Function-Oriented Synthesis of Pentacyclic Triterpenoids and Discovery of an ent-Estrane as a Natural Product-Inspired Androgen Receptor Antagonist.

While pentacyclic triterpenoids have a rich history in chemistry and biology, the challenges associated with their asymmetric synthesis contribute to the current reality that medicinal exploration in the area is largely constrained to natural product derivatization. To address this deficiency, a function-oriented synthesis of pentacyclic triterpenoids was pursued. Overall, we report a divergent synthesis of 26-norgermanicol and 26-norlupeol and we have identified a new class of androgen receptor antagonist that is ∼6× more potent than lupeol.

From Functional Fatty Acids to Potent and Selective Natural-Product-Inspired Mimetics via Conformational Profiling.

Fatty acids play important signaling roles in biology, albeit typically lacking potency or selectivity, due to their substantial conformational flexibility. While being recognized as having properties of potentially great value as therapeutics, it is often the case that the functionally relevant conformation of the natural fatty acid is not known, thereby complicating efforts to develop natural-product-inspired ligands that have similar functional properties along with enhanced potency and selectivity profiles. In other words, without structural information associated with a particular functional relationship and the hopelessly unbiased conformational preferences of the endogenous ligand, one is molecularly ill-informed regarding the precise ligand-receptor interactions that play a role in driving the biological activity of interest. To address this problem, a molecular strategy to query the relevance of distinct subpopulations of fatty acid conformers has been established through "conformational profiling", a process whereby a unique collection of chiral and conformationally constrained fatty acids is employed to deconvolute beneficial structural features that impart natural-product-inspired function. Using oleic acid as an example because it is known to engage a variety of receptors, including GPR40, GPR120, and TLX, a 24-membered collection of mimetics was designed and synthesized. It was then demonstrated that this collection contained members that have enhanced potency and selectivity profiles, with some being clearly biased for engagement of the GPCRs GPR40 and GPR120 while others were identified as potent and selective modulators of the nuclear receptor TLX. A chemical synthesis strategy that exploited the power of modern technology for stereoselective synthesis was critical to achieving success, establishing a common sequence of bond-forming reactions to access a disparate collection of chiral mimetics, whose conformational preferences are impacted by the nature of stereodefined moieties differentially positioned about the C18 skeleton of the parent fatty acid. Overall, this study establishes a foundation to fuel future programs aimed at developing natural-product-inspired fatty acid mimetics as valuable tools in chemical biology and potential therapeutic leads.

Progress Toward the Asymmetric De Novo Synthesis of Limonoids.

Asymmetric de novo construction of limonoids remains a challenging problem in stereoselective synthesis due to the diverse and complex structures associated with this class of natural products. Here, a unique synthetic pathway to an "intact" limonoid system is described. The synthetic route is based on exploiting an oxidative rearrangement reaction of a densely functionalized late-stage intermediate to simultaneously establish the stereodefined C10 quaternary center and an allylic acetate at C12. This is a unique example of a complex rearrangement reaction that proceeds on a system whose presumed intermediate allyl cation is highly hindered and lacks neighboring protons that are otherwise required for cation termination.

Progress Toward the Asymmetric de Novo Synthesis of Lanostanes: A Counter Biomimetic Cucurbitane-to-Lanostane Type Transformation.

An oxidative rearrangement has been established that enables a cucurbitane-to-lanostane type rearrangement that is counter to known biomimetic transformations that proceed in an opposite direction by way of a lanostane-to-cucurbitane transformation. Here, an oxidative dearomatization/Wagner-Meerwein rearrangement with a substrate bearing the characteristic cucurbitane triad of quaternary centers at C9, C13 and C14, and possessing an alkene at C11-C12, proceeds in a manner that selectively shifts the methyl group at C9 to C10 in concert with the establishment of a sterically hindered allylic cation. The major product isolated from this transformation is formed by trapping of the allylic cation by addition of acetate to C12, rather than termination of the cascade by loss of a proton at C8. While proceeding by way of a unique sequence of bond-forming reactions that begins by oxidative dearomatization, this process achieves what we believe is an unprecedented cucurbitane-to-lanostane transformation, generating a product that contains the characteristic lantostane triad of quaternary centers at C10, C13 and C14 while also delivering a functionalized C-ring.

Asymmetric Total Syntheses of Euphol and Tirucallol.

Asymmetric de novo syntheses of euphol and tirucallol have been accomplished by way of a concise sequence of chemical steps featuring several modern stereoselective transformations. The preparative solution described for these complex problems in natural product synthesis departs significantly from biomimetic polyene cyclization chemistry, which has been leveraged to address related tetracyclic triterpenoid targets. In particular, a diastereoselective Friedel-Crafts-type cyclization was employed to establish a tetracycle bearing a stereodefined quaternary center at C9 (steroid numbering) that provided access to intermediates of relevance for introducing the C10 and C14 quaternary centers by sequential stereospecific 1,2-alkyl shifts (C9 → C10 and C15 → C14). Finally, the stereodefined C17 side chain was introduced in a single step by late-stage stereoselective conjugate addition to an intermediate possessing a D-ring enone. Notably, these de novo asymmetric syntheses are the first of their kind, providing completely synthetic access to enantiodefined euphane and tirucallane systems. Overall, each synthesis has been accomplished in fewer than 20 linear chemical steps from a simple Hajos-Parrish-derived ketone through a sequence that features just 15 chromatographic operations.

Toward the Asymmetric de Novo Synthesis of Lanostanes: Construction of 7,11-Dideoxy-Δ5-lucidadone H.

Efforts to establish an asymmetric entry to hexanorlanostanes has resulted in a concise synthesis of 7,11-dideoxy-Δ5-lucidadone H from epichlorohydrin. By exploiting metallacycle-mediated annulative cross-coupling (to establish a functionalized hydrindane) and stereoselective formation of the steroidal C9-C10 bond to establish a stereodefined 9-alkyl estrane, 14 subsequent steps have been established to generate a hexanorlanostane system. Key transformations include formal inversion of the C13 quaternary center, oxidative dearomatization/group-selective Wagner-Meerwein rearrangement, and Lewis acid mediated semi-Pinacol rearrangement.

Toward the Asymmetric Synthesis of Cardenolides and Related Steroidal Systems: syn-SN2' of Organometallics with C14-C17 Vinylepoxides.

In a program aimed at establishing a common sequence of C-C bond-forming reactions for asymmetric construction of tetracyclic triterpenoid natural products and related synthetic systems, effort has been directed toward introducing C17ß-substitution by late-stage functionalization of stereodefined "steroidal" D-ring vinylepoxides (spanning C14-C17). It has been found that cyanocuprates participate in syn-SN2' reactions that result in products bearing various C17ß-substituents and containing a ß-OH at C14.

Progress toward the De Novo Asymmetric Synthesis of Euphanes.

Progress toward an asymmetric synthesis of euphanes is described. A C14-desmethyl euphane system possessing five differentially substituted and electronically distinct alkenes has been prepared. The route employed is based on sequential metallacycle-mediated annulative cross-coupling, double asymmetric Brønsted acid mediated intramolecular Friedel-Crafts alkylation, and an oxidative rearrangement to establish the requisite C10 quaternary center. These studies have also led to the discovery of a novel euphane-based modulator of the Liver X Receptor.

Asymmetric De Novo Synthesis of a Cucurbitane Triterpenoid: Total Synthesis of Octanorcucurbitacin B.

The asymmetric de novo synthesis of a cucurbitane natural product, octanorcucurbitacin B, has been accomplished. Cucurbitanes are a family of structurally complex triterpenoids that characteristically contain three stereodefined quaternary centers at ring fusion carbons positioned about their tetracyclic skeletons (at positions 9, 13, and 14). Taking a diversion from the biosynthetic hypothesis for cucurbitane synthesis, the approach established here provides direct access to the cucurbitane skeleton without having to proceed by way of a lanostane. Using a simple chiral enyne as starting material, a sequence of annulative cross-coupling and intramolecular Heck reaction provides a stereodefined polyunsaturated tetracycle possessing the C9 and C13 quaternary centers. This intermediate was converted to octanorcucurbitacin B through a 12-step sequence that features hydroxy-directed Simmons-Smith cyclopropanation, regioselective deconjugative alkylation, and allylic oxidation.

General Enantioselective and Stereochemically Divergent Four-Stage Approach to Fused Tetracyclic Terpenoid Systems.

Tetracyclic terpenoid-derived natural products are a broad class of medically relevant agents that include well-known steroid hormones and related structures, as well as more synthetically challenging congeners such as limonoids, cardenolides, lanostanes, and cucurbitanes, among others. These structurally related compound classes present synthetically disparate challenges based, in part, on the position and stereochemistry of the numerous quaternary carbon centers that are common to their tetracyclic skeletons. While de novo syntheses of such targets have been a topic of great interest for over 50 years, semisynthesis is often how synthetic variants of these natural products are explored as biologically relevant materials and how such agents are further matured as therapeutics. Here, focus was directed at establishing an efficient, stereoselective, and molecularly flexible de novo synthetic approach that could offer what semisynthetic approaches do not. In short, a unified strategy to access common molecular features of these natural product families is described that proceeds in four stages: (1) conversion of epichlorohydrin to stereodefined enynes, (2) metallacycle-mediated annulative cross-coupling to generate highly substituted hydrindanes, (3) tetracycle formation by stereoselective forging of the C9-C10 bond, and (4) group-selective oxidative rearrangement that repositions a quaternary center from C9 to C10. These studies have defined the structural features required for highly stereoselective C9-C10 bond formation and document the generality of this four-stage synthetic strategy to access a range of unique stereodefined systems, many of which bear stereochemistry/substitution/functionality not readily accessible from semisynthesis.

An Enantiodefined Conformationally Constrained Fatty Acid Mimetic and Potent Inhibitor of ToxT.

The chiral conformation that palmitoleic acid takes when it is bound to ToxT, the master regulator of virulence genes in the bacterial pathogen Vibrio cholerae, was used as inspiration to design a novel class of fatty acid mimetics. The best mimetic, based on a chiral hydrindane, was found to be a potent inhibitor of this target. The synthetic chemistry that enabled these studies was based on the sequential use of a stereoselective annulative cross-coupling reaction and dissolving metal reduction to establish the C13 and C9 stereocenters, respectively.

From an ent-Estrane, through a nat-Androstane, to the Total Synthesis of the Marine-Derived Δ8,9-Pregnene (+)-03219A.

The total synthesis of (+)-03219A, a rare Δ8,9-pregnene isolated from the marine-derived Streptomyces sp. SCSIO 03219, is described that is based on a series of transformations that enable progression from epichlorohydrin to an ent-estrane, then conversion to a nat-androstane, and finally establishment of the natural product target. Key to the success of these studies was implementation of two rearrangement processes to formally invert the quaternary center at C13 and establish the C10 quaternary center.

From Metallacycle-Mediated Annulative Cross-Coupling to Steroidal Tetracycles through Intramolecular C9-C10 Bond Formation.

While semisynthesis is a common platform for medicinal investigation of steroidal systems, varying the nature of substitution and stereochemistry at C9 and C10 remains challenging. It is demonstrated here that de novo synthesis, enabled by a metallacycle-centered annulation reaction, provides a uniquely effective means of addressing this problem. In short, double asymmetric Friedel-Crafts cyclization proved most effective for establishing anti- relative stereochemistry (with respect to C13), while an intramolecular Heck reaction reliably delivered the syn- diastereomers with high selectivity. In addition, these studies reveal that this oxidative rearrangement is effective for establishing a C10 quaternary center boasting variable alkyl or aryl substitution.

Synthesis of Anhydroryanodol.

A stereoselective entry to ryanoids is described that culminates in the synthesis of anhydroryanodol and thus the formal total synthesis of ryanodol. The pathway described features an annulation reaction conceived to address the uniquely complex and highly oxygenated polycyclic skeleton common to members of this natural product class. It is demonstrated that metallacycle-mediated intramolecular coupling of an alkyne and a 1,3-diketone can proceed with a highly functionalized enyne and with outstanding levels of stereoselection. Furthermore, the first application of this technology in natural product synthesis is demonstrated here. More broadly, the advances described demonstrate the value that programs in natural product total synthesis have in advancing organic chemistry, here through the design and realization of an annulation reaction that accomplishes what previously established reactions do not.

Studies Targeting Ryanodol Result in an Annulation Reaction for the Synthesis of a Variety of Fused Carbocycles.

An annulation reaction is described to access a range of polycyclic and highly oxygenated carbocycles. First developed in an approach to the synthesis of ryanodol, metallacycle-mediated annulative diketone-alkyne coupling defines a framework for realization of new retrosynthetic relationships for complex molecule synthesis. In addition to demonstrating this reaction in the context of forging distinct carbocyclic systems, including those featuring a seven-membered ring, the choice of quenching reagent leads to unique reaction outcomes.

A synthesis strategy for tetracyclic terpenoids leads to agonists of ERß.

Natural product and natural product-like molecules continue to be important for the development of pharmaceutical agents, as molecules in this class play a vital role in the pipeline for new therapeutics. Among these, tetracyclic terpenoids are privileged, with >100 being FDA-approved drugs. Despite this significant pharmaceutical success, there remain considerable limitations to broad medicinal exploitation of the class due to lingering scientific challenges associated with compound availability. Here, we report a concise asymmetric route to forging natural and unnatural (enantiomeric) C19 and C20 tetracyclic terpenoid skeletons suitable to drive medicinal exploration. While efforts have been focused on establishing the chemical science, early investigations reveal that the emerging chemical technology can deliver compositions of matter that are potent and selective agonists of the estrogen receptor beta, and that are selectively cytotoxic in two different glioblastoma cell lines (U251 and U87).

Synthesis of C14-Desmethylene Corialactone D and Discovery of Inhibitors of Nerve Growth Factor Mediated Neurite Outgrowth.

An asymmetric synthesis of C14-desmethylene corialactone D is described on the basis of strategic application of a metallacycle-mediated annulative cross-coupling reaction, a Still [2,3]-Wittig rearrangement, and Morken's hydroxyl-directed diboration reaction. While representing a convenient approach to access novel compositions of matter inspired by the sesquiterpenoid natural product class (including classic natural product synthesis targets including the picrotaxanes and dendrobine), these studies have led to the discovery of natural product-inspired agents that inhibit nerve growth factor (NGF)-mediated neurite outgrowth in PC-12 cells.

A Complementary Process to Pauson-Khand-Type Annulation Reactions for the Construction of Fully Substituted Cyclopentenones.

A complementary process to the Pauson-Khand annulation is described that is well suited to forging densely substituted/oxygenated cyclopentenone products (including fully substituted variants). The reaction is thought to proceed through a sequence of metallacycle-mediated bond-forming events that engages an internal alkyne and a ß-keto ester in an annulation process that forges two C-C bonds. A variant of this annulation process has also been established that delivers deoxygenated cyclopentenones that lack the allylic tertiary alcohol.

Toward the Total Synthesis of Ryanodol via Oxidative Alkyne-1,3-Diketone Annulation: Construction of a Ryanoid Tetracycle.

A synthetic strategy conceived with the intent of establishing a novel approach to the de novo construction of ryanoids is described that is based on a recently developed metallacycle-mediated intramolecular oxidative alkyne-1,3-diketone coupling reaction. In short, a one-pot annulation/oxidation sequence is shown to be capable of establishing a densely oxygenated polycyclic intermediate that could be converted to a composition of matter that contains the ABCD tetracyclic ring system present in the ryanoid family of natural products.

Synthesis and Discovery of Estra-1,3,5(10),6,8-pentaene-2,16α-diol.

A metallacycle-centered approach to the assembly of partially aromatic synthetic steroids was investigated as a means to prepare a boutique collection of unique steroidal agents. The synthesis and discovery of estra-1,3,5(10),6,8-pentaene-2,16α-diol (VII) is described, along with structure-activity relationships related to its cytotoxic properties. Overall, VII was found to have a GI50 = 0.2 µg/mL (∼800 nM) in MDA-MB-231 human breast cancer cells, be an efficacious estrogen receptor agonist with potency for ERß > ERα (ERß EC50 = 21 nM), possess selective affinity to the cdc-2-like kinase CLK4 (Kd = 350 nM), and be phenotypically related to paclitaxel by an unbiased panel assessment.