Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Enolates furan

The oxidation of the cyclic enol ether 93 in MeOH affords the methyl ester 95 by hydrolysis of the ketene acetal 94 formed initially by regioselective attack of the methoxy group at the anomeric carbon, rather than the a-alkoxy ketone[35]. Similarly, the double bond of the furan part in khellin (96) is converted ino the ester 98 via the ketene acetal 97[l23],... [Pg.34]

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. [Pg.35]

The cyclic enol ether 255 from the functionalized 3-alkynoI 254 was converted into the furans 256 by the reaction of allyl chloride, and 257 by elimination of MeOH[132], The alkynes 258 and 260, which have two hydroxy groups at suitable positions, are converted into the cyclic acetals 259 and 261. Carcogran and frontalin have been prepared by this reaction[124]. [Pg.501]

The 4-kcto group in the alkyne 262 as an enol form adds to the triple bond to give the furan 263[133], Even the conjugated keto alkyne 264 was converted into the furan 266 via isomerization to the allenyl ketone 265[134],... [Pg.502]

Only the potentially 2,4-dihydroxy derivatives of furan and thiophene are known and these exist in the solid state and in polar solvents as the monoenols (82) (71T3839). However, in non-polar solvents the furan derivatives exist predominantly in the dioxo form (83). The 2,5-dioxo structure (84) is well established for X=0, NR, S and Se (71BSF3547) and there is no evidence for intervention of any enolic species. The formal tautomer (85) of succinimide has been prepared and is reasonably stable (62CI(L)1576). [Pg.37]

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). [Pg.37]

The course of the acid catalyzed dehydration of 1,4-diketones to furans, known as the Paal-Knorr method (1884CB2756), entails the formal addition of the enol of one carbonyl group to the other carbonyl. Examples which illustrate some of the routes used to make the necessary 1,4-diketones are shown in Scheme 13. Few examples are known of the preparation of the other heterocycles by this general approach using isolated intermediates, although some of the ring closures discussed in Section 3.03.3.1.1 are mechanistically equivalent. One example of the preparation of a hydroxypyrrole is included in Scheme 13 <59AC(R)2075). [Pg.97]

FEIST - SENARY Furan Synthesis Synthesis ol lurans by condensation ol an a-halocarbonyl compound with an enol. [Pg.115]

In the chemical shift range for alkenes and aromatic and heteroaromatic compounds enol ether fragments (furan, pyrone, isoflavone, 195-200 Hz) ... [Pg.27]

The 17a-ethynyl compound (59) has been prepared in 88% yield from estr-4-ene-3,17-dione (58) and acetylene, at 2-3 atm pressure in tetrahydro-furan in the presence of potassium t-butoxide. Presumably the A-ring enone system is protected as the enolate anion during the course of the reaction. [Pg.66]

Similarly, 1-alkylpyrroles, indoles, furans, thiophenes [60], a-picoline [61], enols, malonates [76], and organometallic compounds [56, 62] react with acyl imines of trifluoropyruvates to give derivatives of a-trifluoromethyl a-amino acids... [Pg.842]

It was reported that the eorresponding 2-hydroxy-3-aeetyl-benzofuran exists entirely in the enol form (73AJC1079 84H737 87CC1150) [produees deep blue eolor with ferrie ehloride (55JA1623)].This is in aeeordanee with AMI, PM3, and B3LYP/6-31G ealeulations for 3-formyl-2-hydroxybenzo [h]furan as model eompound (99UP1) (see Seheme 10, all values in keal/mol). [Pg.98]

Naphtho[2,l-h]furan-2-(3 -one 28 has been described as a keto tautomer (91JA2301). Naphtho[l,2-h]furan-3-(2//)-ones of type 29 (R = H, Me, Et, Pr, pentyl, heptyl) show keto-enol tautomerism with the enol form predominating (88RRC917). [Pg.99]

Very little is known concerning the simple, monocyclic 3-hydroxy-furans (cf. reference 15). Both the oxo and hydroxy forms of the substituted 3-hydroxyfurans 26 and 27 (R = H, CcHn) have been isolated/ but the individual tautomers slowly undergo interconversion. The enol forms give a positive reaction with ferric chloride, react rapidly with bromine, and form a peroxide with oxygen. From chemical evidence, the benzo derivatives of 3-hydroxyfuran, 28 and 29, appear to exist predominantly in the oxo form, and this is further supported by ultraviolet spectral data. Stefanye and Howard- ... [Pg.6]

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... [Pg.20]

Reacdons of 2-f2-nitrovinyl -l,4-ben2Qquinone with furans, indoles, and endocyclic enol ethers form angular, fused heterocyclic quinoidring systems fsee Eq. 8.13. "... [Pg.237]

High-pressnre promoted cycloadditions of nitroalkenes and enol ethers eliminate the nse of Lewis acids fEq 8 106 "Thus, even sterically hmdered nitroalkenes react with 2,3-thhydro-furan to give the exo cyclic nitronates stereoselecdvely without using Lewis acids... [Pg.279]

Due to the /( -elimination of thiolate, the enolate of (2/ ,5/ )-2-/enolate formed in situ delivers the adducts in >90% diastereoselectivity and about 50% chemical yield115. [Pg.511]

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). [Pg.44]

Diketones give furans when treated with acids. This is actually an example of an intramolecular addition of an alcohol to a ketone, since it is the enol form that adds ... [Pg.1181]

Yosikoshi reported the synthesis of furan derivatives by the reaction of 1,3-diketones with nitroalkenes, in which the Michael addition of the anions of 1,3-diketones and the subsequent intramolecular displacement of the nitro group by enolate oxygen are involved as key steps (Eq. 7.40).42... [Pg.192]


See other pages where Enolates furan is mentioned: [Pg.466]    [Pg.319]    [Pg.36]    [Pg.36]    [Pg.46]    [Pg.57]    [Pg.128]    [Pg.216]    [Pg.232]    [Pg.235]    [Pg.38]    [Pg.46]    [Pg.161]    [Pg.291]    [Pg.112]    [Pg.321]    [Pg.10]    [Pg.490]    [Pg.112]    [Pg.468]    [Pg.1009]    [Pg.867]    [Pg.119]    [Pg.104]    [Pg.61]    [Pg.813]    [Pg.113]   
See also in sourсe #XX -- [ Pg.162 , Pg.180 , Pg.197 , Pg.229 ]




SEARCH



Furan enol ether route

© 2024 chempedia.info