Alder reactions, but also powerful Michael acceptors. The reactivity of these compounds is enhanced by the same strain release in the Diels-Alder cycloadditions as well as in the 1,4-additions, and indeed the borderline between tandem Michael-cyclization and Diels-Alder-type cycloaddition is not well defined in many cases. 

Furo[3,4-J]pyridazines have also been used in Diels-Alder reactions (331a with maleic anhydride, acrylic acid, 1,4-naphthoquinone, dibenzoyl-ethylene, 1,4-benzoquinone, benzo[c]furandione ° 331c with maleic anhydride) 331a has been shown to be more reactive than 331c. 1,3-Diphenylfuro[3,4-b]quinoxaline (335) has been obtained from phthalide 334 (Eq. 17) as a green crystalline, quite stable solid (mp 244-246°C). In DMSO (deep blue solution), 335 reacts instantaneously with such dienophiles... 

Attempts to isolate 2,3-dimethoxyfuran (156) have, as yet, been fruitless (79JCS(P1)1893), but it may be generated in situ and trapped with the propiolate (155) the initial adducts (157) are unstable under the acidic conditions and yield the biphenyls (158) and (159) (Scheme 67). 2,5-Bis(trimethylsilyloxy)furans, readily available from succinic anhydrides in one step, are also more reactive than furan in Diels-Alder reactions (80TL3423). They readily undergo reaction with both DMAD and ethyl acrylate. Thus at 50 °C in carbon tetrachloride the furan (160) with DMAD followed by detrimethylsilylation gave only the quinone (163). At 80 °C, however, the hydroquinone (164) is the major product. Both the intermediates (161) and (162) may be detected by ]H NMR spectroscopy. The formation... 

Asymmetric Diels-Alder reactions. The acrylate of (+ )-l is markedly superior to ( —)-menthyl acrylate in effecting asymmetric induction in Diels-Alder reaction with cyclopentadiene (Lewis acid catalysis, equation I). A chiral intermediate in Corey s prostaglandin synthesis was obtained by the reaction of (-1- )-8-phenylmenthyl acrylate with a cyclopentadiene derivative.1... 

Simple diastereoselectivity may also occur in Diels-Alder reactions between electron-poor dienophiles and cyclopentadiene (Figure 15.30). Acrylic acid esters or fraus-crotonic acid esters react with cyclopentadiene in the presence or absence of A1C13 with substantial selectivity to afford the so-called emfo-adducts. When the bicyclic skeleton of the main product is viewed as a roof the prefix endo indicates that the ester group is below this roof, rather than outside (exo). However, methacrylic acid esters add to cyclopentadiene without any exo.endo-selectivity regardless whether the reaction is carried out with or without added A1C13 (Figure 15.30, bottom). 

Furo[3,4-(/]pyrimidines are ideally suited for participation in Diels-Alder reactions. The reaction of the derivatives (47) with dienophiles leads to tricyclic compounds of the type (48) <9iJOC245>. There is some selectivity, since methyl acrylate forms only two regioisomeric endo products whilst other dienophiles yield both endo and exo adducts (Equation (13)). 

Alkene- and alkyne-substituted Fischer carbenes participate as dienophiles in Diels-Alder reactions. The conditions are usually mild and the reaction proceeds smoothly at room temperature. Similar isomeric ratio and rate acceleration is observed to that of Lewis acid-promoted Diels-Alder reactions between methyl acrylates and dienes when compared to the uncatalyzed reactions. The reactions are endo-selective. Asymmetric Diels-Alder reactions are... 

Cationic ansa metallocenes can be utilized as chiral catalysts in Diels-Alder reactions. For example, in the presence of the cationic zirconocene complex [(ebthi)Zr(Ot-Bu) thf]+, the [4 + 2] cycloaddition of acrolein and cyclopentadiene proceeds efficiently to afford endo and exo cycloadducts (equation 71). In reactions in which methyl acrylate is used as the dienophile, cycloadditions occur with lower levels of enan-tioselection (23% ee), but with significantly higher degrees of diastereoselectivity (17 1 endo, exo). In these processes, recent studies demonstrated the great influence of chiral metallocene structure and the dramatic solvent effect. ... 

The possibility that metallocenes might function as Lewis acids in Diels-Alder reactions was probed with ferrocenium hexafluorophosphate [184]. The answer is affirmative the cycloadditions studied include methacrolein, crotonaldehyde, and methyl vinyl ketone as dienophiles and butadienes and cyclopentadienes as diene components. Yields are in the range 60-80 % with reaction times of 3-36 h at 0 to 20 °C. Fair to good yields were also obtained in reactions of isoprene and cyclopentadiene with acrolein and methyl vinyl ketone in the presence of 1 % [Pd(PPh3)2(MeCN)2](BF4)2 (in CH2CI2, room temperature). Methyl acrylate resulted in low yields, and chiral modification with (5)-BINAP is reported to give the cycloadducts with modest enantioselectivity [164]. 

Prepared by the action of methanolic potassium hydroxide on hexachlorocyclo-pentadiene, the dimethoxy compound is more reactive in Diels-Alder reactions than the 5,5-dichloro derivative and reacts with a number of dienophiles under relatively mild conditions. - The adducts are of interest because they can be aromatized by a simple reaction sequence, as illustrated for the acrylic acid adduct, prepared by... 

Specific anion- and cation-reactant effects have been reported to play a role in Diels-Alder reactions [142-144], in particular for the em/oselectivity. Using the model reaction cyclopentadiene and methyl acrylate (Fig. 10, left) in various ionic liquids, the authors [142] demonstrated that the endo/exo ratio decreased when the reactions were carried out in [C4mim]+ homologues in the order of ... 

Silica-supported Lewis acids (e. g. ZnCl2, Znl2, or TiCl4) were also studied in Diels-Alder reactions of furan [37,38]. Reactions were performed at 25 °C and the best results were obtained in the absence of solvent. The combination of dienophile and supported Lewis acid had a great influence on the outcome of the reactions. Reaction with the chiral dienophile (-)-8-phenylmenthyl acrylate gave the endo adduct in 68 % d. e. and the exo adduct in 70 % d. e. [37]. 

In addition, Diels-Alder reactions have also been studied on R-containing micro-porous materials. Murthy and Pillai (1991) reported the reaction of cyclopentadiene, cyclohexadiene or furan with less reactive dienophiles (acrolein, acrylamide or methyl acrylate). They observed that microporous materials give high yields in Diels-Alder reactions although exclusive endoselectivity was only obtained on a combined catalyst (Ce-Y zeolite with anhydrous ZnBr2). 

K10 montmorillonites exchanged with different cations, dried at 120°C or calcined at 550°C, are used as catalysts in Diels-Alder reactions of methyl and (-)-menthyl acrylates with cyclopentadiene. In general, calcined clays give rise to better conversions and selectivities. Zr(IV) and specially Ti(IV) clays display the best catalytic activities. However, the best asymmetric induction is achieved with Cr(lll) and Ca(ll) calcined clays. Clays containing easily reducible cations behave differently due to the cyclopentadiene polymerization via radical cations. 

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