The multiple multicomponent approach to natural product mimics: tubugis, N-substituted anticancer peptides with picomolar activity

Synthesis and Superpotent Anticancer Activity of Tubulysins Carrying Non-hydrolysable N-Substituents on Tubuvaline

Synthetic tubulysins 24 a-m, containing non-hydrolysable N-substituents on tubuvaline (Tuv), were obtained in high purity and good overall yields using a multistep synthesis. A key step was the formation of differently N-substituted Ile-Tuv fragments 10 by using an aza-Michael reaction of azido-Ile derivatives 8 with the α,β-unsaturated oxo-thiazole 5. A structure-activity relationship study using a panel of human tumour cell lines showed strong anti-proliferative activity for all compounds 24 a-m, with IC₅₀ values in the sub-nanomolar range, which were distinctly lower than those of tubulysin A, vinorelbine and paclitaxel. Furthermore, 24 a-m were able to overcome cross-resistance to paclitaxel and vinorelbine in two tumour cell lines with acquired resistance to doxorubicin. Compounds 24 e and 24 g were selected as leads to evaluate their mechanism of action. In vitro assays showed that both 24 e and 24 g interfere with tubulin polymerization in a vinca alkaloid-like manner and prevent paclitaxel-induced assembly of tubulin polymers. Both compounds exerted antimitotic activity and induced apoptosis in cancer cells at very low concentrations. Compound 24 e also exhibited potent antitumor activity at well tolerated doses on in vivo models of diffuse malignant peritoneal mesothelioma, such as MESOII peritoneal mesothelioma xenografts, the growth of which was not significantly affected by vinorelbine. These results indicate that synthetic tubulysins 24 could be used as standalone chemotherapeutic agents in difficult-to-treat cancers.

Synergy Effects in the Chemical Synthesis and Extensions of Multicomponent Reactions (MCRs)-The Low Energy Way to Ultra-Short Syntheses of Tailor-Made Molecules

Several novel methods, catalysts and reagents have been developed to improve organic synthesis. Synergistic effects between reactions, reagents and catalysts can lead to minor heats of reaction and occur as an inherent result of multicomponent reactions (MCRs) and their extensions. They enable syntheses to be performed at a low energy level and the number of synthesis steps to be drastically reduced in comparison with ‘classical’ two-component reactions, fulfilling the rules of Green Chemistry. The very high potential for variability, diversity and complexity of MCRs additionally generates an extremely diverse range of products, thus bringing us closer to the aim of being able to produce tailor-made and extremely low-cost materials, drugs and compound libraries.

Natural product analogues: towards a blueprint for analogue-focused synthesis

A review of approaches to natural product analogues leads to the suggestion of new methods for the generation of biologically active natural product-like scaffolds.

A multicomponent bicyclization reaction of isocyanide, allenoate, imine and water to synthesize pyrrolidine-fused rings

A multicomponent bicyclization of isocyanide, allenoate, imine, and water has been disclosed. This protocol involves the formation of five chemical bonds (two C–C, two C–N, and one C–O), thus providing a new pathway to fused rings.

Synthesis of acylhydrazino-peptomers, a new class of peptidomimetics, by consecutive Ugi and hydrazino-Ugi reactions

Herein we describe a versatile approach for the synthesis of acylhydrazino-peptomers, a new class of peptidomimetics. The key idea in this approach is based on a simple route using a one-pot hydrazino-Ugi four-component reaction followed by a hydrazinolysis or hydrolysis reaction and subsequent hydrazino-Ugi reaction or classical Ugi reaction for the construction of acyclic acylhydrazino-peptomers. The consecutive multicomponent reactions produced a variety of acylhydrazino-peptomers in moderate to excellent yields (47–90%). These compounds are multifunctional intermediates that can be further functionalized to obtain new peptidomimetics with potential biological activity.

Applications of Convertible Isonitriles in the Ligation and Macrocyclization of Multicomponent Reaction-Derived Peptides and Depsipeptides

Peptide ligation and macrocyclization are among the most relevant approaches in the field of peptide chemistry. Whereas a variety of strategies relying on coupling reagents and native chemical ligation are available, there is a continuous need for efficient peptide ligation and cyclization methods. Herein we report on the utilization of convertible isonitriles as effective synthetic tools for the ligation and macrocyclization of peptides arising from isocyanide-based multicomponent reactions. The strategy relies on the use of convertible isonitriles-derived from Fukuyama amines-and peptide carboxylic acids in Ugi and Passerini reactions to afford N-alkylated peptides and depsipeptides, respectively, followed by conversion of the C-terminal amide onto either N-peptidoacyl indoles or pyrroles. Such activated peptides proved efficient in the ligation to peptidic, lipidic and fluorescently labeled amines and in macrocyclization protocols. As a result, a wide set of N-substituted peptides (with methyl, glycosyl and amino acids as N-substituents), cyclic N-methylated peptides and a depsipeptide were produced in good yields using conditions that involve either classical heating or microwave irradiation. This report improves the repertoire of peptide covalent modification methods by exploiting the synthetic potential of multicomponent reactions and convertible isonitriles.

Synthesis of pretubulysin-derivatives via the TubUgi-approach

The Ugi reaction is found to be a very powerful tool for the synthesis of (pre)tubulysin derivatives, allowing the introduction of various functionalized side chains in only one step. While polar groups such as amides are not well tolerated, unpolar side chains such as allyl or propargyl ether are well accepted. These functionalities also allow subsequent modifications in the side chain, e.g. via ring closing metathesis or Click reaction.

Total Synthesis and Biological Evaluation of Natural and Designed Tubulysins

A streamlined total synthesis of N(14)-desacetoxytubulysin H (Tb1) based on a C-H activation strategy and a short total synthesis of pretubulysin D (PTb-D43) are described. Applications of the developed synthetic strategies and technologies to the synthesis of a series of tubulysin analogues (Tb2-Tb41 and PTb-D42) are also reported. Biological evaluation of the synthesized compounds against an array of cancer cells revealed a number of novel analogues (e.g., Tb14), some with exceptional potencies against certain cell lines [e.g., Tb32 with IC50 = 12 pM against MES SA (uterine sarcoma) cell line and 2 pM against HEK 293T (human embryonic kidney) cell line], and a set of valuable structure-activity relationships. The highly potent cytotoxic compounds discovered in this study are highly desirable as payloads for antibody-drug conjugates and other drug delivery systems for personalized targeted cancer chemotherapies.

Complementary isonitrile-based multicomponent reactions for the synthesis of diversified cytotoxic hemiasterlin analogues

A small family of structural analogues of the antimitotic tripeptides, hemiasterlins, have been designed and synthesized as potential inhibitors of tubulin polymerization. The effectiveness of a multicomponent approach was fully demonstrated by applying complementary versions of the isocyanide-based Ugi reaction. Compounds strictly related to the lead natural products, as well as more extensively modified analogues, have been synthesized in a concise and convergent manner. In some cases, biological evaluation provided evidence for strong cytotoxic activity (six human tumor cell lines) and for potent inhibition of tubulin polymerization.

A three-component reaction for rapid access to underexplored 1,3-thiazine-2-thiones

Driven by the shortage of known effective possibilities for the synthesis of 4-hydroxy-3,4-dihydro-2H-1,3-thiazine-2-thiones on the one hand and the promising potential of these structures as novel drug candidates on the other hand, synthetic access to 4-hydroxy-3,4-dihydro-2H-1,3-thiazine-2-thiones was developed. The desired products could be synthesized effectively and facilely starting from β-chlorovinyl aldehydes with the aid of a new MCR (multicomponent reaction). Furthermore, the 4-hydroxy-3,4-dihydro-2H-1,3-thiazine-2-thiones are shown to be appropriate substrates in the preparation of diverse annulated polycyclic systems.

Aminocatalysis-Mediated on-Resin Ugi Reactions: Application in the Solid-Phase Synthesis of N-Substituted and Tetrazolo Lipopeptides and Peptidosteroids

A new solid-phase protocol for the synthesis of N-substituted and tetrazolo peptides is described. The strategy relies on the combination of aminocatalysis-mediated on-resin Ugi reactions and peptide couplings for the N-alkylation of peptides at selected sites, including the N-terminal double lipidation, the simultaneous lipidation/biotinylation, and the steroid/lipid conjugation via tetrazole ring formation. The solid-phase Ugi four-component reactions were enabled by on-resin transimination steps prior to addition of the acid and isocyanide components. The strategy proved to be suitable for the feasible incorporation of complex N-substituents at both termini and at internal positions, which is not easily achievable by other solid-phase methods.

A sequential Ugi multicomponent/Cu-catalyzed azide-alkyne cycloaddition approach for the continuous flow generation of cyclic peptoids

The development of a continuous flow multistep strategy for the synthesis of linear peptoids and their subsequent macrocyclization via Click chemistry is described. The central transformation of this process is an Ugi four-component reaction generating the peptidomimetic core structure. In order to avoid exposure to the often toxic and malodorous isocyanide building blocks, the continuous approach was telescoped by the dehydration of the corresponding formamide. In a concurrent operation, the highly energetic azide moiety required for the subsequent intramolecular copper-catalyzed azide-alkyne cycloaddition (Click reaction) was installed by nucleophilic substitution from a bromide precursor. All steps yielding to the linear core structures can be conveniently coupled without the need for purification steps resulting in a single process generating the desired peptidomimetics in good to excellent yields within a 25 min reaction time. The following macrocyclization was realized in a coil reactor made of copper without any additional additive. A careful process intensification study demonstrated that this transformation occurs quantitatively within 25 min at 140 °C. Depending on the resulting ring strain, either a dimeric or a monomeric form of the cyclic product was obtained.

An efficient synthesis of [1,3]dioxolo[4,5-g]thieno[3,4-b]quinolin-8(5H)-ones as novel thiazapodophyllotoxin analogues with potential anticancer activity

A catalyst-free synthesis of 6,9-dihydro-[1,3]dioxolo[4,5-g]thieno[3,4-b]quinolin-8(5H)-ones as novel analogues of podophyllotoxins was developed by a three-component reaction of aldehydes, ethyl 2,4-dioxotetrahydrothiophene-3-carboxylate and 3,4-(methylenedioxy)aniline. This methodology not only provides new chemical library for the screening of anticancer activity, but also features excellent isolated yields, short reaction time, simple work up procedure and little environmental impact.

Synthesis of antibacterial 1,3-diyne-linked peptoids from an Ugi-4CR/Glaser coupling approach

A library of ten 1,3-diyne-linked peptoids has been synthesized through an Ugi four-component reaction (U-4CR) followed by a copper-catalysed alkyne homocoupling (Glaser reaction). The short and chemoselective reaction sequence allows generating diverse (pseudo) dimeric peptoids. A combinatorial version allows the one-pot preparation of, e.g., six-compound-libraries of homo- and heterodimers verified by ESI-MS and HPLC. In a preliminary evaluation, some compounds display moderate activity against the Gram-positive bacterium Bacillus subtilis.

Direct transformation of amides: a one-pot reductive Ugi-type three-component reaction of secondary amides

An efficient reductive Ugi-type reaction employing common secondary amides as starting materials has been established. The reaction exhibited a broad substrate scope, good chemoselectivity and functional group tolerance.

Investigation of the Passerini and Ugi reactions in β-lactam aldehydes. Synthetic applications

Ugi and Passerini multicomponent reactions on 4-oxoazetidine-2-carboxaldehydes allow the straightforward synthesis of highly functionalized β-lactams.

Chemistry and biology of tubulysins: antimitotic tetrapeptides with activity against drug resistant cancers

Since their first report in 2000, tubulysins have sparked great interest for development as anti-cancer agents due to their exceptionally potent anticancer activity.

A straightforward approach towards cyclic photoactivatable tubulysin derivatives

The development of a new photolabile protecting group containing an additional allyl functionality allows the synthesis of cyclic photoactivatable natural products. Cyclization occurs between the allyl moiety in the protecting group and a second double bond in the target molecule by means of ring-closing metathesis. Cyclization should increase the metabolic stability towards proteases. On the other hand, the conformational change should cause diminished biological activity. As illustrated for tubulysin derivatives, cyclic and photoactivatable drug candidates can easily be obtained in only two steps from simple building blocks through Ugi reaction and ring-closing metathesis. The photolabile protecting group is introduced by means of the isocyanide component during the Ugi reaction.

Consecutive isocyanide-based multicomponent reactions: synthesis of cyclic pentadepsipeptoids

The synthesis of six cyclic depsipeptoids inspired by the natural depsipeptide sansalvamide A is described. An efficient and fast synthetic strategy was developed using a combination of consecutive isocyanide-based multicomponent reactions (Ugi and Passerini reactions). This methodology can be used to access a variety of cyclic oligodepsipeptoids.

Ugi multicomponent reaction product: the inhibitive effect on DNA oxidation depends upon the isocyanide moiety

The hydroxyl-substituted benzoic acid (as phenyl group A in the product), aniline (as phenyl group B in the product), benzaldehyde (as phenyl group C in the product), and four isocyanides are employed to synthesize bis-amide via an Ugi four-component reaction. The effects of the obtained 20 bis-amides on quenching radicals and inhibiting DNA oxidation are estimated. It is found that the antioxidant effectiveness of bis-amide generated by hydroxyl groups is markedly influenced by the structural feature derived from isocyanide. The phenolic hydroxyl group attaching to phenyl group A plays a major role in scavenging radicals, and the radical-scavenging property is reinforced by the structural moiety introduced from ferrocenylmethyl isocyanide. The same conclusion is also obtained when bis-amides are used to inhibit DNA oxidation. It is still found that the ferrocenylmethyl moiety enhances the antioxidant effect of hydroxyl group at phenyl group A in protecting DNA against the oxidation. Moreover, when the bis-amide is prepared by the same isocyanide, e.g. ethyl isocyanoacetate, it is found that the hydroxyl group at phenyl group C plays the major role in inhibiting DNA oxidation, followed by the hydroxyl groups attaching to phenyl groups B and A.

A multiple multicomponent approach to chimeric peptide-peptoid podands

The success of multi-armed, peptide-based receptors in supramolecular chemistry traditionally is not only based on the sequence but equally on an appropriate positioning of various peptidic chains to create a multivalent array of binding elements. As a faster, more versatile and alternative access toward (pseudo)peptidic receptors, a new approach based on multiple Ugi four-component reactions (Ugi-4CR) is proposed as a means of simultaneously incorporating several binding and catalytic elements into organizing scaffolds. By employing α-amino acids either as the amino or acid components of the Ugi-4CRs, this multiple multicomponent process allows for the one-pot assembly of podands bearing chimeric peptide-peptoid chains as appended arms. Tripodal, bowl-shaped, and concave polyfunctional skeletons are employed as topologically varied platforms for positioning the multiple peptidic chains formed by Ugi-4CRs. In a similar approach, steroidal building blocks with several axially-oriented isocyano groups are synthesized and utilized to align the chimeric chains with conformational constrains, thus providing an alternative to the classical peptido-steroidal receptors. The branched and hybrid peptide-peptoid appendages allow new possibilities for both rational design and combinatorial production of synthetic receptors. The concept is also expandable to other multicomponent reactions.

Multicomponent reactions - opportunities for the pharmaceutical industry

Multicomponent reactions – reactions that combine three or more starting materials in one pot to give a single product that contains essentially all of the atoms of the reactants – offer many exciting opportunities for medicinal chemistry. Their most obvious application is in the drug discovery phase, where they can provide efficient access to large numbers of analogs for lead discovery or optimization. By contrast, the inherent 'greenness' of multicomponent reactions makes them of increasing importance in the sustainable production of pharmaceuticals. We will discuss recent advances in both fields as well as general drivers and barriers for the application of multicomponent reactions in a medicinal chemistry context.

A one-pot multicomponent strategy for stereospecific construction of tricyclic pyrrolo[1,2-a]quinolines

A novel multicomponent strategy for the efficient synthesis of tricyclic pyrrolo[1,2-a]quinolines has been described. The bond-forming efficiency, accessibility and generality of this synthesis make it highly attractive to assemble tri-heterocyclic scaffolds.