Lavendamycin

A Pd-cataly2ed reaction of amines with halides is expected, but actually little is known about the reaction. The CDE ring system of lavendamycin (805) has been constructed by the intramolecular reaction of aryl bromide with aniline derivative in 804, but 1.2 equiv, of Pd(Ph3P)4 is required[679]. 

Fig. 9 Synthesis of lavendamycin methyl ester by Kende and Ebetino... 

The Pictet-Spengler reaction has also seen much use in the synthesis of lavendamycin methyl ester. Both Hibino [31] and Behforouz [32] used it as a key step in their syntheses of this molecule (Fig. 10). In Hibino s synthesis, /i-methyltryptophan ethyl ester 27 was condensed with quinoline aldehyde 28 to give the corresponding lelrahydro- -carbohnc, which was aromatized by heating with palladium on carbon in xylenes, giving /1-carboline 29 in 75% yield. A five-step sequence (which included conversion to the methyl ester for easier comparison with known compounds) yielded bromoquinone 24 in 27% yield for the five steps. This completed Hibino s formal total synthesis of lavendamycin methyl ester, since this was the same intermediate used in Kende s synthesis. 

Behforouz s synthesis employed more highly substituted quinoline aldehyde 30, which, when condensed with ester 21, produced /3-carboline 31 without need of a separate oxidation step. Selective hydrolysis of the acetamide group then provided lavendamycin methyl ester in high yield. A few years later, Behforouz and coworkers reported an improved synthesis of 30, thus boosting the overall yield of their lavendamycin synthesis [33]. 

One of the earliest syntheses of lavendamycin methyl ester, however, did not employ either the Pictet-Spengler or the Bischler-Napieralski reactions for construction of the /J-carboline ring system. Instead, a palladium-promoted ring closure of aryl pyridine 36 (Fig. 12) was used to prepare /1-carboline 37 by Boger and coworkers [35]. Unfortunately, stoichiometric palladium was found to be necessary, in this case 1.5 equivalents of the tetrakis(triphenylphosphine)palladium(0) being used. Friedlander condensation with aldehyde 38 in the presence of benzyltrimethylammonium hydrox-... 

Although lavendamycin was not used clinically due to solubility and toxicity problems [37], interest in lavendamycin analogues continues today. For... 

Boger and co-workers were the first to report the intramolecular amination of aryl halides in their synthesis of lavendamycin [436-438], Thus, biaryl 366 is smoothly cyclized under the action of 1.5 equivalents of Pd(PhjP)4 to P-carboline 367, which comprises the CDE rings of lavendamycin. 

As an intermediate in the synthesis of lavendamycin, the fully aromatic 123 was obtained from 2-methyltryptophan and 8-methoxyquinoline-2-carboxylic acid (84TL923). 

Streptonigrin, lavendamycin, and related natural and synthetic compounds 48... 

Boger reported studies on palladium-mediated cyclization to form the CDE ring system of lavendamycin, as shown in Eq. (2) [74-76]. These reactions were conducted with stoichiometric amounts of [Pd(PPh3)4] (2). When used in a 1 mol% quantity, 2 failed to catalyze these reactions, presumably because of the absence of a base. Until almost 10 years later, no palladium-catalyzed animation chemistry was reported, and few citations of the early amination chemistry existed. 

Additionally, Boger and Panek reported an intramolecular amine arylation mediated by stoichiometric quantities of Pd (0), Eq. (2) [15]. Efforts to render this transformation catalytic in palladium were fruitless, however. The resulting heterocycle was utilized in the total synthesis of lavendamycin. 

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