Diversity-Oriented Synthesis of Diverse Polycyclic Scaffolds Inspired by the Logic of Sesquiterpene Lactones Biosynthesis

Expanding Natural Diversity: Tailored Enrichment of the 8,12-Sesquiterpenoid Lactone Chemical Space through Divergent Synthesis

The divergent synthesis of a non-natural 8,12-sesquiterpenoid lactone collection is described. The synthesis relies on a rationally designed guaianolide scaffold bearing a tertiary hydroxyl as the pinpoint for inducing its selective diversification. Key reactions include an unprecedented Suarez-type CH lactonization and a highly diastereoselective oxy-Cope/ene cascade that allows the introduction of three stereocenters in a single operation. Selective oxidative/reductive and redox neutral transformations follow to highlight the synthesis of naturally unpresented highly substituted 8,12-guaianolides.

Hypocretenolides: collective total syntheses and activities toward metastatic colon cancer

A concise and collective synthetic route to hypocretenolides was developed for the first time. This route features one-pot addition-alkylation and intramolecular 1,3-dipolar cycloaddition to efficiently assemble the 5/7/6 ring system. Our syntheses enabled multigram preparation of hypocretenolide which facilitated further biological evaluation. Preliminary CCK-8 cytotoxic results of hypocretenolide indicated its IC50 values within 1 μM against 4 colon cancer cell lines. Wound healing and transwell assays suggested the promising inhibitory activities of hypocretenolide toward the migratory capabilities of colon cancer cells in vitro. The animal results confirmed that hypocretenolide can inhibit metastasis of colon cancer cells.

Total Syntheses of Paraconic Acids and 1,10-seco-Guaianolides via a Barbier Allylation/Translactonization Cascade of 3-(Bromomethyl)-2(5H)-furanone

A palladium-catalyzed Barbier allylation/translactonization cascade reaction was established for the rapid construction of β,γ-disubstituted α-exo-methylene-γ-butyrolactone, an important motif in sesquiterpenes. Dimethyl zinc played significant roles in both steps for the umpolung of π-allylpalladium as a nucleophile and promoting a Lewis acid-mediated translactonization. This sequence showed a broad substrate scope and was further harnessed for the synthesis of two paraconic acids as well as the first protecting-group-free total synthesis of two 1,10-seco-guaianolides.

Allylative Approaches to the Synthesis of Complex Guaianolide Sesquiterpenes from Apiaceae and Asteraceae

With hundreds of unique members isolated to date, guaianolide lactones represent a particularly prolific class of terpene natural products. Given their extensive documented therapeutic properties and fascinating chemical structures, these metabolites have captivated the synthetic chemistry community for many decades. As a result of divergent biosynthetic pathways, which produce a wide array of stereochemical and oxidative permutations, a unifying synthetic pathway to this broad family of natural products is challenging. Herein we document the evolution of a chiral-pool-based synthetic program aimed at accessing an assortment of guaianolides, particularly those from the plant family Apiaceae as well as Asteraceae, members of which possess distinct chemical substructures and necessitate deviating synthetic platforms. An initial route employing the linear monoterpene linalool generated a lower oxidation state guaianolide but was not compatible with the majority of family members. A double-allylation disconnection using a carvone-derived fragment was then developed to access first an Asteraceae-type guaianolide and then various Apiaceae congeners. Finally, using these findings in conjunction with a tandem polyoxygenation cascade, we developed a pathway to highly oxygenated nortrilobolide. A variety of interesting observations in metal-mediated aldehyde allylation and alkene polyoxygenation are reported and discussed.

Scaffold Diversity Synthesis Delivers Complex, Structurally, and Functionally Distinct Tetracyclic Benzopyrones

Complexity-generating chemical transformations that afford novel molecular scaffolds enriched in sp3 character are highly desired. Here, we present a highly stereoselective scaffold diversity synthesis approach that utilizes cascade double-annulation reactions of diverse pairs of zwitterionic and non-zwitterionic partners with 3-formylchromones to generate highly complex tetracyclic benzopyrones. Each pair of annulation partners adds to the common chroman-4-one scaffold to build two new rings, supporting up to four contiguous chiral centers that include an all-carbon quaternary center. Differently ring-fused benzopyrones display different biological activities, thus demonstrating their immense potential in medicinal chemistry and chemical biology research.

A Double Allylation Strategy for Gram-Scale Guaianolide Production: Total Synthesis of (+)-Mikanokryptin

With over 5000 members isolated to date, sesquiterpene lactones represent a prolific source of medicinal agents with several derivatives in human clinical trials. The guaianolides, a major subset of this group, have been intensely investigated from both medicinal and chemical-synthesis perspectives for decades. To date, the myriad stereochemical permutations presented by this enormous family have precluded the synthesis of many unique members. Herein we report the total synthesis of the trans-fused 8,12-guaianolide (+)-mikanokryptin in 10 steps from (+)-carvone. Notably, this synthesis is the first gram-scale total synthesis of a guaianolide natural product.

Platinum-catalyzed cycloisomerizations of a common enyne: a divergent entry to cyclopropane sesquiterpenoids. Formal synthesis of sarcandralactone A

A common enyne scaffold, resembling the structures of natural elemanes was found to be an excellent substrate for highly regioselective cycloisomerizations to produce diverse cyclopropane sesquiterpenoids. Platinum-catalysis was utilized to produce either lindenane or myliol cores, found in natural products, starting from enyne acetate 10 and its corresponding allene 12 respectively. Based on this concept, a second generation strategy allows the formal synthesis of sarcandralactone A.

Accessing Nature's diversity through metabolic engineering and synthetic biology

In this perspective, we highlight recent examples and trends in metabolic engineering and synthetic biology that demonstrate the synthetic potential of enzyme and pathway engineering for natural product discovery. In doing so, we introduce natural paradigms of secondary metabolism whereby simple carbon substrates are combined into complex molecules through “scaffold diversification”, and subsequent “derivatization” of these scaffolds is used to synthesize distinct complex natural products. We provide examples in which modern pathway engineering efforts including combinatorial biosynthesis and biological retrosynthesis can be coupled to directed enzyme evolution and rational enzyme engineering to allow access to the “privileged” chemical space of natural products in industry-proven microbes. Finally, we forecast the potential to produce natural product-like discovery platforms in biological systems that are amenable to single-step discovery, validation, and synthesis for streamlined discovery and production of biologically active agents.

Metal-catalyzed cycloisomerization as a powerful tool in the synthesis of complex sesquiterpenoids

Metal-catalyzed cycloisomerization reactions can be regarded as “artificial sesquiterpenoid cyclase” tools in modern organic synthesis, and their progress is reviewed.

Divergent pathways to furosesquiterpenes: first total syntheses of (+)-zedoarol and (Rac)-gweicurculactone

Divergent transformation of a non-natural elemane to furo-guaiane and furo-cadinane cores.

Diversity-oriented synthesis of Lycopodium alkaloids inspired by the hidden functional group pairing pattern

Natural products continue to provide a rich source of inspiration for both chemists and biologists. The efficient synthesis of bioactive natural products or natural product-like molecules has offered tremendous opportunities for complex biological processes exploration and drug discovery. However, because natural products usually contain numerous stereogenic centres and polycyclic ring systems, significant synthetic challenges remain. Here we employ the build/couple/pair strategy that is frequently used in diversity-oriented synthesis to obtain skeletally diverse compounds with complexities comparable to natural products. Inspired by the functional group pairing patterns hidden in Lycopodium alkaloids, we efficiently and in parallel construct four natural products, (+)-Serratezomine A, (-)-Serratinine, (+)-8α-Hydroxyfawcettimine and (-)-Lycoposerramine-U, as well as six different unnatural scaffolds, following the advanced build/couple/pair algorithm. This newly developed strategy is expected to be applied to the efficient synthesis of other complex natural products possessing functional group pairing patterns as well as skeletally diverse natural product-like molecules.

Biomimetic synthesis of (+)-ledene, (+)-viridiflorol, (-)-palustrol, (+)-spathulenol, and psiguadial A, C, and D via the platform terpene (+)-bicyclogermacrene

(+)-Bicyclogermacrene is a strained bicyclic and common sesquiterpene found in several essential oils. A short and good yielding synthesis of bicyclogermacrene proceeding in seven steps is reported. This terpene is used as key platform intermediate for a biomimetic access to several aromadendrene sesquiterpenoids, such as ledene, viridiflorol, palestrol, and spathulenol. Furthermore, bicyclogermacrene is shown to be the terpene component in the synthesis of the meroterpenoids psiguadial A, C, and D.

From understanding to prediction: gold- and platinum-based π-acid catalysis for target oriented synthesis

During the last century, conceptual advances in organometallic chemistry were often rapidly embraced by target oriented synthesis. Feedback provided by such preparative scrutiny has greatly benefitted method development; particularly prominent are examples from the entire cross coupling arena, as well as olefin metathesis. Seen against this backdrop, it is somewhat surprising that the explosive growth of research into π-acid catalysis has not yet yielded a matching number of implementations into the synthesis of structurally complex targets of biological significance. In contrast to the massive output of methodological and mechanistic investigations, few studies illustrate the strategic use of gold, silver, or platinum catalysis in late stages of such multistep endeavors. These elaborate and highly precious compounds demand utmost confidence in the reliability and robustness of the method to be applied. In this Account, we analyze the possible reasons for this imbalance, after a short summary of the conceptual basis of carbophilic activation of π-bonds with the aid of soft transition metal cations or complexes. We pinpoint mechanistic subtleties, which, at least in part, produce a great deal of structural diversity but can jeopardize predictive power. With the advances in the understanding of π-acid catalyzed processes in general, however, this uncertainty is gradually vanishing and the entire field is transitioning from comprehension to prediction. This is expected to foster advanced applications, while recent progress in asymmetric gold catalysis further improves the preparative significance. The presented work in this Account illustrates our own commitment to the field as well as our growing confidence in the maturity of platinum and gold catalysis. The carbophilic activation of π-bonds, particularly of alkynes, provides a method to manipulate functional groups that is orthogonal to traditional carbonyl chemistry. We illustrate this notion by presenting a new approach to hydroxypyrone derivatives that has enabled the total synthesis of the fragile polyunsaturated cyclophane derivative neurymenolide A. The synthesis of the pyrrole alkaloid streptorubin by an enyne cycloisomerization is equally instructive. In addition, different manifestations of transannular hydroxyl addition reactions across alkyne partners mark the late stages of our conquests of amphidinolide F, polycavernoside A, and spirastrellolide F. Together with a few model studies and a personal selection of recent highlights from other groups, these examples augur well for future applications of π-acid catalysts in the realm of target oriented synthesis.

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. The literature from January to December 2012 is reviewed, and 471 references are cited.

Hits, leads and drugs against malaria through diversity-oriented synthesis

Malaria is a devastating infectious disease and approximately half the world’s population is at risk. Since vaccination is not yet available, small-molecule-based medicines are currently the best option for the treatment of patients suffering from malaria and combating the spread of infection. Development of resistance against existing drugs has created a need for new types of small molecules to be screened against Plasmodium falciparum, the etiological agent of malaria. The advent of diversity-oriented synthesis has enabled access to novel chemical structures. Evaluation of diversity-oriented synthesis compounds in phenotypic assays for growth inhibition of P. falciparum has resulted in novel hits, leads and even investigational drugs against malaria.