Y-Carbon

It IS convenient to use the Greek letters a p 7 and so forth to locate the carbons m a molecule m relation to the carbonyl group The carbon atom adjacent to the carbonyl carbon is the a carbon atom the next one down the chain is the p carbon and so on Butanal for example has an a carbon a p carbon and a y carbon... 

Some cleavage takes place even if the phenoHc hydroxyl is blocked as an ether link to another phenylpropane unit and quinonemethide formation is prevented. If the a- or y-carbon hydroxyl is free, alkaH-catalyzed neighboring-group attack can take place with epoxide formation and P-aryloxide elimination. In other reactions, blocked phenoHc units are degraded if an a-carbonyl group is present. 

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (45 X = NH) the a carbon signal is shifted by about 20 p.p.m., to ca. S 47.7, while in A-methylpiperidine (45 X = Me) it appears at S 56.7. Quaternization at nitrogen produces further effects similar to replacement of NH by A-alkyl, but simple protonation has only a small effect. A-Acylpiperidines show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 p.p.m., and the mean shift is close to that of the unsubstituted amine (45 X=NH). The nitroso compound (45 X = N—NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 p.p.m.). The (3 and y carbon atoms of piperidines. A- acylpiperidines and piperidinium salts are all upfield of the cyclohexane resonance, by 0-7 p.p.m. 

The ether oxygen of tetrahydropyran (45 X = O) induces a large downfield shift of the a carbons, while the /3 and y carbons move slightly upfield, the y more noticeably. 

Koyama, T., Endo M. and Onuma, Y., Carbon fibers obtained by thermal decomposition of vaporized hydrocarbon, Japan J. Appl. Phys.,1972, 11,445. 

To explain tlie stereodieniistiy of tlie allylic substitution reaction, a simple stereoelectronic model based on frontier molecular orbital considerations bas been proposed fl55. Fig. G.2). Organocopper reagents, unlike C-nudeopbiles, possess filled d-orbitals fd - configuration), wbidi can interact botli witli tlie 7t -fC=C) orbital at tlie y-carbon and to a minor extent witli tlie cr -fC X) orbital, as depicted... 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

Base treatment of the following ,j3-unsaturated carbonyl compound yields an anion by removal of H+ from the y carbon. Why are hydrogens on the y carbon atom acidic ... 

Remarkable cis diastereoselectivity occurs in the addition of l-alkoxy-l-(trialkylsilyloxy)-ethenes to y-silyloxy-substituted cyclo-2-alkenones using mercury(II) iodide as a catalyst265,266. C-C Bond formation syn to the electron-withdrawing silyloxy substituent has been attributed to the stabilization of the emerging cr -orbital at the -carbon by interaction with the ct(CH) bond at the y-carbon atom267. 

Ethers and alcohols (m/z 31. 45, and 59)t Primary straight-chain alcohols Primary alcohols bonded at the y-carbon Formates... 

Pyridoxal phosphate mainly serves as coenzyme in the amino acid metabolism and is covalently bound to its enzyme via a Schiff base. In the enzymatic reaction, the amino group of the substrate and the aldehyde group of PLP form a Schiff base, too. The subsequent reactions can take place at the a-, (3-, or y-carbon of the respective substrate. Common types of reactions are decarboxylations (formation of biogenic amines), transaminations (transfer of the amino nitrogen of one amino acid to the keto analog of another amino acid), and eliminations. 

If the gas consists of a mixture of two or more simple gases e.fj.y carbon dioxide and oxygen), which dissolve in a liquid, we can assume as a first approximation that the amount of each dissolved will be independent of the presence of the other gases, and will be proportional to its partial pressure in the gas mixture standing in equilibrium over the solution (J. Dalton, 1807). 

Let us recall, for instance, the case of the crystalline form of s-PP [113], or of s-PS [114,115], which contain s(2/l)2 helices. In this kind of helix (Fig. 2) there are two non-equivalent sets of methylene carbons in the main chain, the first one on the chain axis and the second one far from the chain axis. While for the methylene on the axis there are two y-carbons (that is carbons separated by three bonds) in G conformation, for the methylene on the periphery of the helix there are two y-carbons in the T conformation. This generates the so called y-elfect, that is a shift difference between the resonances of the two methylene carbons (8.7 ppm for s-PP, 10 ppm for s-PS). 

Early kinetic work127 showed that the formation of both ortho and para products was a first-order process and that the rates of reaction were insensitive to added acid or base and to change of solvent. The activation parameters were of the same order of magnitude for both reactions and the suggestion was made that both had a similar rate-determining step. Schmid et a/.128 showed that the formation of a dienone intermediate in the para rearrangement was also reversible since the radioactivity from allyl 2,6-dimethyl-4-allyl-y-14C phenyl ether LXXXVII became uniformly distributed in the y carbon atoms of the O- and C-allyl groups... 

All the y-sultines were obtained as diastereomeric mixtures (ca 1 1, by NMR), and each one of y-sultines ( + )-49 and ( + )-51 (R = t-Bu) was separated into two diastereomers A and B by column chromatography. The oxidation of y-sultines (— )-49A and (+ )-49B to the corresponding optically active sultones (+ )-52A,B, which lack a chiral sulfur, may be taken as proof that the observed optical activity in the sultines is also due to the y-carbon. This result seems to exclude the intermediacy of vinylsulfene in the reaction mechanism, since its disrotatory closure would lead to racemic y-carbon in the product. 

The identity in sign and similarity in optical rotations of sultones (+)-52A,B, obtained from (—)-49A and (+)-49B, indicate that the absolute configuration of the y-carbon in both sultones as well as in both sultines is the same. In conclusion, the authors suggested113,114 that of the four possibilities shown below, y-sultines 48A-51A and 48B-51B may be assigned the (R)c-(Sf and (R)c-(Rf absolute configurations, respectively. Although initiated by mechanistic interest, this study has also resulted in a new method for selective synthesis of... 

Nucleophilic substitution at an allylic carbon can also take place by an Sn2 mechanism, in which case no allylic rearrangement usually takes place. However, allylic rearrangements can also take place under Sn2 conditions, by the following mechanism, in which the nucleophile attacks at the y carbon rather than the usual... 

Syn elimination and the syn-anti dichotomy have also been found in open-chain systems, though to a lesser extent than in medium-ring compounds. For example, in the conversion of 3-hexyl-4-d-trimethylammonium ion to 3-hexene with potassium ec-butoxide, 67% of the reaction followed the syn-anti dichotomy. In general syn elimination in open-chain systems is only important in cases where certain types of steric effect are present. One such type is compounds in which substituents are found on both the P and the y carbons (the unprimed letter refers to the branch in which the elimination takes place). The factors that cause these results are not completely understood, but the following conformational effects have been proposed as a partial explanation. The two anti- and two syn-periplanar conformations are, for a quaternary ammonium salt ... 

When R is vinylic it is possible, by using a combination of an alkyllithium and r-BuOK, to get migration to the y carbon (as well as to the a carbon), producing an... 

It was suggested that reaction occurred by the transfer of a hydrogen atom to the y-carbon of the allyl group, i.e.,... 

Carbon atoms are numbered from the carboxyl carbon (carbon No. 1). The carbon atoms adjacent to the carboxyl carbon (Nos. 2, 3, and 4) are also known as the a, P, and y carbons, respectively, and the terminal methyl carbon is known as the CO or n-carbon. 

Carbanion 239 derived from vinyl sulfone 238 was shown to be alkylated regiospecifi-cally at the a-carbon atom The carbanions 240 and 241 derived from <5-ketosulfones were shown to be alkylated at both a- and y-carbons . Alkylation of 240 gave a-alkylated products predominantly, while the a/y ratio varied markedly by change of the alkylating agent in the reaction with 241. For example, methylation with methyl iodide gave the products in the a/y ratio of 72 28, while the a/y ratio was 27 73 in the ethylation with ethyl bromide ... 

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