Fragmentation reactions radical cyclization

In a rare study of a radical cyclization of fragmentation-derived alkene radical cations, it was discovered that the stereochemistry of the precursor can have significant consequences on the outcome of the reaction. Thus, a gluco-... 

This radical cyclization strategy was utilized for the synthesis of the smaller fragment silyl ether 54 as well (Scheme 8). Evans aldol reaction of the boron eno-late derived from ent-32 with aldehyde 33, samarium(III)-mediated imide methyl ester conversion, and protecting group exchange led to tosylate 51. Elaboration of 51 to ketone 53 was achieved under the conditions used for construction of the second tetrahydrofuran moiety of 49 from 46. A highly diastereoselective reduc-... 

Intramolecular group-transfer reactions can involve either direct displacements or addition-fragmentation reactions that are discussed in Section 4.2.4. A direct displacement is shown in the reaction of aryl radical 1, produced from the corresponding bromide. Cyclization of 1 is fast enough to compete efficiently... 

To summarize, an atom transfer sequence has two basic requirements (i) a rapid, exothermic addition, cyclization or fragmentation reaction must convert a starting radical into a more reactive (less resonance... 

Conducting radical cyclizations by fragmentation reactions offers a powerful alternative to the tin hydride method. Instead of. obtaining reduced products, one obtains products of substitution an alkene is regenerated in the fragmentation step. 

In cases where the fragmentation reactions are not prohibitively rapid, it may be possible to conduct either cyclization or fragmentation by selection of appropriate reagents and conditions. Scheme 47 provides an example where the same two intermediate radicals are involved.200-201 The cyclized product is formed in the presence of tin hydride, but the fragmented product is produced under the atom transfer conditions. 

When a cyclization reaction precedes a fragmentation, carbon-carbon bonds are exchanged rather than created. Nonetheless, such sequences are very useful for reorganizing the carbon skeleton of a molecule, especially when easily fragmentable alkoxyl radicals are formed in the cyclization step. Depending on... 

Feldman239,240 and Oshima and Utimoto241 have introduced a very sophisticated sequence of reactions that effects a radical annulation starting from a vinylcyclopropane. Scheme 59 provides two examples and a likely mechanism for this sequence, which involves fragmentation, addition and cyclization. The... 

Fig. 72 Cu(I)-catalyzed [S-fragmentations and its application as a trigger for a radical cyclization reaction...

Radical cyclizations may occur by different means including hydride-mediated additions, radical addition of RX across a multiple bond and radical addition followed by fragmentation. Of these, the tin hydride mediated addition is the most common. Additionally, radical cyclizations may occur in tandem or three reactions may occur together within the same molecule. 

Since the computational studies support the general perception of good or poor leaving groups in self-terminating radical reactions, but clearly contradict the experimental findings, radical cyclization cascades initiated by N-centered radical addition to alkynes are not terminated by homolytic p-fragmentation. 

Unimolecular reactions of organic radical cations are fragmentation, rearrangement and cyclization, as illustrated by the following generic examples. The specific details of each of these transformations are included in Section 7.4.6. (Note Ar and R represent aryl and alkyl groups, respectively.)... 

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