Beilstein J. Org. Chem.2014,10, 2930–2954, doi:10.3762/bjoc.10.312
yields (up to 95%, Scheme 4).
McBurney et al. prepared various N-heterocycles, using oximecarbonates as excellent precursors for the photoinduced generation of iminyl radicals, whereby at standard photolysis conditions, 3-substituted 6-methylphenanthridines were obtained in good to quantitative yields
-halobenzyl)arylamines, oximecarbonates, isocyanobiphenyls, etc.). Phenanthridines are usually obtained within 2–3 reaction steps, by application of different radical initiators. An intriguing alternative is the radical generation by UV irradiation with or even without a photocatalyst. The major advantage of
PDF
Graphical Abstract
Scheme 1:
The Grignard-based synthesis of 6-alkyl phenanthridine.
Beilstein J. Org. Chem.2013,9, 1083–1092, doi:10.3762/bjoc.9.120
DFT computations, which gave insights into factors influencing the two cyclisation modes.
Keywords: cyclisation; EPR spectroscopy; free radicals; heterocycles; oximecarbonates; Introduction
Radical cyclisations onto aromatic acceptors take place readily, even though disruption of the 6π-electron
carbocation, followed by proton loss. Kinetic or other data to help predict which mode would be favoured for a novel iminyl radical is essentially nonexistent.
We discovered recently that oximecarbonates ArC(R)=N–OC(O)OR’ are clean and convenient precursors for iminyl as well as O-centred radicals [26][35
radical intermediates to be directly monitored by EPR spectroscopy. We have now prepared a representative set of oximecarbonates with the aim of studying competition between ortho- and spiro-ring closures of the released iminyl radicals. Precursors 1a–f, 2a,b, 3 and 4, consist of O-ethoxycarbonyl