- Efficient Solution-Phase Parallel Synthesis of 4-Substituted N-Protected Piperidines
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Practical conditions for the synthesis of 4-substituted N-protected piperidines through CuCN·2LiBr-catalyzed organozinc additions to 1-acylpyridinium salts and subsequent hydrogen-transfer hydrogenation have been established. The reaction sequence is performed at room temperature and provides 4-substituted N-protected piperidines in excellent overall yields without the isolation of intermediate dihydropyridines, In those cases in which the organozinc addition results in mixtures of 2- and 4-substituted dihydropyridines, the 2-substituted isomers are efficiently scavenged with maleic anhydride and subsequently removed by a mild extractive workup with NaHCO3 (sat.). The N-acyl group can conveniently be exchanged from N-phenoxycarbonyl to N-tBoc, thus allowing orthogonal deprotection strategies. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.
- Wang, Xiaoyang,Kauppi, Anna M.,Olsson, Roger,Almqvist, Fredrik
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p. 4586 - 4592
(2007/10/03)
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- Versatile Synthesis of Dihydroquinolines and Quinoline Quinones Using Cyclobutenediones. Construction of the Pyridoacridine Ring System
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1-BOC-2-lithio-1, 4-dihydropyridines were condensed with 3,4-disubstituted cyclobutenediones to produce 1,2-adducts. Neat thermolysis under oxygen-free conditions produced substituted 1,4-dihydroquinoline hydroquinones in which the tert-butoxy residue of the BOC group was displaced by a phenolic residue, generating an oxazolone ring that functioned to protect both rings of the dihydroquinoline hydroquinone from untimely oxidation. Oxidative aromatization with concomitant loss of the oxazolone ring was achieved using 2 equiv of o-chloranil in acetic acid and provided substituted quinoline quinones in good yields. By use of this strategy, a concise synthesis of the pyridoacridine ring system was achieved.
- Zhang, Dawei,Llorente, Isidro,Liebeskind, Lanny S.
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p. 4330 - 4338
(2007/10/03)
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