Linear cascade reactions

Linear Cascade Reactions Involving (a-Trantam mases I 75... 

Other things that are particularly controlled in condensation polymerization are chain topology and cascade reactivity of monomers. In general condensation polymerization, not only linear polymers but also cyclic polymers can be produced because both ends of the polymer are reactive during polymerization. When cascade reactions selectively occur on a monomer, condensation polymerization proceeds in a way different from general polymerization behavior based on the principle of Carothers and Flory. Thus, in condensation polymerization of AA and BB monomers, if AA monomers successively react with both the functional groups on a BB monomer, a poly-... 

The addition of organometals to non-activated carbon-carbon double or triple bonds, although a breakthrough in polymer chemistry about 70 years ago, has now become an efficient way of creating linear or cyclic substituted structures. The new organometallic thus formed can be quenched by various electrophiles, or can be used, in the case of polyunsaturated systems, to create polycycles via cascade reactions. 

Cascade release facilitated by linear autodegradation reactions... 

Cascade Release Facilitated by Linear Autodegradation Reactions... 

Time-resolved fluorescence spectra clearly indicate a linear cascade of excitation-energy transfer from BChl c in the chlorosome rods, through the BChl a-containing baseplate, to the core-antenna BChl a in the membrane, and finally to the reaction center. Overall excitation transfer efficiency from BChl c to the core antenna (B806-866) in Cf. aurantiacus cells has been reported to be 69 13% at 50 °C, but only -15% at 4 K. Excitation transfer from B806 to B866 within the core antenna is 100%. When the reaction center is closed, the 883-nm fluorescence decays less rapidly in -250 ps. 

Generation of non-stabilized alkyl radicals. Malacria has developed several new cascade reactions taking advantage of selective hydrogen atom abstractions by alkenyl radicals [145]. Recently, he achieved the preparation of a linear triquinane from an acyclic precursor. The last steps of the reaction sequence are a hydrogen abstraction by a vinyl radical at the /i-position of the sulfone followed by the elimination of a sulfonyl radical, affording the final product in 50% yield (Scheme 44)... 

Based on a rare example of asymmetric 1,6-addition to linear 2,4-dienals proceeding with high 6-site and stereoselectivity, a novel aminocatalytic vinylogous cascade reaction was also developed to yield valuable tetrahy-drofuran spirooxindole derivatives (13AGE10780). 

The McDonald group reported a second generation of abudinol B that employed two cascade reactions in a linear sequence (Scheme 4.36) [71]. The left tricycle was formed from diepoxide 169 through the use of an enolsilane as the... 

In an effort to develop new cascade reactions, Zhang et al. envisioned that a linear aldehyde can also be genaated in situ via an extra iminium catalysis from an ot,p-unsaturated aldehyde prior to the triple cascade reaction. Therefore, there would be a possibility of extending the triple cascade reactions to four-component cascade reactions. Based on this design, a four-component quadruple cascade reaction through iminium-enamine-iminium-enamine sequential activation initiated by oxa-Michael addition of alcohol to acrolein in moderate yield (about 50%), excellent diastereoselectivities (>20 1), and excellent enantioselectivities (>99% ee) was accomplished (Scheme 1.33) [47]. 

Using these two groups of multienzymatic systems as a basis, tiiere have been several attempts to classify such cascade reaction schemes [19, 24]. Three basic cascade schemes can be distinguished, namely, (1) linear, (2) parallel and cyclic, and (3) orthogonal. The motivation for implementahon in each case is a little different ... 

Linear cascade. Those cascades which add value to the reaction by virtue of extending the reaction sequence (either combining the previous step or the subsequent step) are referred to as linear cascades (Figure 20.2a). 

Intramolecular reaction can be used for polycyclization reaction[275]. In the so-called Pd-catalyzed cascade carbopalladation of the polyalkenyne 392, the first step is the oxidative addition to alkenyl iodide. Then the intramolecular alkyne insertion takes place twice, followed by the alkene insertion twice. The last step is the elimination of/3-hydrogen. In this way, the steroid skeleton 393 is constructed from the linear diynetriene 392(276]. 

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