Solid-Phase Synthesis of 1-Substituted Tetrahydroisoquinoline Derivatives Employing BOC-Protected Tetrahydroisoquinoline Carboxylic Acids

Mechanistic studies on the cis to trans epimerization of trisubstituted 1,2,3,4-tetrahydro-beta-carbolines

It is well-known that N(b)-benzyltryptophan alkyl esters undergo the Pictet-Spengler reaction with aldehydes to furnish both cis- and trans-1,2,3,4-tetrahydro-beta-carbolines, with the trans isomer predominating. Epimerization at C-1 took place under acidic conditions to produce, exclusively, the thermodynamically more stable trans diastereomer via internal asymmetric induction. Recent kinetic experiments provided insight into the cis to trans epimerization mechanism involved in the Pictet-Spengler reaction of 1,2,3-trisubstituted tetrahydro-beta-carbolines. Since the epimerization reaction had been shown to be sensitive to electronic effects at C-1, the rate data for a series of 1-phenyl-substituted 1,2,3,4-tetrahydro-beta-carbolines was investigated via a Hammett study. Analysis of the data supported the presence of a positively charged intermediate with a rho value of -1.4, although the existence of an iminium ion intermediate or a carbocationic intermediate could not be determined from this data alone. Analysis of the rate of epimerization demonstrated first-order kinetics with respect to TFA following the initial protonation of the substrate. This observation was consistent with the formation of a doubly protonated intermediate as the rate-determining step in the carbocation-mediated cis to trans epimerization process. In addition, the observed first-order rate dependence was inconsistent with the retro-Pictet-Spengler mechanism since protonation at the indole-2 position was not rate determining as demonstrated by kinetic isotope effects. Based on this kinetic data, the retro-Pictet-Spengler pathway was ruled out for the cis to trans epimerization of 1,2,3-trisubstituted 1,2,3,4-tetrahydro-beta-carbolines, while the olefinic mechanism had been ruled out by experiments carried out in TFA-d.

Study of the cis to trans isomerization of 1-phenyl-2,3-disubstituted tetrahydro-beta-carbolines at C(1). Evidence for the carbocation-mediated mechanism

The present study was undertaken to shed light on the mechanism of the epimerization of cis-1,2,3-trisubstituted tetrahydro-beta-carbolines into the trans isomers via a potential carbocationic intermediate at C(1). In order to study the pathway involved in C(1)-N(2) bond cleavage, the electronic character of the carbon atom at C-1 was altered by substitution of electron-rich and electron-poor phenyl rings at this position. This provided direct evidence of the effects of charge at the proposed site of the carbocationic intermediate. In this regard, a diverse set of 1-(phenyl substituted)-2-benzyl-3-ethoxycarbonyl-1,2,3,4-tetrahydro-beta-carbolines has been synthesized via the Pictet-Spengler reaction by condensation of l-tryptophan derivatives with electron-poor and electron-rich aromatic aldehydes. The epimers involved in the isomerization mechanism were investigated by dynamic (1)H and (13)C NMR spectroscopic and X-ray crystallographic analyses. The kinetic studies, which involved conversion of cis diastereomers into their trans counterparts, were carried out in dilute TFA/CH(2)Cl(2). The 1-(4-methoxyphenyl) cis diastereomer epimerized at a much faster rate into the corresponding trans diastereomer than the related 1-(4-nitrophenyl) cis diastereomer epimerized. These observations provide support for the carbocationic intermediate in the C(1)-N(2) scission process. The understanding of this epimerization process is of importance when Pictet-Spengler reactions are carried out under acidic conditions during the synthesis of indole alkaloids.

Convenient Synthesis of a Library of Discrete Hydroxamic Acids Using the Hydroxythiophenol (Marshall) Resin

Several resins have reportedly been used to synthesize hydroxamic acids except for the hydroxythiophenol (Marshall) resin. Herein we report the use of the Marshall resin to synthesize hydroxamic acids from carboxylic acids and its application to convert a library of fourteen discrete aliphatic and aromatic carboxylic acids including N-protected amino acids to their corresponding hydroxamic acids in good yields.

Efficient solid-phase synthesis of a library of imidazo[1,2-a]pyridine-8-carboxamides

A versatile method for the solid-phase synthesis of imidazo[1,2-a]pyridine-based derivatives, imidazo[1,2-a]pyridine-8-carboxamides, has been developed. They were obtained by treatment of the amino group of the polymer-bound 2-aminonicotinate with different alpha-haloketones, followed by halogenation at the 3-position of the polymer-bound imidazo[1,2-a]pyridine. The derived polymer-bound imidazo[1,2-a]pyridines 5, 6, and 7 were finally cleaved from the solid-support with an excess of primary or secondary amines. The final crude products were purified from excess amines by solid-supported liquid-liquid extraction (SLE).