Radical cyclizations carbocyclization

Moreover, the reaction with Y = OCH3 and the stereochemical control of analogous hex-5-enyl radical cyclizations has also been studied. This method constitutes part of a synthetic route from carbohydrates to optically active carbocycles.[74],[75]... 

Free-radical cyclizations using ethyl radicals generated by EtsB/air system or stannyl radicals systems provide a range of carbocyclic and heterocyclic hydroxylamines (equation 77). Stereoselectivity in these reactions is variable but can be semiquaUtatively predicted by Beckwith-Houk models . Depending on the substitution pattern of the emerging cyclic system, stereoselectivity can be very high, especially in fused polycyclic systems (equation... 

Typically a hept-6-enyI radical will cyclize much slower than the corresponding hex-5-enyl radical, and often significant proportion of the primary radical is trapped before cyclization, and the olefinic product results. However, with the appropriate substituents or an activated acceptor, hepenyl radical cyclization can be used to prepare 6-membered carbocycles from carbohydrate precursors. For example, Redlich et al. [35] reported that 1,2-dideoxyhept-l-enitol derivatives can be cyclized to carbahexose derivatives [Eq. (II)]. Even though structural requirements (protecting groups, configuration of atoms in the carbon chain, and... 

T. V. RajanBabu, From carbohydrates to optically active carbocycles, 1 Stereochemical control in sugar hex-5-enyl radical cyclization, J. Am. Chem. Soc. 109 609 (1987). 

Marco-Contelles and de Opazo utilised 8-ewd< -trig and 1-exo-Aig cyclizations, mediated by B113S11H, in their construction of chiral, polyfunctionalised medium-sized carbo-cycles670. For example, treatment of iodide (104) with BujSnll under standard radical conditions afforded 7-membered carbocycles (105) in moderate yield (equation 79). Prado and coworkers reported the synthesis of benzolactams by 11 -endo aryl radical cyclization, an example of which is highlighted in equation 80675. 

Preparation of highly functionalized medium-sized carbocycles (210) and (212), derived from carbohydrates, can also be carried out with the same procedure via 8-endo-trig and 1-exo-trig manners, as shown below (eqs. 3.81, 3.82). Moreover, preparation of (3-C-disaccharide (214) proceeds selectively via 8-endo-trig radical cyclization of two temporarily tethered monosaccharide (213), by means of the addition of a carbohydrate-... 

In a rather comprehensive work aimed at synthesizing diverse carbapentofuranoses, Desire and Prandi [8] exploited a cobalt-catalyzed radical cyclization/oxygenation of a wide number of 6-iodohex-l-enitols to assemble the carbocyclic frame of the targets. As an example, Scheme 4 displays how 4a-carba-a-D-arabinofuranose 27 was prepared by starting with readily available glucopyranoside 24. 

Quite recently, Marco-Contelles [73] has made use of free radical cyclization and ring-closing metathesis in order to develop useful synthetic protocols to access a number of chiral non-racemic, densely oxygenated medium-sized carbocycles from carbohydrate precursors. 

Martinez-Grau, A. et al. Carbocycles from Carbohydrates via Free Radical Cyclization New Synthetic Approaches to Glycomimetics. 1998 [93]... 

The formation of ring systems by the anionic cyclization of olefinic alkyl, aryl and vinyl-lithiums is an interesting synthetic transformation that provides a regiospecific and highly stereoselective route to five-membered carbocycles and heterocycles99. Most importantly, it is possible to functionalize the initially formed cyclization product by a tandem reaction with electrophiles, a reaction that is not generally possible in the case of radical cyclizations. 

Radical cyclization reactions have become a powerful tool for the constniction of carbocyclic and heterocyclic frameworks. Irradiation of 368 in the presence of benzophenone (BP) for 20 min provided the spiro 1,3-dithiolane 370 in 63% yield along with the substtate (14%) via the radical intermediate 369 (Scheme 47). The reaction carried out in the presence of acetone or acetophenone, instead of BP, resulted only in isomerization of the olefin geometry <1996T9713>. 

Free radical cyclizations have been used extensively to convert carbohydrates into carbocycles [39,40]. The general strategy consists of opening the sugar, transforming the carbonyl into a radical acceptor (alkene or alkyne) and converting one of the hydroxyls into a nucleophilic radical suitable to achieve cychzation. 

Alonso, R A, Burgey, C S, Rao, B V, Vite, G D, Vollerthun, R, Zottola, M A, Fraser-Reid, B, Carbohydrates to carbocycles — synthesis of the densely functionalized carbocyclic core of tetrodotoxin by radical cyclization of an anhydro sugar precursor, J. Am. Chem. Soc., 115, 6666-6672, 1993. 

Carbocycles from Unsaturated Halo Sugars by Hex-5-enyl Radical Cyclization [8]... 

A. Carbocycles from unsaturated halo sugars by hex-5-enyl radical cyclization 558... 

Synthesis of carbocyclic systems intramolecular free-radical cyclization, the Diels-Alder reaction, and ring-closing metathesis... 

In 1989 Curran and co-workers reported on a photocatalytically induced free-radical cyclization leading to various cyclic, bi-, or polycyclic carbocycles (fused and spiro) via isomerization of unsaturated iodides (alkenes, alkynes) [63]. This corresponds to the nonreductive variant of the tin hydride method. Under sunlight irradiation and in the presence of 10 mol% hexabutylditin, a-iodo esters, ketones, and malonates are efficiently transformed via an iodide atom transfer chain mechanism (eq. (4)). 

The first examples of radical cyclizations to afford medium-sized carbocycles via radical-initiated oxirane cleavage have been described <93JOCi2i5>. Rawal and co-workers have made contributions to this area in a series of synthetically useful examples featuring intramolecular cyclizations <90JOC5181, 92TL3439, 92TL4687, 93TL2899) (Scheme 36). 

The tandem aldol condensation-radical cyclization sequence for the elaboration of functionalized bicyclo[3.3.0]octane systems has been developed [48]. Conjugate addition of Me.AlSePh to dimethylcyclopentenone followed by trapping of the resultant enolate with aldehyde afforded the trans.erythro aldol predominantly which then underwent radical cyclization with Bu SnH and catalytic AIBN yielding the bicyclic ketol stereospecifically. This approach represents a highly convergent method for the annulation of carbocycles leading to the polyquinane sesquiterpenes. 

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