Hydrogen atom transfer reactions intramolecular cyclization

Similar reactions occur with TV-nitrosoamines R2-NNO and /V-nitrosoamides RC0(R )NN0.70 Apart from the intramolecular hydrogen atom transfer, diverse intramolecular radical additions (cyclizations) and other rearrangements have also been used to obtain cross-reaction products between NO and the radicals resulting from the transformation of the primarily formed R to another radical R. 69 70 Giving evidence for the persistent radical effect, the yields of the desired products are large and those of the selftermination of the transient intermediates are low,... 

Useful synthetic methodologies are based on the cyclization or rearrangement of the nitrogen-centered radicals generated in the reaction of the appropriate amides with (diacetoxyiodo)benzene in the presence of iodine [652-655]. Specific examples are illustrated by the synthesis of bicyclic spirolactams 622 from amides 621 [653] and preparation of the oxa-azabicyclic systems (e.g., 624) by the intramolecular hydrogen atom transfer reaction promoted by carbamoyl and phosphoramidyl radicals generated from the appropriately substituted carbohydrates 623 (Scheme 3.244) [654],... 

As with carbocation-initiated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant provide potential reaction sites. Cyclization may be followed by a second intramolecular step or by an intermolecular addition or alkylation. Intermediate radicals can be constructed so that hydrogen atom transfer can occur as part of the overall process. For example, 2-bromohexenes having radical stabilizing substituents at C(6) can undergo cyclization after a hydrogen atom transfer step.348... 

An interesting intramolecular radical cyclization followed by enantioselective hydrogen atom transfer has recently been reported (Scheme 11) [40]. This reaction is carried out in the presence of a chiral complexing agent 38, which... 

It was reported that Pd(0)-catalyzed coupling reactions of allenic alcohols, amines and acids with hypervalent iodonium salts afforded cyclized heterocyclic tetrahydrofurans, tetrahydropyrans, pyrrolidines, piperidines, or lactones under mild conditions <99SL324>. Intramolecular 1,5-hydrogen atom transfer radical cyclization reaction of pyrrolidine derivatives was examined. Reaction of 3,4-dialiyloxy-JV-(0-bromobenzyl)pyrtolidine gave hexahydro-... 

Cyclizations of aromatic diazonium salts138 (intramolecular Meerwein arylations) are pieparatively related to atom transfer reactions because a radical cyclization is terminated by the transfer of an atom or group other than hydrogen. However, the two methods are not mechanistically related. In the atom transfer method, the atom that is transferred to the cyclic product always derives from the radical precursor, but in the cyclizations of aryldiazonium salts, die atom or group transferred derives from an added reagent. This means that many different products can be prepared from a single diazonium precursor, but it... 

Intramolecular 1,5-hydrogen atom transfer cyclizations.3 In reactions with low concentrations of the tin hydride (14,313), an original vinyl radical can undergo a 1,5-hydrogen shift to produce a new radical from a C—H bond, which then undergoes cyclization (equation I). 

The (3-metaloxy radical was first exploited for synthetic purposes in C—H and C—C bond-forming reactions by Nugent and RajanBabu through the use of titanocene(III) chloride as an electron-transfer reagent [5]. They established that the (3-titaniumoxy radicals formed after electron transfer can be reduced by hydrogen atom donors, e. g. 1,4-cy-clohexadiene or tert-butyl thiol, that they add to a,(3-unsaturated carbonyl compounds, and that they can react intramolecularly with olefins in 5-exo cyclizations. 

Coverage in this chapter is restricted to the use of alkenes or alkynes as enophiles (equation 1 X = Y = C) and to the use of ene components in which a hydrogen is transferred. Coverage in Sections 1.2 and 1.3 is restricted to ene components in which all three heavy atoms are carbon (equation 1 Z = C). Thermal intramolecular ene reactions of enols (equation 1 Z = O) with unactivated alkenes are presented in Section 1.4. Metallo-ene reactions are covered in the following chapter. Use of carbonyl compounds as enophiles, which can be considered as a subset of the Prins reaction, is covered in depth in Volume 2, Chtqiter 2.1. Addition of enophiles to vinylsilanes and allylsilanes is covered in Volume 2, Chapter 2.2, while addition of enophiles to enol ethers is covered in Volume 2, Chapters 2.3-2.S. Addition of imines and iminium compounds to alkenes is presented in Volume 2, Part 4. Use of alkenes, aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.9. Coverage of singlet oxygen, azo, nitroso, S=N, S=0, Se=N or Se=0 enophiles are excluded since these reactions do not result in the formation of a carbon-carbon bond. 

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