Nitronates radical cyclization

The initially formed nitronate radical reacts with olefin R C=CR" to give an elongated radical A whose successive cyclization and oxidation affords target nitronates (5g) in 18% to 81% yields. Manganese triacetate is regenerated by electrochemical methods. 

Pseudocopsinine (215) was isolated from Vinca erecta. Parsons et al. devised a concise synthesis of the pseudocopsinine framework 214 utilizing a 5-exo/5-exo double tandem radical cyclization [83-84]. The cyclization precursor 213 was readily accessible via a [3 + 2] nitrone cyclization. As depicted in Scheme 40, reaction between the pyrrolidine aldehyde 210 ando-... 

The Wittig-Horner olefination of the aldehyde 28 provided alkenes 29 which were subjected to radical cyclization leading to benzofused tricyclic j6-lactams 30, obtained as single diastereomers (Scheme 8) [36]. A convenient, direct regio- and stereoselective route to optically pure unusually fused or bridged tricyclic /3-lactams has been developed by the use of intramolecular nitrone-alkene cycloaddition reactions. For example, the aldehyde 21 can be transformed into nitrone 31 which subsequently was used for a variety... 

Clerici and Porta reported that phenyl, acetyl and methyl radicals add to the Ca atom of the iminium ion, PhN+Me=CHMe, formed in situ by the titanium-catalyzed condensation of /V-methylanilinc with acetaldehyde to give PhNMeCHMePh, PhNMeCHMeAc, and PhNMeCHMe2 in 80% overall yield.83 Recently, Miyabe and co-workers studied the addition of various alkyl radicals to imine derivatives. Alkyl radicals generated from alkyl iodide and triethylborane were added to imine derivatives such as oxime ethers, hydrazones, and nitrones in an aqueous medium.84 The reaction also proceeds on solid support.85 A-sulfonylimines are also effective under such reaction conditions.86 Indium is also effective as the mediator (Eq. 11.49).87 A tandem radical addition-cyclization reaction of oxime ether and hydrazone was also developed (Eq. 11.50).88 Li and co-workers reported the synthesis of a-amino acid derivatives and amines via the addition of simple alkyl halides to imines and enamides mediated by zinc in water (Eq. 11.51).89 The zinc-mediated radical reaction of the hydrazone bearing a chiral camphorsultam provided the corresponding alkylated products with good diastereoselectivities that can be converted into enantiomerically pure a-amino acids (Eq. 11.52).90... 

Lastly, the radical inter- and intramolecular cyclizations in the presence of one-electron oxidizing agents as a procedure for the synthesis of five-membered cyclic nitronates can be considered. Radical oxidation of a-nitro ketones (19) in the presence of disubstituted olefins under the action of Mn(OAc)3 was documented (72a) (Scheme 3.22, Eq. 1). 

Cyclization of nitro-stabilized radicals provides another method for the generation of cyclic nitronates (221). Oxidation of the aci-foim of nitroalkanes with ceric ammonium nitrate generates the ot-carbon centered radical, which in the presence of an alkene, leads to the homologation of the a-radical. In the case of a tethered alkene of appropriate length, radical addition leads to a cyclic nitronate (Scheme 2.20). 

---