The palladium-catalyzed oxidation of hydroxy-enamines, which were obtained by the condensation of P-aminoalcohols and carbonyl compounds, proceeded to give the corresponding polysubstituted pyrroles and 4,5,6,7-tetrahydroindoles in good yields. The treatment of o-(2hydroxyethyl)aniline with the palladium catalyst also gave indole in 78% yield. Copyright Q 1996 Elsevier Science Ltd Highly functionalized pyrroles are subunits of considerable importance in heme, chlorophyll, related natural products such as the bile pigments, vitamin B12 and the pyrrole alkaloids from marine sources.1 Recently, there has been considerable interest in the development of methodology for accessing these compounds. Therefore, since Knorr reported the first synthesis of pyrroles at the end of the last century,2 a variety of syntheses for substituted pyrroles has been described. 1 Also, alternative pyrrole syntheses have been developed. For example, the pyrrole synthesis based on the Lewis acid-mediated reaction of readily available 2acetoxypropanal N, N-dimethylhydrazone
with cyclic and open chain silyl enol ethers was reported by Enders et aZ.3 On the other hand, Furstner et al. described the low-valent titanium-mediated approach to pyrroles. 4 In addition, the pyrrole synthesis via [2+3] cycloaddition reactions of S-methyl N-(benzotriazol-l-ylmethyl)thioimidate with a&unsaturated esters, ketones, nitriles, and vinylpyridines was described.5 The convenient synthesis of 2-cyanopyrroles from isocyanoacetonitrile has also been reported.6
The efficient palladium-catalyzed oxidation of alcohols to carbonyl compounds with carbon tetrachloride and aryl bromide was reported by Tsuji et al.7 and Yoshida et al.,8 respectively. Further, Choudary ef al. reported the palladium acetate-catalyzed oxidation of primary and secondary alcohols to aldehydes and ketones at room temperature under phase transfer conditions. 9 During our research on the palladium-catalyzed reactions, we have found that the oxidation of P-hydroxy-enamine and o-(2-hydroxyethyl)aniline by utilizing the palladium-catalyzed oxidation systems reported by Yoshida and Tamam* gave the corresponding pyrroles and indoles. In this communication, we wish to describe the alternative facile and efficient synthesis of pyrroles and indoles via the palladium oxidation of P-hydroxy-enamine and o-(2-hydroxyethyl)aniline. : , R4-NH2 D Scheme 1