Sundaresan Prabhakar

A NEW ENANTIOSELECTIVE SYNTHESIS OF N-ARYLAZIRIDINES BY PHASE-TRANSFER CATALYSIS

Abstract Chiral N-arylaziridines are obtained from N-acyl-N-arylhydroxylamines using quaternary salts of Cinchona alkaloids as phase-transfer catalysts.

Synthesis of phenanthridines by radical CarylCaryl coupling

Abstract Treatment of N-(o-bromobenzyl)anilines with 1-3 equivalent amounts of n-tributyltinhydride, in the presence of 0.5 to 0.6b mol equiv. of AIBN, results in the formation of phenanthridines in good yields. The mechanism of the oxidation step is probed with a deuterated aniline derivative and azobiscyclohexylcarbanonitrile (ABCN) as the initiator. It is shown that the carbon centred radical derived from the latter does not act as the hydrogen abstracting species.

Asymmetric synthesis of N-aryl aziridines

Abstract The reactions of a variety of N-arylhydroxamates as nitrogen transfer reagents to acryloyl derivatives of (−)-8-phenylmenthol, (−)-quinine and (−)-Oppolzers sultam acting as Michael acceptors was studied. Poor to modest diastereoselection was observed in the formation of aziridines. The absolute structure of one of the pure diastereomers secured from Oppolzers auxiliary was established by X-ray crystallography and hence the absolute configuration of the derived methyl-N-phenylaziridine-2-carboxylate could be assigned. Whilst only poor facial selectivity was observed for chiral hydroxamic acid prepared from dehydroabietic acid, moderate to good enantioselection of aziridines could be achieved with the chiral quaternary salts based on cinchona alkaloids, especially with that of cinchonine. A model is presented to explain the origin of enantioselection and a mechanism is proposed for the aziridination reaction.

New syntheses of the amaryllidacaea alkaloids vasconine assoanine, oxoassoanine, pratosine and ismine by radical cyclisation

Abstract N-(2-bromo-4,5-dimethoxybenzyl)-N-(2′-β-hydroxyethylphenyl)amine when subjected to the action of AIBN and tri- n -butyltin hydride (TBTH) in benzene, under reflux, yielded the corresponding β-4-phenanthridinylethyl alcohol. While reduction of the tetracyclic quaternary salt obtained by treatment of this alcohol with PBr 3 gave assoanine, its oxidation with alkaline ferricyanide led to a separable mixture of pratosine and oxoassoanine. The σ radical, similarly generated from 2-bromo-4,5-methylenedioxybenzyl-2′-aminophenylether, afforded the biphenyl alcohol, N-norismine.

Reactions of hydroxylamines with ethyl cyanoformate. Preparation of aminonitrones and their synthetic applications

Abstract The reaction of hydroxylamines with ethyl cyanoformate 1 leads to the formation of carbethoxyamino nitrones 2 . UV spectroscopy provided information that suggested the nitrone structure for these compounds in solution. X-ray analysis of 2a confirmed that it exists entirely in the nitrone form in the solid state. These nitrones are shown to be excellent starting materials for a variety of nitrogen containg compounds, namely, imidazoles, benzimidazoles, benzimidazolones, oxadiazolones, N-arylamidines and quinoxalone.

Aryl-aryl coupling induced by n-tributylstannyl hydride an efficient phenanthridine synthesis

Abstract N -( o -Bromobenzyl) anilines undergo smooth cyclisation, via dihydrophenanthridines, to phenanthridines by intramolecular addition of aryl radicals to the o position of the aryl ring bearing the nitrogen atom.

Reactions between hydroxylamines and aroyl cyanides

Abstract Aromatic hydroxylamines and sterically bulky aliphatic hydroxylamines react with aroyl cyanides to give exclusively the O -acyl derivatives.

A new synthesis of (−)-debromoflustramine B, (+)-ent-debromoflustramine B and (+)-debromoflustramide B

Abstract The first synthesis of (−)-debromoflustramine B is reported. Appropriate structural modifications of an optically pure Barton ester, obtained in five steps from N-acetyl- l -tryptophan methyl ester, lead to the alkaloid.

Synthetic applications of N-Aryl-O-acyl hydroxymic acids. A convenient route to 3-substituted N-Benzoyl oxindoles

Abstract The enol silylethers of N -acyloxybenzanilides undergo smooth rearrangement to afford o -amino-benzoyl-phenylacetic acids and thence to oxindoles by dehydration.

Diterpenoids from Salvia candelabrum

Abstract From the aerial parts of Salvia candelabrum two new rearranged abietane diterpenoids, candesalvones A and B, have been isolated besides β-sitosterol, nepeticin [lup-20(29)-ene-3β,11α-diol], candelabrone [11,12,14-trihydroxy-8,11,13-abietatriene-3,7-dione) and large amounts of ursolic and oleanolic acids. The structures of candesalvone A[11,12,14-trihydroxy-19(4→3)-abeo-3,8,11,13-abietatetraen-7-one] and candesalvone B [11,12,14-trihydroxy-7-oxo-3,4-seco-4(18),8,11,13-abietatetraen-3-oic acid] were established by chemical and spectroscopic means. In addition, the root bark of the same species afforded the previously known abietane derivatives 7α-acetoxyroyleanone, 12-O-methylpisiferic acid and sugiol.