Furan dipole

Furan, 3-bromo-C7, 4, 587 <75JCS[Pg.21]

PE, 4. 26 73MI30102, 74MI30100), 587 <73JA291> Furan, tetrahydro-, tin tetrachloride complex dipole moment, 4, 555 B-63MI31000)... 

H NMR, 4, 558 <65ACS485) Furan-2-carbaldehyde (furfural) dipole moment, 4, 555 (B-63MI31000, 65MI31001) AHe, 4, 543 (72T3015)... 

H NMR, 4, 557, 566 <75CS(7)21I) Furan-2-carboxamide dipole moment, 4, 555 [Pg.24]

Furan-2-carboxylic acid dipole moment, 4, 555 ... 

Fu ran-2(3 H)-one, 3-b romo-4-methoxy-dipole moment, 4, 555 B-63MI3I000) Furan-2(3f/)-one, 4,5-dihydro-dipole moment, 4, 555 (B-63MI3I000)... 

Thieno[3,4-c]furan dipole moment, 4, 1049 (74JA1817) Thieno[2,3-d]pyridazine... 

Furan, 2,5-bis(trimethylsilyloxy)-cycloaddition reactions, 4, 625 Diels-Alder reactions, 4, 77 synthesis, 1, 417 Furan, bromo-dipole moments, 4, 553 Furan, 2-bromo-electron diffraction, 4, 537 reactions, 4, 78 synthesis, 4, 604 Furan, 3-bromo-electron diffraction, 4, 537 Furan, 2-bromomethyl-5-nitro-reactions... 

Furan, 2,5-diacetoxy-2,5-dihydro-dipole moments, 4, 556 Furan, 3,4-diacetyl-2,5-dimethyl-structure, 4, 539... 

Furan-2(3H)-one, 4,5-dihydro-5-methyl-dipole moments, 4, 556 Furan-2(3H)-one, 5-methyl-reactions, 4, 648 Furan-2(5H)-one, 5-alkoxy-synthesis, 4, 135 Furan-2(5H)-one, 5-ethyl-synthesis, 4, 135... 

The electric dipole moments in units 1 X 10 18 e. s. u. of these molecules and their derivatives by hydrogenation measured19 in benzene solution are the following furan, 0.670 2,5-di-hydrofuran, 1.53 tetrahydrofuran, 1.68 pyrrole, 1.80 pyrroline, 1.42 pyrrolidine, 1.57 thiophene, 0.54 and tetrahydrothiophene, 1.87. We now give a very rough interpretation of these quantities based on the bond moments given... 

Eor other examples of dipoles adding across a rr-bond of a furan see Heinze I, Eberbach W (1988) Tetrahedron Lett 2051... 

To investigate the feasibility of employing 3-oxidopyridinium betaines as stabilized 1,3-dipoles in an intramolecular dipolar cycloaddition to construct the hetisine alkaloid core (Scheme 1.8, 77 78), a series of model cycloaddition substrates were prepared. In the first (Scheme 1.9a), an ene-nitrile substrate (i.e., 83) was selected as an activated dipolarophile functionality. Nitrile 66 was subjected to reduction with DIBAL-H, affording aldehyde 79 in 79 % yield. This was followed by reductive amination of aldehyde x with furfurylamine (80) to afford the furan amine 81 in 80 % yield. The ene-nitrile was then readily accessed via palladium-catalyzed cyanation of the enol triflate with KCN, 18-crown-6, and Pd(PPh3)4 in refluxing benzene to provide ene-nitrile 82 in 75 % yield. Finally, bromine-mediated aza-Achmatowicz reaction [44] of 82 then delivered oxidopyridinium betaine 83 in 65 % yield. 

The intramolecular cycloaddition reactions of the nitrile oxides 357 (n = 1, 2, 3, 9), obtained in situ from the 2,5-difunctional furan hydroximoyl chlorides or nitro compounds (415) has specific features because of the 2,5-arrangement of two open chains bearing acetylenic and fulminic moieties. Only with 357 (n = 3) is the expected furanoisoxazolophane 358 formed, in acceptable yield. Compound 357 ( =9) gives a complex product mixture whereas 357 ( = 1, 2) gives rise to the exclusive reaction of the dipole with a double bond of the furan system. 

Note that the dipoles of furan and thiophene are opposite in direction to that in pyrrole. In furan and thiophene, there is a greater inductive effect opposing the resonance effect, whereas in pyrrole the resonance... 

One of the earliest uses for rhodium(II)-catalyzed dipoles was demonstrated in Davies furan synthesis [22]. Isomiinchnones were also shown to produce substituted furans [115]. Additional furan syntheses have been described using silylacetates [116], unsaturated esters [117], and fluoroalkyl diazo acetates [118]. The synthesis of furofuranones and indenofuranones 35 from a-diazo ketones having pendant alkynes has also been reported (Eq. 6) [119]. Other fused heterocyclic systems include furo[3,4-c]furans [120, 121] furo[2,3-b]furans [122] as well as thiobenzofurans [123], and benzoxazoles[124] have also been synthesized with this methodology. 

The intramolecular cycloaddition of nitrile oxides to substituted furans was reported to occur with low stereoselectivity (274). Inserting a stereogenic unit within the chain connecting the dipole and dipolarophile did not increase the stereoselectivity (274). 

In the full account of this work, Padwa et al. (41) demonstrated that the 1,3-dipolar cycloaddition is an endo cycloaddition and the regiochemistry is consistent with that of a HOMO-dipole controlled process as judged from the products 91 and 92 that arise from the reaction between isomiinchnone 90 and methyl propiolate and phenyl vinyl sulfone, respectively (Scheme 10.15). Iso-miinchnone 90 is also trapped with DMAD to give the expected furan in 41% yield. 

Measurements of dipole moments, Kerr constants, and dielectric absorption have been employed (81RCR336) widely to obtain information on the conformational equilibrium in acyl heterocycles. Details on conformer structures and populations depend on the choice of additive scheme, group moments, or polarizability tensor in the case of Kerr constants. Several early conclusions, especially for furan- and thiophene-2-carboxaldehyde, appeared contradictory, owing to the choice of these quantities. A more precise definition of polarizability tensors for several heterocycles and a choice of group moments and additive schemes tested on a large amount of available experimental results and supported by accurate theoretical calculations have led to more confidence in the use of experimental dipole moments and Kerr constants in conformational analysis. A limitation of the method is that the... 

The results of dipole moment and Kerr constant elaboration indicated (84JST(116)377) that in the 2-pivaloyl derivatives of furan and pyrrole the... 

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