Total synthesis of pactamycin and pactamycate: a detailed account

Synthesis of the Cyclopentane Core Skeleton of Cranomycin and Jogyamycin

Cranomycin and jogyamycin, two aminocyclopentitol natural products, possess complex structures and potential medicinal properties. This review describes synthetic studies about the process of making an advanced intermediate of cranomycin and jogyamycin. This highly functionalized intermediate, featuring three contiguous amine-substituted stereocenters, was constructed from cyclopentadiene through a series of reactions including the nitroso Diels-Alder reaction, nitrogen radical cyclization reaction, 1,2-nitrogen migration, and stereoselective nitrogen addition.

Direct access to tetrasubstituted cyclopentenyl scaffolds through a diastereoselective isocyanide-based multicomponent reaction

An efficient strategy combining the stereocontrol of organocatalysis with the diversity-generating character of multicomponent reactions is described to produce structurally unique, tetrasubstituted cyclopentenyl frameworks. An asymmetric Michael addition-hemiacetalization between α-cyanoketones and α,β-unsaturated aliphatic aldehydes was performed for constructing cyclic hemiacetals, which were next employed as chiral bifunctional substrates in a new diastereoselective intramolecular isocyanide-based multicomponent reaction. This approach furnished a diversity of structurally complex compounds - including peptidomimetics and natural product hybrids in high stereoselectivity (up to >99% ee and up to >99 : 1 dr) and in moderate to high yields.

Nucleophilic transformations of azido-containing carbonyl compounds via protection of the azido group

Nucleophilic transformations of azido-containing carbonyl compounds are discussed. The phosphazide formation from azides and di(tert-butyl)(4-(dimethylamino)phenyl)phosphine (Amphos) enabled transformations of carbonyl groups with nucleophiles such as lithium aluminum hydride and organometallic reagents. The good stability of the phosphazide moiety allowed us to perform consecutive transformations of a diazide through triazole formation and the Grignard reaction.

Strategies for the Syntheses of Pactamycin and Jogyamycin

Pactamycin and jogyamycin are aminocyclopentitol natural products, where each core carbon bears a stereodefined alcohol or amine moiety. Their structural complexity, coupled with the diversity of functional groups coexisting in a condensed space, make them fascinating synthetic targets in their own right. Pactamycin and its derivatives bind to the 30S ribosomal subunit and display activity against parasites responsible for drug-resistant malaria and African sleeping sickness; however, efforts to develop their therapeutic potential have been hampered by their cellular toxicity. Interestingly, bioengineered analogues display differences in selectivity and toxicity towards mammalian cells, spurring efforts to develop flexible strategies to thoroughly probe structure-activity relationships (SAR), particularly in analogues lacking the C7 hydroxyl group of pactamycin. This review compares and contrasts approaches towards pactamycin and jogyamycin, including two successful total syntheses of the former. The implications of each route for preparing analogues to inform SAR and lead to compounds with increased selectivity for binding malarial over human ribosomes are briefly discussed.

Synthetic Studies on Pactamycin: A Synthesis of Johnson's Intermediate

A formal total synthesis of pactamycin (1) has been accomplished by face-selective and regioselective nitroso Diels-Alder (NDA) reaction of acyl nitroso compound 14, which contains a camphorsultam chiral auxiliary, and chiral cyclopentadiene 12. Construction of the chiral secondary alcohol of 12 was performed by (S,S)-Ts-DENEB catalyst-mediated reduction, and the NDA adduct 15a was readily converted to Johnson's intermediate 21.

Total Synthesis of Pactalactam, an Imidazolidinone-Type Pactamycin Analogue

The first total synthesis of pactalactam was accomplished using substrate-controlled stereoselective aziridination and regioselective aziridine ring-opening to construct three continuous amino groups on an octasubstituted cyclopentane core. The cyclopentane framework was obtained by ring-closing metathesis and aldol coupling using a l-threonine-derived oxazoline compound. Cyclic urea formation, m-acetylphenyl group introduction by Chan-Lam coupling, and primary alcohol-selective acylation yielded the reported pactalactam structure. The presence of pactalactam in the fermentation broth of pactamycin-producing bacteria was also confirmed.

Oxidative allene amination for the synthesis of nitrogen-containing heterocycles

The prevalence of stereochemically complex amines in natural products, pharmaceuticals and other bioactive compounds, coupled with the challenges inherent in their preparation, has inspired our work to develop new and versatile methodologies for the synthesis of amine-containing stereotriads ('triads'). The key step is a highly chemo-, regio-, and stereoselective transition-metal catalyzed nitrene transfer reaction that transforms one of the cumulated double bonds of an allene precursor into a bicyclic methyleneaziridine intermediate. This account summarizes our strategies to rapidly elaborate such intermediates into stereochemically rich, densely functionalized amine triads, nitrogen heterocycles, aminated carbocycles and other useful synthetic building blocks.

Synthesis of the Aminocyclitol Core of Jogyamycin via an Enantioselective Pd-Catalyzed Trimethylenemethane (TMM) Cycloaddition

The use of β-nitroenamines as a new class of acceptors in the enantioselective Pd-catalyzed trimethylenemethane cycloaddition afforded differentiated 1,2-dinitrogen bearing cyclopentanes with three contiguous stereocenters. The utility of these acceptors was demonstrated with the efficient construction of the core of jogyamycin and aminocyclopentitols. Further elaboration of the cycloadducts provided a concise synthetic approach toward joygamycin.

Synthetic Studies Toward Pactamycin Highlighting Oxidative C-H and Alkene Amination Technologies

A strategy enabled by C-H and alkene amination technologies for synthesizing the aminocyclitol natural product, pactamycin, is disclosed. This work features two disparate approaches for assembling the five-membered ring core of the target, the first of which utilizes acyl anion catalysis and a second involving β-ketoester aerobic hydroxylation. Installation of the C3-N bond, one of three contiguous nitrogen centers, is made possible through Rh-catalyzed allylic C-H amination of a sulfamate ester. Subsequent efforts are presented to introduce the C1,C2 cis-diamino moiety en route to pactamycin, including carbamate-mediated alkene aziridination. In the course of these studies, assembly of the core of C2- epi-pactamycate, which bears the carbon skeleton and all of the requisite nitrogen and oxygen functional groups found in the natural product, has been achieved.

Approach to pactamycin analogues using rhodium(ii)-catalyzed alkene aziridination and C(sp3)–H amination reactions

Application of dirhodium(ii)-catalyzed nitrene transfers allows for the preparation of a platform bearing the triamino moiety present in pactamycin.

Synthetic Study on Pactamycin: Stereoselective Synthesis of the Cyclopentane Core Framework

The cyclopentane core framework 23 of pactamycin (1) was synthesized in 14 steps from symmetric cyclohexadiene 11. Our synthetic strategy features Rh-mediated catalytic desymmetrization of 10 via aziridination and then regioselective ring-opening reaction of sulfonylaziridine 9 with NaN₃, ring-contraction of cyclohexene 14 by ozonolysis followed by intramolecular aldol reaction, and stereoselective construction of the sequential tetrasubstituted carbons by dihydroxylation and methylation reaction. Stereospecific incorporation of amine on tetrasubstituted carbon was achieved by Curtius rearrangement and subsequent carbamide formation.

Appreciation of symmetry in natural product synthesis

This review defines symmetric molecules from a synthetic perspective and shows various strategies that take advantage of molecular symmetry to construct them.

Total synthesis and structure-activity relationship studies of a series of selective G protein inhibitors

G proteins are key mediators of G protein-coupled receptor signalling, which facilitates a plethora of important physiological processes. The cyclic depsipeptides YM-254890 and FR900359 are the only known specific inhibitors of the G_q subfamily of G proteins; however, no synthetic route has been reported previously for these complex natural products and they are not easily isolated from natural sources. Here we report the first total synthesis of YM-254890 and FR900359, as well as of two known analogues, YM-385780 and YM-385781. The versatility of the synthetic approach also enabled the design and synthesis of ten analogues, which provided the first structure-activity relationship study for this class of compounds. Pharmacological characterization of all the compounds at G_q-, G_i- and G_s-mediated signalling provided succinct information on the structural requirements for inhibition, and demonstrated that both YM-254890 and FR900359 are highly potent inhibitors of G_q signalling, with FR900359 being the most potent. These natural products and their analogues represent unique tools for explorative studies of G protein inhibition.

Synthetic Study of Pactamycin: Enantioselective Construction of the Pactamycin Core with Five Contiguous Stereocenters

A synthetic study of pactamycin is described. Enantioselective construction of the aminocyclopentitol core of pactamycin bearing five contiguous stereocenters was achieved based on an organocatalytic asymmetric aziridination of 2-cyclopentene-1-one, a regio- and diastereoselective 1,3-dipolar cycloaddition, and a rhodium-catalyzed C-H amination reaction.

Diastereoselective Synthesis of the Aminocyclitol Core of Jogyamycin via an Allene Aziridination Strategy

Oxidative allene amination provides rapid access to densely functionalized amine-containing stereotriads through highly reactive bicyclic methyleneaziridine intermediates. This strategy has been demonstrated as a viable approach for the construction of the densely functionalized aminocyclitol core of jogyamycin, a natural product with potent antiprotozoal activity. Importantly, the flexibility of oxidative allene amination will enable the syntheses of modified aminocyclitol analogues of the jogyamycin core.

Novel Pactamycin Analogs Induce p53 Dependent Cell-Cycle Arrest at S-Phase in Human Head and Neck Squamous Cell Carcinoma (HNSCC) Cells

Pactamycin, although putatively touted as a potent antitumor agent, has never been used as an anticancer drug due to its high cytotoxicity. In this study, we characterized the effects of two novel biosynthetically engineered analogs of pactamycin, de-6MSA-7-demethyl-7-deoxypactamycin (TM-025) and 7-demethyl-7-deoxypactamycin (TM-026), in head and neck squamous cell carcinoma (HNSCC) cell lines SCC25 and SCC104. Both TM-025 and TM-026 exert growth inhibitory effects on HNSCC cells by inhibiting cell proliferation. Interestingly, unlike their parent compound pactamycin, the analogs do not inhibit synthesis of nascent protein in a cell-based assay. Furthermore, they do not induce apoptosis or autophagy in a dose- or a time-dependent manner, but induce mild senescence in the tested cell lines. Cell cycle analysis demonstrated that both analogs significantly induce cell cycle arrest of the HNSCC cells at S-phase resulting in reduced accumulation of G₂/M-phase cells. The pactamycin analogs induce expression of cell cycle regulatory proteins including master regulator p53, its downstream target p21^Cip1/WAF1, p27^kip21, p19, cyclin E, total and phospho Cdc2 (Tyr15) and Cdc25C. Besides, the analogs mildly reduce cyclin D1 expression without affecting expression of cyclin B, Cdk2 and Cdk4. Specific inhibition of p53 by pifithrin-α reduces the percentage of cells accumulated in S-phase, suggesting contribution of p53 to S-phase increase. Altogether, our results demonstrate that Pactamycin analogs TM-025 and TM-026 induce senescence and inhibit proliferation of HNSCC cells via accumulation in S-phase through possible contribution of p53. The two PCT analogs can be widely used as research tools for cell cycle inhibition studies in proliferating cancer cells with specific mechanisms of action.

Preparation and biological evaluation of synthetic and polymer-encapsulated congeners of the antitumor agent pactamycin: insight into functional group effects and biological activity

The synthesis and biological analysis of a number of novel congeners of the aminocyclopentitol pactamycin is described. Specific attention was paid to the preparation of derivatives at crucial synthetic branch points of the parent structure, and biological assays revealed a number of insights into the source of pactamycin's biological activity. Additionally, the encapsulation of pactamycin and select derivatives into the PRINT© nanoparticle technology was investigated as a proof-of-concept, and evidence of bioactivity modulation through nanoparticle delivery is demonstrated. This work has provided heretofore unrealized access to a large number of novel compounds for further evaluation.

Asymmetric synthesis of the aminocyclitol pactamycin, a universal translocation inhibitor

An asymmetric total synthesis of the aminocyclopentitol pactamycin is described. The title compound is delivered in 15 steps from 2,4-pentanedione. Critical to this approach was the exploitation of a complex symmetry-breaking reduction strategy to assemble the C1, C2, and C7 relative stereochemistry within the first four steps of the synthesis. Multiple iterations of this reduction strategy are described, and a thorough analysis of stereochemical outcomes is detailed. In the final case, an asymmetric Mannich reaction was developed to install a protected amine directly at the C2 position. Symmetry-breaking reduction of this material gave way to a remarkable series of stereochemical outcomes leading to the title compound without recourse to nonstrategic downstream manipulations. This synthesis is immediately accommodating to the preparation of structural analogs.

Recent Applications and Developments of Organic Azides in Total Synthesis of Natural Products

Organic azides have been exploited since their discovery because of their high reactivities. Various organic reactions using azides have been synthetically applied in chemical biology pharmaceuticals medicinal and agricultural areas. In this review we present some recent applications and developments of organic azides in the total synthesis of natural products (mostly within five years) especially alkaloids. We focus not only on application examples of organic azides but also show their preparation methods including recently reported procedures concerning their decomposing and reducing methods in the syntheses of bioactive molecules.

Enantioselective synthesis of pactamycin, a complex antitumor antibiotic

Medicinal application of many complex natural products is precluded by the impracticality of their chemical synthesis. Pactamycin, the most structurally intricate aminocyclopentitol antibiotic, displays potent antiproliferative properties across multiple phylogenetic domains, but it is highly cytotoxic. A limited number of analogs produced by genetic engineering technologies show reduced cytotoxicity against mammalian cells, renewing promise for therapeutic applications. For decades, an efficient synthesis of pactamycin amenable to analog derivatizations has eluded researchers. Here, we present a short asymmetric total synthesis of pactamycin. An enantioselective Mannich reaction and symmetry-breaking reduction sequence was designed to enable assembly of the entire carbon core skeleton in under five steps and control critical three-dimensional (stereochemical) functional group relationships. This modular route totals 15 steps and is immediately amenable for structural analog synthesis.

Diastereoselective synthesis of vicinal tertiary diols

A strategy for the synthesis of differentiated vicinal tertiary diols is described. The key step is a high-yielding, diastereoselective LaCl3·2LiCl-mediated addition of a Grignard or organolithium reagent to ketone 2a. The reaction is believed to proceed via a 1,3-chelated intermediate. One of the adducts has been transformed into a functionalized cyclopentenone resembling the core structure of pactamycin.

Mutasynthesis of fluorinated pactamycin analogues and their antimalarial activity

A mutasynthetic strategy has been used to generate fluorinated TM-025 and TM-026, two biosynthetically engineered pactamycin analogues produced by Streptomyces pactum ATCC 27456. The fluorinated compounds maintain excellent activity and selectivity toward chloroquine-sensitive and multidrug-resistant strains of malarial parasites as the parent compounds. The results also provide insights into the biosynthesis of 3-aminobenzoic acid in S. pactum.