Phenylsulfonyl oxime ether

Recent advances in radical reactions have greatly benefited from the efficiency of organotin reagents as mediators. Radical reaction of alkyl iodides with trifluoromethyl phenylsulfonyl oxime ether 69 and hexamethylditin at 300 nm in benzene afforded the corresponding trifluoromethyl oxime ethers 70 in high yields (Scheme 37) . 

Kim and Kim [14] have reported the addition of alkyl radicals derived from alkyl iodides to phenylsulfonyl oxime ether 107 attached to Wang resin (Scheme 22). Treatment of 107 with a range of alkyl iodides and hexam-ethylditin with irradiation at 300 nm gave the expected adducts such as 109, in moderate to good yield after cleavage from the solid support. 

The authors also investigated the feasibility of a radical cyclization-capture sequence [14] using immobilized phenylsulfonyl oxime ether 107 and iodide 110 (Scheme 23). Upon treatment with hexamethylditin, 110 undergoes a sequence of two 5-exo cyclizations followed by capture of the resultant radical by resin 107. Cleavage from the support gave 111 in moderate yield. 

Scheme 22 Alkyl radical additions to phenylsulfonyl oxime ethers by Kim and Kim [ 14]...

The Naito group has also prepared pyrrolidines [23] on solid phase by a combination of intermolecular radical addition to alkene 43 (Scheme 10) followed by intramolecular oxime ether cyclization to yield 44. These reactions proceeded sluggishly with triethylborane at room temperature, while the analogous solution-phase process was kinetically much faster. Radical additions to the phenylsulfonyl oxime ether 45 were reported by Jeon et al. [24]. Yields were better with primary and secondary alkyl iodides, and the tandem cyclization sequence with iodide 46 to afford bicyclic 47 was also accomplished, albeit in modest yield. 

The addition of alkyl radicals to vinyl sulfones to give functionalized alkenes via an addition-elimination sequence has been investigated by Russell and coworkers in some details [132, 147]. This reaction has recently been extended to unsaturated sulfimides, allowing the synthesis of styryl tetrahydrofurans and tetrahydropyrans [148]. The extension of this approach to phenylsulfonyl oxime ethers and heteroaromatic aryl sulfones (/p o-substitution) has recently been obtained with success [149, 150]. An example which comes from the work of Kim et al. is reported in equation (76) [149]. In this radical sequence, the alkyl radical generated photochemically from an alkyl iodide, in the presence of 1.2 equivalent of hexabutylditin, adds readily to the C=N bond of the oxime ether to... 

Recently, Kim and colleagues have described a new efficient method for the preparation of a-keto esters 48 via a free-radical acylation approach using (phenylsulfonyl) methoxycarbonyl oxime ether 46 as carbonyl equivalent radical acceptor (Scheme 28). The oxime 46 was conveniently prepared from readily available methylphenylsulfonyl acetate 44 by a two-step sequence (via oxime 45) as shown in Scheme 28. Nitrosation of 44 with isoamyl nitrite in the presence of sodium methoxide gave oxime 47 in 78% yield. 

The combination of carbon monoxide with sulfonyl oxime ethers allow for a set of multicomponent coupling reactions involving consecutive Cl/Cl-type coupling, a rare class of radical multicomponent reactions. In Scheme 6.27, examples of three-, four-, and five-component coupling reactions are shown [46], In these reactions, allyltin is not incorporated into the product, but serves as an acceptor of the phenylsulfonyl radical and a source of the tributyltin radical, which delivers the radical chain. 

This free-radical acylation approach is extended for the synthesis of a-keto esters and ketones using phenylsulfonyl methoxycarbonyl oxime ether 5 [23] and bis-methanesulfonyl oxime ether 6, respectively (Scheme 6) [24], 5 is more reactive and effective than 2b. For instance, radical reaction of tert-butyl iodide with 5 gave tert-butyl oxime ester in 65% yield, whereas the use of 2b gave the corresponding tert-butyl oxime ether in 15% yield. In free-radical-mediated ketone synthesis via a sequential radical acylation approach, 6 is used as a carbonyl equivalent geminal radical acceptor. This method works well with primary alkyl iodides but somewhat less efficiently with secondary iodides and can be applied to prepare unsymmetrical acyclic ketones as well as cyclic ketones. It is noteworthy that stable allylic and benzylic radicals react smoothly with 6. 

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