Total synthesis of the antitumor antibiotic (±)-streptonigrin: first- and second-generation routes for de novo pyridine formation using ring-closing metathesis.

The total synthesis of (±)-streptonigrin, a potent tetracyclic aminoquinoline-5,8-dione antitumor antibiotic that reached phase II clinical trials in the 1970s, is described. Two routes to construct a key pentasubstituted pyridine fragment are depicted, both relying on ring-closing metathesis but differing in the substitution and complexity of the precursor to cyclization. Both routes are short and high yielding, with the second-generation approach ultimately furnishing (±)-streptonigrin in 14 linear steps and 11% overall yield from inexpensive ethyl glyoxalate. This synthesis will allow for the design and creation of druglike late-stage natural product analogues to address pharmacological limitations. Furthermore, assessment of a number of chiral ligands in a challenging asymmetric Suzuki-Miyaura cross-coupling reaction has enabled enantioenriched (up to 42% ee) synthetic streptonigrin intermediates to be prepared for the first time.

Total synthesis of (±)-streptonigrin: de novo construction of a pentasubstituted pyridine using ring-closing metathesis.

The synthesis of the potent antitumor agent (±)-streptonigrin has been achieved in 14 linear steps and 11% overall yield from ethyl glyoxalate. The synthesis features a challenging ring-closing metathesis reaction, followed by elimination and aromatization, to furnish a key pentasubstituted pyridine fragment.

Heteroaryl cross-coupling as an entry toward the synthesis of lavendamycin analogues: a model study.

ABC analogues of the antitumor antibiotic lavendamycin, which contain the key metal chelation site and redox-active quinone unit essential for biological activity, were prepared via the palladium(0)-catalyzed cross-coupling reaction of various 2-haloheteroaromatics with 2-stannylated pyridines and quinolines. Using the Stille reaction, 2-bromo substituted quinolines and 1-bromoisoquinolines were found to undergo efficient coupling with 2-pyridinylstannanes to provide unsymmetrical heterobiaryl derivatives. While the Stille reaction using the reverse coupling partners (i.e., 2-quinolinylstannanes and haloheteroaromatics) had not received much attention in the literature, we found that this alternative coupling reaction efficiently provided several new heterobiaryl derivatives. The gold-catalyzed intramolecular cycloisomerization of N-(prop-2-ynyl)-1H-indole-2-carboxamide smoothly afforded a beta-carbolinone derivative that was subsequently used for a Pd(0)-catalyzed cross-coupling directed toward the synthesis of lavendamycin analogues.

Novel immunoconjugates comprised of streptonigrin and 17-amino-geldanamycin attached via a dipeptide-p-aminobenzyl-amine linker system.

Cytotoxic agents streptonigrin and 17-amino-geldanamycin were linked to monoclonal antibodies (mAbs), forming antibody-drug conjugates (ADCs) for antigen-mediated targeting to cancer cells. The drugs were conjugated with a linker construct that is labile to lysosomal proteases and incorporates a valine-alanine-p-aminobenzyl (PAB)-amino linkage for direct attachment to the electron-deficient amine functional groups present in both drugs. The resulting ADCs release drug following internalization into antigen-positive cancer cells. The drug linkers were conjugated to mAbs cAC10 (anti-CD30) and h1F6 (anti-CD70) via alkylation of reduced interchain disulfides to give ADCs loaded with 4 drugs/mAb. The streptonigrin ADCs were potent and immunologically specific on a panel of cancer cell lines in vitro and in a Hodgkin lymphoma xenograft model. We conclude that streptonigrin ADCs are candidates for further research, and that the novel linker system used to make them is well-suited for the conjugation of cytotoxic agents containing electron-deficient amine functional groups.

Gold-catalyzed cycloisomerization of N-Propargylindole-2-carboxamides: application toward the synthesis of lavendamycin analogues.

A series of N-propargylindole-2-carboxamides were found to undergo a AuCl 3-catalyzed cycloisomerization to give beta-carbolinones in high yield. The corresponding beta-chlorocarboline derivative was prepared and used for Pd(0)-catalyzed cross-coupling chemistry directed toward the synthesis of lavendamycin analogues.

Lavendamycin antitumor agents: structure-based design, synthesis, and NAD(P)H:quinone oxidoreductase 1 (NQO1) model validation with molecular docking and biological studies.

A 1H69 crystal structure-based in silico model of the NAD(P)H:quinone oxidoreductase 1 (NQO1) active site has been developed to facilitate NQO1-directed lavendamycin antitumor agent development. Lavendamycin analogues were designed as NQO1 substrates utilizing structure-based design criteria. Computational docking studies were performed using the model to predict NQO1 substrate specificity. Designed N-acyllavendamycin esters and amides were synthesized by Pictet-Spengler condensation. Metabolism and cytotoxicity studies were performed on the analogues with recombinant human NQO1 and human colon adenocarcinoma cells (NQO1-deficient BE and NQO1-rich BE-NQ). Docking and biological data were found to be correlated where analogues 12, 13, 14, 15, and 16 were categorized as good, poor, poor, poor, and good NQO1 substrates, respectively. Our results demonstrated that the ligand design criteria were valid, resulting in the discovery of two good NQO1 substrates. The observed consistency between the docking and biological data suggests that the model possesses practical predictive power.

Total synthesis of streptonigrone.

A total synthesis of streptonigrone, 1, is described, which incorporates a one-step synthesis of substituted pyridones devised in our laboratory. Other aspects of the synthesis that differentiate the present approach from previous ones are the use of a Conrad-Limpach reaction, rather than the customary Friedländer methodology, to assemble the quinoline segment of 1, and the implementation of an anionic sequence for the functionalization of a key pyridone intermediate.

Synthesis and evaluation of antitumor activity of novel N-acyllavendamycin analogues and quinoline-5,8-diones.

A series of 7-N-acyllavendamycins with zero, one or two substituents at the C-2', C-3', and C-11' were synthesized through short and efficient methods. Pictet-Spengler condensation of 7-N-acylamino-2-formylquinoline-5,8-diones with tryptamine or tryptophans produced the desired lavendamycins. Screening data on a panel of three ras oncogene-transformed cell lines and the non-transformed parent cell line showed that a significant number of these analogues are potent antitumor agents and appear to be particularly active against K-ras transformed cells. Compared with the corresponding quinolinediones, these novel lavendamycins are much more inhibitory toward the transformed cells indicating that the beta-carboline moiety of the lavendamycin analogues plays an important role in its potency and selective toxicity.

Novel lavendamycin analogues as antitumor agents: synthesis, in vitro cytotoxicity, structure-metabolism, and computational molecular modeling studies with NAD(P)H:quinone oxidoreductase 1.

Novel lavendamycin analogues with various substituents were synthesized and evaluated as potential NAD(P)H:quinone oxidoreductase (NQO1)-directed antitumor agents. Pictet-Spengler condensation of quinoline- or quninoline-5,8-dione aldehydes with tryptamine or tryptophans yielded the lavendamycins. Metabolism studies with recombinant human NQO1 revealed that addition of NH2 and CH2OH groups at the quinolinedione-7-position and indolopyridine-2'-position had the greatest positive impact on substrate specificity. The best and poorest substrates were 37 (2'-CH2OH-7-NH2 derivative) and 31 (2'-CONH2-7-NHCOC3H7-n derivative) with reduction rates of 263 +/- 30 and 0.1 +/- 0.1 micromol/min/mg NQO1, respectively. Cytotoxicity toward human colon adenocarcinoma cells was determined for the lavendamycins. The best substrates for NQO1 were also the most selectively toxic to the NQO1-rich BE-NQ cells compared to NQO1-deficient BE-WT cells with 37 as the most selective. Molecular docking supported a model in which the best substrates were capable of efficient hydrogen-bonding interactions with key residues of the active site along with hydride ion reception.

Total synthesis of novel 6-substituted lavendamycin antitumor agents.

[structure: see text] Novel 6-substituted lavendamycins have been synthesized for the first time. The key step in these syntheses is a Pictet-Spengler condensation (Scheme 1). Efficient methods for the synthesis of each compound, including a novel reaction for the facile introduction of alkylamino groups at the C-6 position of the lavendamycin system, are discussed. Possible mechanisms for these reactions are also presented.

Novel lavendamycin analogues as potent HIV-reverse transcriptase inhibitors: synthesis and evaluation of anti-reverse transcriptase activity of amide and ester analogues of lavendamycin.

Novel lavendamycins including two water soluble derivatives were synthesized via short and efficient methods. Pictet-Spengler condensation of 7-N-acylamino-2-formylquinoline-5,8-diones with tryptophans produced lavendamycin esters or amides 11-17. Lavendamycins 18-21 were obtained, respectively, by further transformations of 13-15 and 17. Several lavendamycins were found to be potent HIV reverse transcriptase inhibitors with very low toxicity in vitro and in vivo. Several compounds also acted either additively or synergistically to inhibit enzyme activity together with AZT-triphosphate.

Siloxane-based cross-coupling of bromopyridine derivatives: studies for the synthesis of streptonigrin and lavendamycin.

Highly functionalized 4-bromopyridines were prepared and found to undergo fluoride-promoted, Pd-catalyzed cross-coupling with aryltrialkoxysilanes to give sterically demanding biaryls. The 3-nitro-4-bromopyridine derivative coupled in good yield with TBAT (tetrabutylammonium triphenyldifluorosilicate) to provide a biaryl adduct that serves as a model system for the total synthesis of the antitumor antibiotics streptonigrin and lavendamycin. [reaction: see text]

Transformation of streptonigrin into streptonigrone; synthesis and biological evaluation of antibiotics streptonigrin and streptonigrone alkyl ethers.

A method of synthesis of antibiotic streptonigrin 8'-O-alkyl ethers by alkylation of streptonigrin diphenylmethyl ester and consequent deprotection of carboxylic group with CF3COOH is developed. An attempt to deblock carboxylic group of 8'-O-methylstreptonigrin diphenylmethyl ester by hydrogenation over Pd produced 8'-O-methylstreptonigrone. Similarly streptonigrin was transformed into streptonigrone over Pd-black in H2 stream. Methylation of streptonigrone afforded 5',5'-N-dimethyl-2',8'-O-dimethylstreptonigrone and 1',5',5'-tri-N-trimethyl-8'-O-methylstreptonigrone. Alkyl streptonigrin ethers demonstrated lower antibacterial activity in vitro than the parent antibiotic.

Streptonigrin and related compounds. 5. Synthesis and evaluation of some isoquinoline analogues.

A series of analogues of streptonigrin, in which the quinoline of ring B is replaced by isoquinoline and the substituted pyridine of ring C is replaced by phenyl, nitrophenyl, aminophenyl, or benzyl functions, have been prepared. Thus, 1-substituted isoquinoline-5,8-diones with 7-amino or 6-(alkylamino) groups were prepared. The various quinones were evaluated for antimicrobial activity against Bacillus subtilis and root-growth inhibitory activity against Lepidium sativum. The effect of specific structural changes on these activities was examined with streptonigrin for comparison. The necessity of an aminoquinone function for activity is confirmed. With regard to the antibacterial activity, the isoquinoline analogues appear to be less active compared to the quinoline derivatives. However, the higher degree of antibacterial activity of the 1-benzylisoquinolines and the 1-nitrophenylisoquinolines compared to the 1-phenyl isoquinolines is noteworthy. In contrast to the results seen with the antibacterial activity, most of the isoquinoline analogues showed activity comparable to, or even higher than, that of streptonigrin in the root-growth inhibition assay. The 1-nitrophenylisoquinolines again appear to be the most active. The equal or greater potency of the benzyl analogue in comparison with the phenyl analogue was unexpected and questions the need for the extended conjugation and the geometry required for metal binding as considered earlier. It also opens up new possibilities for structural variation.