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Heteroatom radical cyclizations

Examples of heteroatomic radical cyclizations leading to bridged rings will now be described. Oxabicyclic compounds 306 (R = H) have been obtained by photolysis of the nitrites (305) (oxime not isolated) (Scheme 121). When R = Me compounds 307 are obtained and, although yields are modest, this shows that intramolecular addition of an alkoxy radical is possible and that it gives exclusively the (Cy 5) radical in the Cy5/Cy6 case. [Pg.228]

Heteroatom radical addition-cyclization and its synthetic application 99H(50)505. [Pg.214]

As with the other procedures for the preparation of six-membered heterocyclic systems which proceed via formation of only one ring bond there are relatively few methods which involve formation of a ring bond y to the heteroatom and which can best be classified as [6 + 0] processes rather than [4 + 2], [3 + 3], etc, processes. Of those which can be so represented, however, a number are important processes which are widely used for the synthesis of saturated, partially saturated and aromatic six-membered heterocyclic systems and their benzo derivatives. Mechanistically, the nucleophile —> electrophile approach is by far the most common, but in contrast to the reactions discussed in the previous three sections, radical cyclizations are of considerable utility here. [Pg.73]

This chapter begins with an introduction to the basic principles that are required to apply radical reactions in synthesis, with references to more detailed treatments. After a discussion of the effect of substituents on the rates of radical addition reactions, a new method to notate radical reactions in retrosynthetic analysis will be introduced. A summary of synthetically useful radical addition reactions will then follow. Emphasis will be placed on how the selection of an available method, either chain or non-chain, may affect the outcome of an addition reaction. The addition reactions of carbon radicals to multiple bonds and aromatic rings will be the major focus of the presentation, with a shorter section on the addition reactions of heteroatom-centered radicals. Intramolecular addition reactions, that is radical cyclizations, will be covered in the following chapter with a similar organizational pattern. This second chapter will also cover the use of sequential radical reactions. Reactions of diradicals (and related reactive intermediates) will not be discussed in either chapter. Photochemical [2 + 2] cycloadditions are covered in Volume 5, Chapter 3.1 and diyl cycloadditions are covered in Volume 5, Chapter 3.1. Related functional group transformations of radicals (that do not involve ir-bond additions) are treated in Volume 8, Chapter 4.2. [Pg.716]

The following introduction will briefly recount some of the key features of radical cyclizations with an emphasis on basic concepts that control regio- and stereo-selectivity. More details will be provided in the following sections, which describe specific types of reactions. The factors affecting the cyclization reactions to carbon-heteroatom multiple bonds are treated separately in Section 4.2.5, and the cyclizations of heteroatom-centered radicals are contained in Section 4.2.4. [Pg.780]

The placement of heteroatoms adjacent to the radical center often slows the rate of cyclization and, although low concentration techniques are required, good yields of cyclic products can still be obtained. Hart s systematic studies on the cyclizations of acylamino radicals were among the first to demonstrate the preparative utility of radical cyclizations.78 Scheme 19 provides some recent examples. In general,... [Pg.794]

Intramolecular addition of heteroatom radicals to olefins constitutes a convenient method for the synthesis of heterocycles. The photochemical ring closure reaction of oxyl radical derived from 44 provides access to tetrahydrofuran 45 [95JOC6706]. The regioselectivity in this cyclization is excellent, however, the stereoselectivity is only modest. The stereoselectivity was dependent on the temperature of the reaction. [Pg.19]

Early examples of intramolecular aryl radical addition reactions to heteroatom containing multiple bonds included cyclizations on N=N and C=S moieties [52, 53]. Recently, cyclizations to imines have been used as part of a new enantio-selective approach to indolines (Scheme 8). In the first step of the sequence, the required ketimines 19 were obtained by phase-transfer catalyzed alkylation of 2-bromobenzyl bromides 20 with glycinyl imines 21 in the presence of a cincho-nidinium salt [54], Due to the favorable substitution pattern on the imine moiety of 19, the tributyltin hydride mediated radical cyclization to 22 occurred exclusively in the 5-exo mode. The indoline synthesis can therefore also be classified as a radical amination. [Pg.38]

Examples of stereoselective cyclizations involving heteroatom radicals are rare. Tandem oxy radical cyclization and diastereoselective hydrogen atom transfer reactions, however, have also been studied (Eq. (13.58)) [74]. These reactions take advantage of chirality at the y carbon to induce anti-/ cycloaddition. Hydrogen... [Pg.534]

Although the examples shown for the most part use AIBN and tributyltin hydride in refluxing benzene, other solvents can be used. For instance, radical cyclization of 165 in aqueous ethanol using a combination of phosphoric acid and a base led to 166 in 83% yield. In this example, it is clear that heteroatoms can be part of the chain that links the radical precursor and the alkene. Stork introduced a modification that produces protected lactones via radical cyclization. In a synthesis of 12a-deoxytetracycline, Stork converted 167 to 168 under standard conditions, in 90% yield. 0... [Pg.1176]

A typical radical cyclization involves the attack of a radical center on an sp carbon of a double bond (or other unsaturated group) in the chain. If the chain includes one or more heteroatoms, then a heterocycle will be formed. In Scheme 4.44, we examine the cyclization of a radical to form a 5-membered ring (a pyrrolidine) by this process. Note the practice of showing single-electron processes with single-barbed arrows ( fish hooks ). [Pg.82]

Regardless of the precise structure of the chosen half southern synthon, the two main problems to be solved are the establishment of the carbon skeleton and the introduction of the necessary chirahty into the molecule. The published approaches have introduced chirality either by resolution, by starting with a chiral precursor, or via use of asymmetric synthesis techniques. The carbon skeleton has been established by use of a wide variety of techniques including the Diels-Alder and other cycloaddition reactions, heteroatom induced cyclizations, intramolecular Michael or Aldol cyclizations, intramolecular ether formation, and radical cyclization. [Pg.82]

In addition, Chattopadhyay et al. have estabhshed a straightforward three-step synthetic route to dibenzo-fused nine-membered oxacycles 408 using sequential MBH reaction and radical cyclization on salicylaldehyde derivatives as described in Scheme 3.180. This methodology provides the possibihty of obtaining synthetically challenging nine-membered dibenzo-heteroeyeles incorporating nitrogen or sulfur heteroatom. " ... [Pg.289]

In Cramer s [60] heteroatom-nucleophile-induced C-C fragmentations, an allenic hydroxamic acid 162 was employed for a domino oxidative radical cyclization to afford the densely functionalized 1,2-oxazine 166 in 52% yield... [Pg.164]

Radical Cyclizations. Several examples of radical cycliza-tions have been reported during the selenosulfonation of cyclic or acyclic dienes. When the tether linking the two alkene moieties includes a heteroatom, the procedure affords the corresponding heterocycles. Illustrative examples are shown in eqs I4 i7.3,22d,22b,22f jjj gq quatemization of the nitrogen atom with a Lewis acid affects the yield and stereochemistry of the radical cyclization. Similar cychzations occur when unconjugated enynes or bisallenes are subjected to selenosulfonation. [Pg.463]

In general many of the features which govern the cyclization of carbon radicals are met with again in reactions involving heteroatomic radicals. But it is also true that the nature of Z may introduce subtle, or dramatic, changes in I I... [Pg.159]

Intramolecular addition of heteroatomic radicals to a double bond is, in most of the cases studied, a more general and more efficient process than intramolecular addition of carbon radicals. It is thus possible to propose new methods for the preparation of heterocyclic compounds and to consider that these processes are involved in important biogenetic schemes such as penicillin or prostaglandin biosynthesis. It is not always easy to conclude that a free radical process is involved because of the possibility of competitive ionic cyclizations. [Pg.192]

Finally, it must be said that while the main features concerning the cyclization of unsaturated 0-, N-, or S-centered radicals are beginning to be understood, very little is known about the behavior of other heteroatomic radicals, although some very interesting features emerge from the first reports published. [Pg.192]

Very little is known of the intramolecular addition of other heteroatomic radicals to triple bonds, although very promising results have been reported by Markl. By AIBN initiation, phenylphosphine (X = P) and phenylarsine (X = As) add to hexa-l,5-diyne (162). The yields of cyclized products 163 are quite good (17% for X = As and 33%, for X = P), if one considers the multistep pathway probably involved (Scheme 73). It must also be emphasized that the (Cy7) compound 163 is exclusively obtained in this Cy6/Cy7 case and this is clearly reminiscent of the results obtained with ethylenic phosphines (Section VIII.5.A). [Pg.198]


See other pages where Heteroatom radical cyclizations is mentioned: [Pg.2091]    [Pg.143]    [Pg.73]    [Pg.150]    [Pg.153]    [Pg.744]    [Pg.783]    [Pg.792]    [Pg.223]    [Pg.33]    [Pg.279]    [Pg.1439]    [Pg.10]    [Pg.155]    [Pg.156]    [Pg.777]    [Pg.950]    [Pg.1122]    [Pg.1177]    [Pg.1179]    [Pg.226]    [Pg.815]    [Pg.196]   
See also in sourсe #XX -- [ Pg.1176 , Pg.1177 , Pg.1178 , Pg.1179 , Pg.1180 ]




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