- Observable enols of anhydrides: Claimed literature systems, calculations, and predictions
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The literature describing the observation of enols of carboxylic anhydrides and mixed carboxylic-sulfuric anhydrides was examined. In the phenylbutyric anhydride system, the alleged enol was shown to be ethylphenylketene, and the monoenol EtC(Ph)=C(OH)OC(=O)CH(Ph)Et (5) and the dienol (6) should not be observed according to calculations. Calculations also show that the claimed enols H2C=C(OH)OSO2Y, Y= SO3-, Ac (15) and the enol of 2H-pyran-2,6(3H)-dione (7) are too unstable to be observed. The bulky enols of β,β-ditipylacetic formic (35a) or trifluoroacetic (35b) anhydride were calculated to be unstable with pKEnol=7.7 (6.2). The suggestion that compounds with the 3-acyl or 3-aroyl-2H-pyran-2,6(3H)-dione skeleton are enolic was examined. In the solid state, all the known structures show that enolization takes place on C(5)=O. However, B3LYP/6-31G** calculations show that, for 3-acetyl-4-methyl-2H-pyran-2,6(3H)-dione (10, R1=Me, R2=H), which is completely enolic, the enol on the acetyl group (cf. 12) is only 0.9 kcal/mol more stable than the enol on the anhydride (cf. 11). Calculations also revealed that 3-(trifluoroacetyl)-2H-pyran-2,6(3H)-dione (28) should exist in nearly equal amounts of the enol of anhydride (cf. 30) and the enol of the acyl group (cf. 29), whereas the enol of anhydride (cf. 32) is the only stable species for 3-(methoxycarbonyl)-2H-pyran-2,6(3H)-dione (31). Furan-2,5-diol (27) and 5-hydroxyfuran-2-one (26) are calculated not to give observable isomers of succinic anhydride (25) (pKEnol=30 and 18, resp.) in spite of the expected aromatic stabilization of 27. Surprisingly, the calculations reveal that the enol (NC)2C=C(OH)OCHO (38) is less stable than its tautomeric anhydride (37) (pKEnol=1.6). Comparison of calculated pKEnol values for (NC)2CHC(=O)X (41) and MeC(=O)X indicates that the assumption that substitution by two β-CN groups affects similarly all the systems regardless of X is incorrect. A pKEnol((NC)2CHC(=O)X) vs. pKEnol(MeC(=O)X) plot is linear for most substituents with severe and mild negative deviations, respectively, for X=NH2 and MeO. Appropriate isodesmic reactions have shown that the β,β-(CN)2 substitution increases the stabilization of the enol of amide (X=NH2) by 14.6 kcal/mol over that for the anhydride (X=OCHO), whereas the amide form is 7.1 kcal/mol less destabilized than for the anhydride. The pKEnol value for (MeOCO)2CHCOOCHO (43) is 3.6, i.e., stabilization by these βelectron-withdrawing groups is insufficient to make the enols observable.
- Rappoport, Zvi,Lei, Yi Xiong,Yamataka, Hiroshi
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Read Online
- A method for determining the enantiomeric purity of profens
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A simple method for determining the enantiomeric purity of profens (based on the carbon skeleton of 2-phenylpropionic acid) is discussed. The enantiomeric purity of a given profen can be determined by stereospecific DCC self-coupling to give a statistical diastereoisomeric mixture of racemic and meso- anhydrides. The relative ratio of diastereoisomers formed can be related to the enantiomeric excess of the original carboxylic acid.
- Coulbeck, Elliot,Eames, Jason
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body text
p. 635 - 640
(2009/08/15)
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- Homobenzotetramisole-catalyzed kinetic resolution of α-Aryl-, α-Aryloxy-, and α-Arylthioalkanoic acids
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Effective kinetic resolutions of α-aryl-, αaryloxy-, and α-arylthioalkanoic acids have been achieved via in situ generation of their symmetrical anhydrides and enantioselective alcoholysis in the presence of homobenzotetramisole (HBTM) 3.
- Yang, Xing,Birman, Vladimir B.
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supporting information; experimental part
p. 2301 - 2304
(2010/01/19)
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- Terreusinone, a novel UV-A protecting dipyrroloquinone from the marine algicolous fungus Aspergillus terreus
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A novel chiral dipyrrolobenzoquinone derivative, terreusinone (1), has been isolated as a potent UV-A protectant from the marine algicolous fungus Aspergillus terreus. The structure and absolute stereochemistry of the new compound was established by spectral interpretation, Horeau's method and quantum chemistry calculations as 2,6-bis[(1R)-1-hydroxyisobutyl]-1H,5H-pyrrolo[2,3-b]indole-4,8-dione (1). Compound 1 exhibited a UV-A absorbing activity with ED50 value of 70 μg/mL.
- Lee, Sang Mi,Li, Xi Feng,Jiang, Hualiang,Cheng, Jia Gao,Seong, Seeyearl,Choi, Hong Dae,Son, Byeng Wha
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p. 7707 - 7710
(2007/10/03)
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- Preparation of 5-substituted 2-methyl-1,3,4-oxadiazoles from 5- substituted tetrazoles and acetic anhydride
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Readily available tetrazoles can be transformed into the corresponding 2- methyl-1,3,4-oxazodiazoles by heating their acetic anhydride solution. The procedure is simple, short, gives high yields, and is applicable to large scale syntheses.
- Jursic,Zdravkovski
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p. 1575 - 1582
(2007/10/02)
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- Kinetic Resolution of Racemic Carboxylic Acids and Alcohols with Homochiral Alcohols and Carboxylic Acids, Respectively, and the Mukaiyama or Palomo Reagents
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The Mukaiyama and Palomo reagents have been used for the kinetic resolution of racemic carboxylic acids or alcohols with homochiral alcohols or carboxylic acids, respectively, in the presence of triethylamine.Thus, enantiomerically enriched carboxylic acids (e.e.68percent), alcohols (e.e.41percent) or diastereoisomerically enriched esters (d.e.84percent) are obtained.
- Mazon, Angel,Najera, Carmen,Yus, Miguel,Heumann, Andreas
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p. 1455 - 1466
(2007/10/02)
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- A NEW SYNTHESIS OF CARBOXYLIC AND CARBONIC ACID ANHYDRIDES USING PHASE TRANSFER REACTIONS
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Acyl chlorides and alkylchloroformates smoothly reacted with one molar equivalent of sodium hydroxide, using liquid-liquid phase transfer conditions to afford high yields of the corresponding symmetrical carboxylic and carbonic hemiester anhydrides.Unstable anhydrides such as 4-nitrobenzoic, 2-furoic and methacrylic anhydrides, which are otherwise difficult to obtain, were easily prepared by this method.The reaction mechanism does not seem to involve intermediate hydrolysis of half the acid chloride into the corresponding sodium carboxylate.
- Plusquellec, Daniel,Roulleau, Fabienne,Lefeuvre, Martine,Brown, Eric
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p. 2471 - 2476
(2007/10/02)
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