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Cyclic dienophile

Diels-Alder disconnection of (S) reveals a cyclic dienophile (11) in which Me and H must be ais since they are also cis in (S), and the one step synthesis duly gi ves c-ir, product. [Pg.182]

Reactions of furan (5) under solvent-free conditions, catalyzed by Montmorillonite K10, have been described by Cintas [27]. The reaction with methyl vinyl ketone (32) produced Michael addition in positions 2 and 5, whereas reaction with symmetrically substituted cyclic dienophiles produced a mixture of the endo and exo adducts with the kinetically favored endo adduct predominating, except when maleic anhydride (39) was used as the dienophile (Scheme 9.2). [Pg.304]

The reactivity of compound 113 toward reactive linear and cyclic dienophiles was reported in a study directed to find a model systems for the proposed [4+2] cycloaddition in the biosynthesis of the natural products brevianamides, paraherquamides, and marcfortines. With DMAD and diethyl azodicarboxylate the formation of 114 and 115 was almost quantitative after 48 h at 80 °C (Cbz = Carbobenzyloxygroup). When relatively unreactive dienophiles such as cyclopentene and cyclohexene were used, harsh reaction conditions and/or a Lewis acid catalyst are necessary for the formation of 116a and 116b (Scheme 16). In contrast, the analogous intramolecular reaction carried out on compound 117 takes place within a few hours at room temperature, even in the absence of a Lewis acid catalyst, to give 118 in 42% yield (Scheme 16) <2000T6345>. [Pg.512]

Unlike thermal homo Diels-Alder reactions in which endo adducts predominate330, the nickel catalyzed reactions of acyclic electron-deficient dienophiles afford the exo isomers as the major cycloadducts. This has been explained by unfavorable steric interactions within intermediate 559 leading to the endo adduct. Cyclic dienophiles, on the contrary, give predominantly the endo isomer, which has again been explained by unfavorable steric interactions within exo 559. The preferred conformation of the dienophile, s-cis or s-trans, has also been suggested to play a role328. [Pg.458]

A semiempirical AMI study of the inverse-electron-demand Diels-Alder reaction of 4-substituted 6-nitrobenzofurans with enol ethers and enamines favours a stepwise mechanism involving short-lived diradical intermediates. The inverse-electron-demand intermolecular Diels-Alder reactions of 3,6-bis(trifluoromethyl)-l,2,4,5-tetra-zine with acyclic and cyclic dienophiles followed by the elimination of N2 produce 4,5-dihydropyridazines, which cycloadd further to yield cage compounds. The preparation of jS-carbolines (90) via an intramolecular inverse-electron-demand Diels-Alder... [Pg.471]

There are two possible configurations, endo and exo, for bridged bicyclic compounds resulting from the reaction of a cyclic diene and cyclic dienophile. A substituent on a bridge is endo if it is closer to the longer of the two other bridges, and it is exo if it is closer to the shorter bridge. Most of these reactions result in an endo product. However, if this reaction is reversible, and thermodynamically controlled, the exo product is formed. [Pg.280]

Known as the endo rule, it was first formulated by Alder and Stein530,532 and explained as the maximum accumulation of double bonds. Now endo selectivity is considered to be the result of stabilizing secondary orbital interactions.533 The endo rule, however, strictly applies only for cyclic dienophiles. [Pg.334]

The cyclic dienophile 2,5-dihydrofuran reacted with (343) in the presence of a catalytic amount of trichloroacetic acid to furnish pyridine (349) in 58% yield. The ether was cleaved with 48% hydrobromic acid to its dibromide (350) and then hydrolyzed to pyridoxine (Scheme 75). [Pg.445]

Dienophiles like phosphaalkyne 44a or activated alkynes and triphosphadewarbenzene 45 undergo a [2+2+2] homo-Diels-Alder reaction that leads to C3P4 cage 46. Cyclic dienophiles yield pentacycles (Scheme 13) <1997JOM(529)215, 1999S1363>. [Pg.884]

The stereochemistry at the ring junction must be ds because the cyclic dienophile can have only a ds double bond. Hydrogenation removes the double bond in the product and shows just how useful the Diels-Alder reaction is for making saturated rings, particularly when there is some stereochemistry to be controlled. [Pg.910]

The cycloaddition of methyl acrylate with 7V-thiobenzoyl-7V,7V-dimethylformamidine 363 under pressure affords the 4-dimethylamino-5,6-dihydro-4//-l,3-thiazine 364. On treatment with methyl iodide and triethylamine, 364 eliminates trimethylamine to give 6//-l,3-thiazine 365 which can also be obtained directly through the cycloaddition of methyl acrylate and the methiodide salt of the formamidine <1987SC1971>. The same methodology has been extended to other 1,3-thiazines by varying the formamidine or electron-deficient cyclic dienophile (Scheme 170) <1988SUL205>. [Pg.751]

When the trifluoromethyl group is the only electron-withdrawing substituent of the cyclic dienophile, such as in 4-(trifluoromethyl)-1,2-dihydronaphthalene, high pressure is required for the reaction to occur.19 The yield is improved when the reaction is performed with the tricarbonyl chromium complex 21 of 4-(trifluoromethyl)-l,2-dihydronaphthalene.5 20 In the reaction with l-methoxy-3-(trimethylsiloxy)buta-l,3-diene (Danishefsky s diene, 4), the cycloadducts are obtained as a 1 1 mixture ofcnr/o/c-vo-addition products. They are easily converted into the (trifluoromethyl)phenanthrenone 22 by the action of bromotrimethylsilane. [Pg.525]

Alder s endo rule specifies a preference for endo (C) over exo (D) addition. However, this rule appears to be strictly applicable only to the addition of cyclic dienophiles (e.g. maleic anhydride, p-qui-nones) to cyclic dienes (e.g. cyclopentadienes). [Pg.318]

A cyclic dienophile forms a bicyclic product. A bicyclic system in which the two rings share a common C-C bond is called a fused ring system. The two H atoms at the ring fusion must be cis, because they were cis in the starting dienophile. A bicyclic system of this sort is said to be ds-fused. [Pg.590]

This reaction is not polar in that no charged intermediates are formed. Neither is it radical because no unpaired electrons are involved. It is instead known as a concerted reaction, or one in which several bonds in the transition state are simultaneously made and broken. When acyclic diene and a cyclic dienophile react with each other as in the present reaction, more than one stereoisomer may be formed. The isomer that predominates is the one which involves maximum overlap of pi electrons in the transition state. The transition state for the formation of the e reaction involves a sandwich with the diene directly above the dienophile. To form the exo-isomer the diene and dienophile would need to be arranged in a stair-step fashion. [Pg.285]

Gupta, R C, Raynor, C M, Stoodley, R J, Slawin, A M Z, Williams, D J, Asymmetric Diels-Alder reactions. Part 1. Diastereofacial reactivity of ( )-3-trimethylsilyloxybuta-l,3-dienyl 2,3,4,6-tetra-O-acetyl-(3-D-glucopyranoside towards cyclic dienophiles, J. Chem. Soc. Perkin. Trans 1, 1773-1785, 1988. [Pg.498]

The use of cyclic dienophiles such as maleic anhydride, A -phenylmaleimide or quinones induces stereoselective cycloadditions, especially when R = Me (Figure 9.49). These results have been interpreted by the approach of the dienophile to... [Pg.591]

An important group of cyclic dienophiles are butenolides. Optically active y-substituted butenolides can be prepared from naturally occurring chiral compounds, such as D-mannitol [(5)-5-hy-droxy-2-penten-4-olidc]18, D-ribonolactone [(5)-5-hydroxy-2-penten-4-olide and its derivatives]19 or L-tartaric acid [(5)-2-penten-4-olide known as /i-angelica lactone]20. Chiral butenolides are extremely selective dienophiles in thermal [4 + 2] cycloadditions to acyclic dienes. [Pg.569]

The 1-azadienes 22a-c react smoothly at room temperature with cyclic dienophiles 23 and 24 affording, in good yields, adducts 25 and 26, respectively, with excellent diastereoselectivity76. [Pg.708]

Thus, pyridines 21 condensed at the c face are obtained when 1,2,4-triazines are reacted with cyclic dienophiles.386,387 405,437 Starting from condensed 1,2,4-triazines and alkene dienophiles, pyridines 22 condensed at the b face are synthesized and pyridines 23 condensed at the b and rfface are isolated when a condensed 1,2,4-triazine is reacted with a cyclic dienophile.403... [Pg.652]

Figure 1 presents a collection of rate constants for cyclic dienophiles in the reaction with tetrazine diester (48). [Pg.936]

Figure 1 Rate constants (10 21 mol s 20°C, dioxan) for the reaction of different cyclic dienophiles with... Figure 1 Rate constants (10 21 mol s 20°C, dioxan) for the reaction of different cyclic dienophiles with...
Figure 2 Plot of log k [1 mol s ], 20°C, dioxane, for (4 + 2) cycloaddition reactions of 1,2,4,5-tetrazines with cyclic dienophiles against n, number of ring carbons. Figure 2 Plot of log k [1 mol s ], 20°C, dioxane, for (4 + 2) cycloaddition reactions of 1,2,4,5-tetrazines with cyclic dienophiles against n, number of ring carbons.
With cyclic dienophiles, rings C and D in the cycloadduct are joined in cis fashion. With acyclic dienophiles containing H-configured C=C bonds, an adduct in which the atom groups necessary for construction of the D ring are oriented, trans is produced see Chapter 3 in [33d]. [Pg.18]


See other pages where Cyclic dienophile is mentioned: [Pg.446]    [Pg.358]    [Pg.88]    [Pg.578]    [Pg.41]    [Pg.272]    [Pg.591]    [Pg.227]    [Pg.406]    [Pg.568]    [Pg.601]    [Pg.624]    [Pg.637]    [Pg.405]    [Pg.535]    [Pg.937]    [Pg.95]   


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