Intermediates cleavage

The retro-Claisen reaction occurs by initial nucleophilic addition of a cysteine -SH group on the enzyme to the keto group of the /3-ketoacyl CoA to yield an alkoxide ion intermediate. Cleavage of the C2-C3 bond then follows, with expulsion of an acetyl CoA enolate ion. Protonation of the enolate ion gives acetyl CoA, and the enzyme-bound acyl group undergoes nucleophilic acyl substitution by reaction with a molecule of coenzyme A. The chain-shortened acyl CoA that results then enters another round of tire /3-oxidation pathway for further degradation. 

The butyllithium promoted rearrangement of 4,5-benzo-3-thiatricyclo[4.1.0.02 7]heptenes 20209 apparently involves the intermediate cleavage of a C — S bond leading to a cyclobutenyli-dene, and the cobalt(II) tetraphenylporphyrin [TPPCo(II)] catalyzed isomerization of bicy-clobutanecarbonitrile 22 210 is initiated by radical attack of the catalyst at C3. 

Cleavage of Si—P bonds is fast Ge—P and Sn—P bonds are less reactive PhjSnPPhj is inert to hot HjO. The stepwise nature of multiple cleavage reactions can be used to prepare intermediate cleavage products, e.g., under controlled conditions both Me3SiP(H)Ph ... 

Accurate measurement of proinsulin has been difhcult for several reasons the blood concentrations are low antibody production is difficult most antisera cross-react with insulin and C-peptide, which are present in much higher concentrations the assays measure intermediate cleavage forms of proinsulin and reference preparations of pure proinsulin are not readily available. However, a more sensitive nonequiUb-rium RIA method for measuring proinsiilin was developed by adsorbing the initial antiserum with biosynthetic human C-peptide coupled to agarose to eliminate cross-reactivity with C-peptide.An enzyme-linked immunosorbent assay (ELISA) has been described that employs an antibody to C-peptide as the coating antibody and antiinsulin antibody for detection. The detection limit is 0.25 pmol/L. ... 

Nucleophilic aminocarbenes 7 have been discussed as intermediate cleavage products in the reaction of l,2-diamino-l,2-diheteroethenes 5 with electrophiles they are also formed by a-elimination from 1,3-heteroazolidines, by deprotonation of 1,3-heteroazonium salts, and by extrusion of nitrogen from 2-azido-l,3-heteroazonium salts. Support for the actual occurrence of these carbene intermediates is provided by the isolation of the stable bis-l,3-(l-adaman-tyl)imidazolidin-2-ylidene in the deprotonation of bis-l,3-(l-adamantyl)imidazolium chloride with sodium hydride in the presence of a catalytic amount of dimethyl sulfoxide anion. This synthetic strategy has also been applied to other stable l,3-imidazoIidin-2-ylidenes. ... 

Distinct mechanisms may be operative for the bimetallic versus the monometallic forms of these enzymes.125-128 Generally, mechanistic pathways have been proposed wherein the (3-lactam carbonyl is polarized via interaction with a zinc center. Following nucleophilic attack by a zinc-bound hydroxide and formation of a tetrahedral intermediate, cleavage of the C-N bond occurs. Protonation of the (3-lactam nitrogen atom then occurs, along with dissociation of the carboxylate moiety from the zinc center resulting in the release of the ring-opened product. 

Note The existence of 4.12f can be accounted for by formation of a [5]paracyclophane intermediate, cleavage of the para bridge and insertion of diethyl ether which serves as a solventf. ... 

Rh(I) catalysts under H2 effect the reductive cychzation of diynes and enynes [119]. In the example below (1.39), Rh(C0D)20Tf (COD = 1,5-cyclooctadiene) is converted by H2 gas to a Rh hydride, which is thought to undergo oxidative cycliza-tion to produce a rhodacyclopentene intermediate. Cleavage of the Rh-C bonds by reductive elimination to form a vinyl rhodium species, and hydrogenolysis of that intermediate, forms the Rh hydride and the cychzed product. 

The tail-to-tad coupling of two geranylgeranyl diphosphate units proceeds via a cyclopropane intermediate. Cleavage of the diphosphate from the primary cyclopropane derivative, ring-opening of the resulting cyclopropyhnethyl carbo-... 

Via intermediates Cleavage of methyl ethers via formoxy compds. 

Rule 4. New functional properties of the intermediate cleavage products are aimed at the restoration of the biological system (Le Chatelier s principle in biology). 

Thus, it appears that a normal complement of the final virion structural polypeptides El, E2, and C is not required for the cellular protein inhibition, although intermediate cleavage products or an uncleaved precursor polypeptide may be involved. Which virus-specific polypeptide is responsible for damaging the sodium pump and altering the permeability of the host cell membrane can only be determined by further experimentation. 

On the other hand, Atkins (1976) has shown that RNA t5 mutants and those RNA t mutants of SIN virus which make only the uncleaved precursor to the structural proteins at the restrictive temperature fail to inhibit DNA synthesis. These data suggest one or more of the structural proteins or intermediate cleavage products is responsible. Therefore, further studies are necessary to determine whether a virus coded nonstructual or structural protein is responsible for inhibition of host DNA synthesis. 

Experimental evidence on the relationship between structure and reactivity in Norrish type II photochemistry of spirobenzoyladamantanes in the solid state has been produced. Geometric factors clearly determine partitioning between cleavage, cychzation, and hydrogen transfer reactions of the 1,4-biradical intermediate Cleavage becomes predominant when the singly occupied orbitals overlap poorly with one another but well with the central CC bond, which, as earlier sections indicate, has been presumed for years based on ketone structure. 

We prefer, therefore, such an intermediate cleavage to occur at 10-10a or 10-9a in the center pyrazine subnucleus, which is well known to be rapidly reversible from the old chemical synthesis of the flavin nucleus. 

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