Oxime ethers, intermolecular radical

The C=N bond of simple imines possesses modest reactivity toward intermolecular radical additions, so such acceptors have rarely been exploited. To enhance their reactivity toward nucleophilic radicals, electron-withdrawing groups at the imine carbon have been effective, as demonstrated by Bertrand in radical additions to a-iminoesters prepared from chiral amines [25]. Also, more reactive oxime ethers have been exploited extensively for radical addition, mainly through the longstanding efforts of Naito [26]. In most cases, stereocontrol has been imparted through the substituents on the imino carbon chiral O-substituents on oximes for stereocontrol were ineffective, presumably due to poor rotamer control [27, 28]. 

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. 

DCA-sensitized irradiation of oxime ethers 39e and 40c afforded the corresponding cyclopropanes 41e and 42c in low yield. However, for unknown reasons, irradiation of compound 39f under these conditions gave a complex mixture of products in which the corresponding 1-ADPM product was not present. Irradiation of oxime ether 40c also afforded the [4-1-4]-cycloadduct 60." The mechanism shown in Scheme 15 could justify the formation of 60. This involves the generation of a radical-cation/radical-anion pair 61. Intramolecular cyclization of the radical-cation within the solvent cage generates the intermediate 62, which undergoes intermolecular cycHzation, to yield the observed product (Scheme 15). 

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