Cascade reactions radical compounds

The reaction of compound 364 with a radical initiator gave a mixture of diastereomeric benzo[tf]quinolizidines 365 and 366 through the radical cascade process summarized in Scheme 83 <1999TL1149>. 

Chain growth polymerizations (also called addition polymerizations) are characterized by the occurrence of activated species (initiators)/active centers. They add one monomer molecule after the other in a way that at the terminus of each new species formed by a monomer addition step an activated center is created which again is able to add the next monomer molecule. Such species are formed from compounds which create radicals via homolytic bond scission, from metal complexes, or from ionic (or at least highly polarized) molecules in the initiating steps (2.1) and (2.2). From there the chain growth can start as a cascade reaction (propagation 2.3) upon manifold repetition of the monomer addition and reestablishment of the active center at the end of the respective new product ... 

The simultaneous construction of the B and C rings leading to compound 33 was accomplished by a radical cascade reaction. The mechanistic details of this cascade are summarized in Scheme 6, where the reaction of 34 with phenyl isonitrile (8) is shown [12]. First, a trimethylstan-nyl radical, derived from hexamethyldistannane, attacks the C-Br bond of 34. The resulting pyri-done radical 35 reacts intermolecularly with the isonitrile 8 to yield the radical intermediate 36. 

However, oxidations based on addition of O-centered radicals to unsaturated compounds appear to be a highly desirable synthetic goal, especially when the new C—O bond could be formed under the mild conditions that are typical for radical reactions. If this radical addition would involve C = C triple bonds, the resulting reactive vinyl radical would be highly suitable for the promotion of intramolecular cascade reactions. 

McCarroll AJ, Walton JC (2001) Angew Chem Int Ed 40 2224 Dhimane A-L, Fensterbank L, Malacria M (2001) Polycyclic Compounds via Radical Cascade Reactions. In Renaud P, Sibi MP (eds) Radicals in organic synthesis, vol 2. WUey, Weinheim, p 350... 

Despite the toxicity of organoselenium compounds, they are routinely applied as precursors for radical reactions. This is due to the unrivaled quality of organoselenium compounds in term of stability, ease of preparation and ease of homolysis of the carbon-selenium bond. Their rich chemistry will continue to make them particularly attractive for the development of new radical processes. This is particularly true when complex systems are examined, such as radical precursors for cascade reactions. Moreover, processes where organoselenium derivatives are used in catalytic amounts will become more attractive and environmentally more friendly. 

The uptake of ozone relates directly to its reactions with substrates present in the lung lining fluid, a mechanism referred to by Postlewaite as reactive absorption [3]. The uptake of ozone is thus related not only to its concentration but also availability of substrates within the RTLF [4]. As these are numerous, ozone does not actually transit RTLF and hence cannot interact directly with the pulmonary epithelium. Rather it is consumed during reactions with compounds in this compartment (Fig. 2). Therefore, cellular responses to ozone are not a result of the direct reaction of ozone with cell surface component/receptors but rather are mediated through a cascade of secondary, free radical derived, ozonation products [2,4]. 

Another interesting example is a cascade reaction of the compound 428 in the presence of 2,3,5,6-tetramethylterephthalonitrile as an electron acceptor and a co-sensitizer (biphenyl). The product 429 is obtained via a radical cation 430 in 23% chemical yield (Scheme 6.203).1234... 

Nitrogen centered radicals have received considerable attention in recent years. In particular, amidyl radicals have been shown to enter into cascade reactions to form pyrrolizidinone and indolizidinone derivatives. Thus, heating the (9-benzoyl hydro-xamic acid derivative 512 with Bu3SnH in the presence of AIBN produced 513 as a 3 2-mixture of diastereomers (Scheme 88) (99SL441). Separation of compound 513 from the tin residues was difficult, and the isolated yield (17%) was consequently low. When 514 was subjected to identical conditions, a 2 1-mixture of indolizidine 515 and pyrrolizidine 516 was isolated in 42% yield, along with the monocyclic... 

In this account, we have described the synthesis of polycyclic compounds by radical cascade reactions. Most of the contributions have appeared during the last decade. 

Interestingly, a cascade alkoxyl radical fragmentation-peroxidation-hydrogen abstraction reaction occurs in some cases when a hemiacetal is treated with DIB/I2 under oxygen pressure. This reaction may have interesting applications in synthetic organic chemistry. We have used it in a one-step synthesis of A and A rings of the tetranortriterpene limonene and related compounds (Eq. 19, Scheme 6) [48]. 

In this radical cascade reaction, an excellent example of a sequential-reaction , [8] the simultaneous construction of the B- and C-rings of camptothecin was accomplished. The mechanism for this radical reaction is best explained by the transformation of the structurally less complex bromoalkyne 12 into the tetracyclic compound 17. First, a tri-methylstannyl radical produced from hexame-thyldistannane attacks the C-Br bond of compound 12. The resulting pyridone radical 13 reacts intermolecularly with the isonitrile 10 to yield the radical intermediate 14. An intra-... 

Radical-based carbonylation procedures can be advantageously mediated by (TMSlsSiH. Examples of three-component coupling reactions are given in Reactions (74) and (75). The cascade proceeds by the addition of an alkyl or vinyl radical onto carbon monoxide with formation of an acyl radical intermediate, which can further react with electron-deficient olefins to lead to the polyfunctionalized compounds. ... 

Another effective radical cascade strategy started from bromomethyldi-methylsilyl propargyl ethers. " The synthesis of functionalized cyclopenta-none 108 was achieved as a single diastereomer, starting from the reduction of bromoderivative 107 in the presence of (TMSlaSiH (Reaction 83). When different substituents are used in the skeleton, as in compound 109, a completely different reaction pattern resulted (Reaction 84). 

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