Terrahedron. Vol. 53, No. I, 269.284. 1997 pp. Copyright 0 1996 Elsewer Scmce Ltd Printed in Great Britain. All nghtr reserved PII: SOO40-4020(96)00983-O 0040.4020/97 $17.00 + 0.00 Synthesis of Phenanthridines by Radical Caryl-Cary~ Coupling Ana M. Rosa, Ana M. Lobe*, Paula S. Branco, Sundaresan Prabhakar* and Ant6nio M. D. L. Pereira (in part) Scc@o de Quimica Orglnica Apkada, Lkpariamento de Quimica, and SINTOR-UMNOVA, campus Faculdadc dc CiEncias e Tecnologia, Universidadc Nova de Lisboa, Quinta da Terre, 2825 Monte da Caparica, Portugal Abstract: Trcatmcnt of N-(o-bromobcnzyl)anilincs wilh 1-3 equivalenl amounts of n-tributyltinhydride. in the prcscnce of 0.5 LO 0.6 mol cquiv. of AIBN, rcsul~s in the formation of phcnanthridines in good yields. The mechanism of lhe oxidation step is probed wilh a deutcrated aniline derivative and azobiscyclohexylcarbanonitrile (ABCN) as the initiator. I1 is shown thal the carbon cenued radical dcrivcd from the latter dots not act as Ihc hydrogen abstracting spccics. Copyright 0 1996 Elsevier Science Ltd Carbon radicals, thermally generated from organic halides or chalcogens by the use of stannylhydride and an initiator have proven themselves to be versatile intermediates in organic synthesis.’ Although the addition of such radicals to a normal olefin lacking any electronic bias in an intermolecular process is considered to be synthetically unattractive, the same is not true for the intramolecular reactions and such processes have been advantageously utilised for the establishment of Caryl-Caryl bond.2 We report here full details pertaining to a practical synthesis 3a,b of polyalkoxyphenanthridines,3~~4 and discuss in some detail the mechanistic aspects of the reactions. The requisite starting materials for phenanthridines, the N-(o-bromobenzylidene)anilines (l), are readily obtained as crystalline solids from the corresponding aromatic aldehydes and anilines by condensation of the two components in boiling ethanol. The various Schiff’s bases 1 thus prepared are collected in Table 1. The conversion of these imines into the corresponding secondary amines, namely, the N-(o- bromobenzyl)anilines 2, was achieved by treatment of the former with sodium borohydride ( 3-4 equiv. ) in MeOH at 0°C and the course of reduction conveniently followed by the disappearance of the yellow colour due to starting materials. The various N-bromobenzyl derivatives 2 thus obtained are collected in Table 2. Wherever difficulties were encountered in the preparation of the imines or when the requisite D- bromobenzyl halide was available commercially, the alternative method involving mono N-alkylation of the NH compounds was employed. Compounds 2j-2n ( Table 2 ) were secured by this procedure. When equimolecular quantities of the bromoaniiine 2a and n-BujSnH ( TBTH ), in benzene under reflux, was treated with the conventional catalytic quantity of AIBN ( 0.1 equiv. ) to initiate the radical process, the yield