Preferential cleavage mechanism

A. Mechanisms of Preferential Cleavage through Neighboring Hydroxy]... 

The assistance from a hydroxyl group in the hydrolysis of an amide bond either via the (largely reversible) hydroxyoxazolidine or the (largely irreversible) lactone mechanism leads to preferential cleavage of peptide bonds next to serine and threonine, on the one hand, and to lactones of... 

Any mechanism written for this preferential cleavage must take into account the double cleavage on both sides of the aspartyl residue that leads eventually to liberation of free aspartic acid (XXI). The nucleophilic concerted interactions (XVI-XVIII) pictured in the proposed mechanism may lead to a bicyclic orthoesteramide (XVII) which would collapse with the release of aspartic acid (XXI). Although the details of this tentative mechanism remain to be established, there is ample precedent for the formation of anhydride intermediates (XX) in intramolecular electro-... 

The radical mechanism involves the preferential cleavage of the formal bond and formation of 1,3-oxathiolane, as well as the cyclic monomer (l,3-dioxa-6,7-dithionane), as the main products. 

Because chalcopyrite does not have preferential cleavage plane, many different surfaces are exposed when the mineral is broken, increasing the difficulty to determine experimentally their properties. In fact, the previous study about the reconstruction of both terminations of the (001) surface was extended to other surfaces like (100), (111), (112), (101) and (110) providing general information about the chalcopyrite surfaces. Three reconstruction mechanisms arose from this investigation. Slab models were constructed to describe properly the surfaces and the convergence of the k point grid was checked for each case. Table 2 summarizes the main characteristics of these reconstructed surfaces. 

As a side reaction, the Norrish type I reaction is often observed. The stability of the radical species formed by a-cleavage determines the Norrish type 1/Norrish type II ratio. For example aliphatic methyl ketones 10 react by a Norrish type II-mechanism, while aliphatic tcrt-butyl ketones 11 react preferentially by a Norrish type I-mechanism. 

Although, as stated above, we wiU mostly focus on hydrolytic systems it is worth discussing oxidation catalysts briefly [8]. Probably the best known of these systems is exemphfied by the antitumor antibiotics belonging to the family of bleomycins (Fig. 6.1) [9]. These molecules may be included in the hst of peptide-based catalysts because of the presence of a small peptide which is involved both in the coordination to the metal ion (essential co-factor for the catalyst) and as a tether for a bisthiazole moiety that ensures interaction with DNA. It has recently been reported that bleomycins will also cleave RNA [10]. With these antibiotics DNA cleavage is known to be selective, preferentially occurring at 5 -GpC-3 and 5 -GpT-3 sequences, and results from metal-dependent oxidation [11]. Thus it is not a cleavage that occurs at the level of a P-O bond as expected for a non-hydrolytic mechanism. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

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