Dimers cychc

The dimeric, cychc anhydrides reviewed by McDonald, and formed during the acetolysis and subsequent hydrolysis of fructans, have now been obtained in much higher yields by the use of fuming nitric acid. However, during these reactions, the fructofuranose form was already present, before ring-closure between adjacent molecules occurred to form the stable ring... 

Many of the physical properties are not affected by the optical composition, with the important exception of the melting poiat of the crystalline acid, which is estimated to be 52.7—52.8°C for either optically pure isomer, whereas the reported melting poiat of the racemic mixture ranges from 17 to 33°C (6). The boiling poiat of anhydrous lactic acid has been reported by several authors it was primarily obtained duriag fractionation of lactic acid from its self-esterification product, the dimer lactoyUactic acid [26811-96-1]. The difference between the boiling poiats of racemic and optically active isomers of lactic acid is probably very small (6). The uv spectra of lactic acid and dilactide [95-96-5] which is the cycHc anhydride from two lactic acid molecules, as expected show no chromophores at wavelengths above 250 nm, and lactic acid and dilactide have extinction coefficients of 28 and 111 at 215 nm and 225 nm, respectively (9,10). The iafrared spectra of lactic acid and its derivatives have been extensively studied and a summary is available (6). 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

In the absence of air or peroxides, only cycHc dimers are formed in the thermal dimerization of isoprene (33). Six cycHc dimers are formed in good yields four substituted cyclohexenes (3—6) and two dimethylcyclooctadienes (7—8). The latter two are, of course, not Diels-Alder dimers. There is some evidence that the isoprene dimerization mechanism differs from the usual Diels-Alder route. 

Most ozonolysis reaction products are postulated to form by the reaction of the 1,3-zwitterion with the extmded carbonyl compound in a 1,3-dipolar cycloaddition reaction to produce stable 1,2,4-trioxanes (ozonides) (17) as shown with itself (dimerization) to form cycHc diperoxides (4) or with protic solvents, such as alcohols, carboxyUc acids, etc, to form a-substituted alkyl hydroperoxides. The latter can form other peroxidic products, depending on reactants, reaction conditions, and solvent. 

The monomer, CPD, obtained via cracking of the dimer, DCPD, and the dimer both have extensive uses. Cyclopentadiene is probably the most widely studied conjugated, cycHc diolefin system. Eleven review articles dealing with the chemistry of cyclopentadiene have been pubHshed (1—11). An article dealing specifically with European uses of DCPD has also been pubHshed (12). The discovery ia 1951 of stable metal derivatives has given additional impetus to the study of the chemistry of cyclopentadiene. Eive review articles have been pubHshed on this subject (13—17). 

Dimer acids are relatively high mol wt (ca 560) and yet are Hquid at 25°C. This Hquidity is a consequence of the many isomers present, most with branching or cycHc stmctures. 

The coupling reaction of 1 (M=Zn) affords CPO 3 (M=Zn) in 55% yield in the presence of template 2 however, the absence of 2 decreases the yield to 34% [22]. With the increase of yield of 3, template 2 induces the selectivity of the reaction the yield of the by-product (cychc dimer 4 (M=Zn)) was changed from 23% (with no template) to 6% (in the presence of template). A similar CPO formation reaction was reported for the corresponding ruthenium porphyrins (3, M=Ru(CO)), in which the stability constant of the Ru-N coordination bond is 10 larger than that of the Zn-N coordination bond [23]. Although the transition state of the CPO produced by the ruthenium-based substrate is expected to be more stable than that produced by ZnPor, the yield of 3 (M=Ru(CO)) is only... 

Florian, J., Johnson, B. G., 1995, Structure, Energetics, and Force Fields of the Cychc Formamide Dimer MP2, Hartree-Fock, and Density Functional Study , J. Phys. Chem., 99, 5899. 

Gonzales, L., Mo, O., Yafiez, M., 1999, Density Functional Theory Study on Ethanol Dimers and Cychc Ethanol Trimers , J. Chem. Phys., Ill, 3855. 

This protein kinase (known as protein kinase A or PK-A) has an R2C2 quaternary structure that binds 3, 5 -cAMP at its dimeric regulatory (R) subunit with resultant release of two catalytic (C) subunits. The free energy of hydrolysis of the cychc nucleotide activator is large (AG 13 kcal/mol) and allows the 3, 5 -cAMP to be virtually irreversibly converted to AMP by the action of a specific phosphodiesterase. This protein kinase, originally discovered by the Nobelists Edwin Krebs and Edward Fischer, is now considered to be the prototype for over two thousand members of the protein kinase superfamily. 

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

The bis(imino)phosphoranes are prepared by a reaction sequence involving iminophosphine and cyclophosphate-trazenes, but readily dimerize to cychc four-coordinate compounds (diiminocyclodiphosphazanes) even with bulky organic substituents at nitrogen R = tert-Hu, CEts, 1-Ad (Scheme 12). ... 

Cyclophosph(V)azanes with five-coordinate phosphorus are mostly dimeric, four-membered ring derivatives, [X3PNR]2 (6). Several reviews cover the subject. A simple preparation of these compounds involves the reaction of PCI5 with primary amines (or their hydrochlorides), the so-called Kirsanov reaction. Weakly basic amines form monomeric Cl3P=NR, but with more basic amines (e.g. with R = alkyl), the products are the cyclic dimers. Various anilines give either a monomer or a cychc dimer, depending upon their basicity. ... 

Hydrogen-bond self-assembly may lead to formation of carbon-free cychc sttuctures containing oxygen and hydrogen in a ring, such as tetrameric [Ph3SiOH]4 or more complex cyclic sttuctures such as dimeric... 

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