Cascade fragmentation reactions

In another example, Postel and co-workers used functionalized carbohydrates to produce P-substituted polyoxygenated furans. Treatment of functionalized benzylidene precursors with TMS-N3 in the presence of dibutyltin oxide gave the corresponding furan in 10% yield. The authors used a computational approach to rationalize furan formation via a series of cascade fragmentation reactions obtained during 1,5-C-H insertion from alkylidenecarbenes. 

Besides these primary reactions, various secondary reactions take place in which ions or excited molecules participate. The final resulf of these three events is that, through the diverse primary and secondary fragmentations, radicals are formed and fhe complefe cascade of reactions triggered by the primary excitation of molecules may take up to several seconds. The energy deposited does not always cause change in the precise position where it was originally deposited, and it can migrate and affect the product yield considerably. 

Fragmentation of the molecules of macrobicyclic boron-capped iron(II) tris-dioximates in the gas phase occurs mainly by an alternative scheme compared with the reactions in solution [292]. Excited molecules of the complexes that gained from a cascade of reactions sufficient energy for rupture of bonds with fragmentation occurred by similar routes for all complexes considered (Table 28). 

Pharmacological inhibition of the key enzymes responsible for the consecutive activation of cascade of reactions, including aprotinin, tranexamic acid, aminocapron acid, Cl-esterase inhibitor, antioxidants, and free radical scavengers are also under evaluation to improve hemocompatibility. Research into the modification of surfaces with integrin (receptors that link cellular components to extracellular matrix proteins) fragments, such as peptides with RGD amino acid sequence, has been conducted to increase hemocompatibility and biocompatibility. Further research into the improvement of device... 

A C-radical originated by a yff-fragmentation is able to participate in cascade reactions. For example, a double /9-fragmentation reaction has been described (Eq. 3)... 

The addition of thiyl and sulfonyl radicals, either in stoichiometric or in catalytic amounts, is used as the first step of tandem processes which have been widely applied to prepare carbocyclic compounds. The catalytic procedure will be further described in Section 5.5.4.3 during the discussion of cascade processes involving fragmentation reactions. The mechanism of the stoichiometric procedure, which can involve thiols, disulfides, or any sulfonyl radical precursor, is represented in Scheme 7. It consists of (i) the initial addition step, (ii) the cyclization with formation of the final radical species, and (iii) the quenching of the final radical to give the product. 

An extremely interesting example of cascade radical reactions has been reported recently <93TL5653>. The initial radical generation from the 3-bromothiophene (565) is brought about by tributyltin hydride. This is then followed by cyclization, fragmentation, recyclization by the 5-exotrig mode and a final fragmentation. After all this, the product (566) is obtained in 74% yield (Scheme 120). 

Cascade Diels-Alder/Schmidt reactions [402] allow the synthesis ofpolycycHc compounds, which lead to useful precursors for the synthesis of natural products. Finally, fragmentation reactions can occur, and in some cases the corresponding nitriles are obtained [403],... 

A number of new resist materials which provide very high sensitivities have been developed in recent years [1-3]. In general, these systems owe their high sensitivity to the achievement of chemical amplification, a process which ensures that each photoevent is used in a multiplicative fashion to generate a cascade of successive reactions. Examples of such systems include the electron-beam induced [4] ringopening polymerization of oxacyclobutanes, the acid-catalyzed thermolysis of polymer side-chains [5-6] or the acid-catalyzed thermolytic fragmentation of polymer main-chains [7], Other important examples of the chemical amplification process are found in resist systems based on the free-radical photocrosslinking of acrylated polyols [8]. 

Fig. 8.2 Principle of the MALDI process. Initially, analyte and matrix are co-crystal I ized. After evaporation of the solvent, a nanosecond laser pulse is directed onto the crystalline surface, and both matrix and analyte molecules are desorbed. A complex reaction cascade leads to the formation of charged analyte molecules that reach the mass spectrometer without significant fragmentation.

About 20 different proteins are included in the complement system Proteins C1-C9, factors B and D, and a series of regulatory proteins. All these proteins are made in the liver, and they circulate freely in the blood and extracellular fluid. Activation of the complement system involves a cascade of proteolytic reactions. In addition to forming membrane attack complexes, the proteolytic fragments released during the activation process promote dilation of blood vessels and the accumulation of phagocytes at the site of infection. 

Meijere et al. [117] group have investigated the direct synthesis of 3-spiro-cyclopropanated p-lactams 184 (Scheme 43) using novel three-component cascade reaction. Earlier, Alberto Brandi and coworkers [118-120] have applied nitrones 181 and bicyclopropylidene 182 to obtain cylcoadducts 183, which were further fragmented under acidic conditions to afford spiro-p-lactams 184. However, this reaction required longer reaction time for the formation of cycloadducts 183. 

Base-catalyzed hydration of conjugated carbonyls, followed by retro-aldol fragmentation has been a common strategy for studying the reaction cascade (1-4). The kinetically important step in the base-catalyzed hydration of an alpha/beta unsaturated carbonyl is similar to a nucleophilic substitution reaction at carbon 3. The reaction cascade proceeds rapidly from the conjugated carbonyl through its hydration and subsequent fragmentation. 

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