Synthesis of Neurotrophic Seco-prezizaane Sesquiterpenes (1R,10S)-2-Oxo-3,4-dehydroneomajucin, (2S)-Hydroxy-3,4-dehydroneomajucin, and (-)-Jiadifenin

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 C17β-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 C17β-substituents and containing a β-OH at C14.

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.

The search for, and chemistry and mechanism of, neurotrophic natural products

Abstract

Neurotrophic factors, now termed neurotrophins, which belong to a class of polypeptidyl agents, have been shown to potentially be beneficial for the treatment of neurodegenerative diseases such as Alzheimer’s disease, because endogenous neurotrophic factors (NGF, BDNF, NT3, NT4) have been recognized to play critical roles in the promotion of neurogenesis, differentiation, and neuroprotection throughout the development of the central nervous system. However, high-molecular weight proteins are unable to cross the blood–brain barrier and are easily decomposed by peptidase under physiological conditions. To address this issue, small molecules that can mimic the functions of neurotrophic factors would be promising alternatives for the treatment of neurodegenerative disease. We have continued to search for natural products having typical neurotrophic properties, which can cause neurogenesis, enhance neurite outgrowth, and protect neuronal death using three cellular systems (PC12, rat cortical neurons, and MEB5 cells). In this review, we summarize the neurotrophic activities and synthesis of dimeric isocuparane-type sesquiterpenes from the liverwort, Mastigophora diclados, the mechanism of neurotrophic neolignans, magnolol, honokiol and their sesquiterpene derivatives, and introduce unique neurotrophin-mimic natural products, including seco-prezizaane-type sesquiterpenes from the Illicium species, vibsane-type diterpenes from Viburnum awabuki, and miscellaneous natural products with neurotrophic effects discovered by us.

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Titanium Alkoxide-Based Regioselective Alkyne-Alkyne Reductive Coupling Mediated by In Situ Generated Arylamidate

Titanium alkoxide-based alkyne-alkyne reductive coupling mediated by in situ generated arylamidate is described. A high level of regioselectivity is achieved in 37 examples, where (E,E)-dienes are exclusively formed. To the best of our knowledge, this study represents the first example of an apparent amide and carbamate directing effect in metal-mediated reductive coupling.

Titanium catalysis for the synthesis of fine chemicals – development and trends

Atlas as a Titan(ium) is holding the earth-abundant chemistry world. Titanium is the second most abundant transition metal, is a key player in important industrial processes (e.g. polyethylene) and shows much promise for diverse applications in the future.

Harpagide, a natural product, promotes synaptic vesicle release as measured by nanoelectrode amperometry

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive loss of dopaminergic (DAergic) neurons and low level of dopamine (DA) in the midbrain. Recent studies suggested that some natural products can protect neurons against injury, but their role on neurotransmitter release and the underlying mechanisms remained unknown. In this work, nanoelectrode electrochemistry was used for the first time to quantify DA release inside single DAergic synapses. Our results unambiguously demonstrated that harpagide, a natural product, effectively enhances synaptic DA release and restores DA release at normal levels from injured neurons in PD model. These important protective and curative effects are shown to result from the fact that harpagide efficiently inhibits the phosphorylation and aggregation of α-synuclein by alleviating the intracellular reactive oxygen level, being beneficial for vesicle loading and recycling. This establishes that harpagide offers promising avenues for preventive or therapeutic interventions against PD and other neurodegenerative disorders.

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.

Development of a Terpene Feedstock-Based Oxidative Synthetic Approach to the Illicium Sesquiterpenes

The Illicium sesquiterpenes are a family of natural products containing over 100 highly oxidized and structurally complex members, many of which display interesting biological activities. This comprehensive account chronicles the evolution of a semisynthetic strategy toward these molecules from (+)-cedrol, seeking to emulate key aspects of their presumed biosynthesis. An initial route generated lower oxidation state analogs but failed in delivering a crucial hydroxy group in the final step. Insight gathered during these studies, however, ultimately led to a synthesis of the pseudoanisatinoids along with the allo-cedrane natural product 11- O-debenzoyltashironin. A second-generation strategy was then developed to access the more highly oxidized majucinoid compounds including jiadifenolide and majucin itself. Overall, one dozen natural products can be accessed from an abundant and inexpensive terpene feedstock. A multitude of general observations regarding site-selective C(sp³)-H bond functionalization reactions in complex polycyclic architectures are reported.

Modern applications of low-valent early transition metals in synthesis and catalysis

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

Contemporary Synthetic Strategies toward seco-Prezizaane Sesquiterpenes from Illicium Species

Since the elucidation of the structure of anisatin in the late 1960s, sesquiterpene lactones from various Illicium species of plants have captivated synthetic chemists worldwide, resulting in a large body of synthetic work. In particular, Illicium sesquiterpenes containing the seco-prezizaane carbon framework have seen immense interest in recent years owing to desirable structural and medicinal attributes. This synopsis will focus on recently developed synthetic strategies to access these compact, highly oxidized terpenoids.

Quantifying orientational regeneration of injured neurons by natural product concentration gradients in a 3D microfluidic device

Regeneration of injured neurons in complicated three-dimensional (3D) microenvironments is a key approach for treating neurodegenerative diseases. Microfluidics provides a versatile tool to recapitulate cellular microenvironments in vitro, but it still remains a big challenge to construct a microfluidic platform incorporating extracellular matrix (ECM) structures and highly controlled 3D gradients of soluble factors to study the regeneration of injured neurons. In this work, we developed a microfluidic device which can provide multiple adjustable gradients in a 3D ECM to investigate the regeneration of injured central nervous system (CNS) neurons in response to natural small molecules. With interconnecting but independently controlled central channels, asymmetrically designed side channels and a series of microgrooves connecting the central channels, spatially and temporally controlled 3D biochemical gradients can be generated inside collagen hydrogel in the central channels. This allows quantitative analysis of guided axon growth and the orientational regeneration of injured dopaminergic neurons by 3D chemical gradients of three natural molecules. This study demonstrates a promising microfluidic platform for the generation of highly controlled 3D biochemical gradients in an ECM to quantitatively study neuronal responses, thereby potentially facilitating drug screening and optimization of treatment protocols for neurodegenerative diseases.

Total synthesis of complex terpenoids employing radical cascade processes

Covering: 2011-2017Radical cyclizations have a rich history in organic chemistry and have been particularly generous to the field of natural product synthesis. Owing to their ability to operate in highly congested molecular quarters, and with significant functional group compatibility, these transformations have enabled the synthesis of numerous polycyclic terpenoid natural products over the past several decades. Moreover, when programmed accordingly into a synthetic plan, radical cascade processes can be used to rapidly assemble molecular complexity, much in the same way nature rapidly constructs terpene frameworks through cationic cyclization pathways. This review highlights recent total syntheses of complex terpenoids (from 2011-2017) employing C-C bond-forming radical cascade sequences.

A Highly Chemo-, Regio-, and Stereoselective Metallacycle-Mediated Annulation Between a Conjugated Enyne and an Ene-Diyne

Alkoxide-directed metal-centered intermolecular [2+2+2] annulation is shown to chemo-, regio-, and stereoselectively engage two polyunsaturated substrate in productive cyclization chemistry. This annulation process is unique in the field, revealing that it is possible to selectively engage three of five π-systems residing in the coupling partners in initial [2+2+2] reaction, and demonstrating that one of the two remaining π-systems (the TMS-alkyne) can ultimately serve to simply generate a new metallacyclopentene of great potential value in additional metallacycle-mediated coupling chemistry.

Total Syntheses of (-)-Majucin and (-)-Jiadifenoxolane A, Complex Majucin-Type Illicium Sesquiterpenes

We report the first chemical syntheses of both (-)-majucin and (-)-jiadifenoxolane A via 10 net oxidations from the ubiquitous terpene (+)-cedrol. Additionally, this approach allows for access to other majucin-type sesquiterpenes, like (-)-jiadifenolide, (-)-jiadifenin, and (-)-(1R,10S)-2-oxo-3,4-dehydroxyneomajucin (ODNM) along the synthetic pathway. Site-selective aliphatic C(sp3)-H bond oxidation reactions serve as the cornerstone of this work which offers access to highly oxidized natural products from an abundant and renewable terpene feedstock.

Synthesis of Highly Oxygenated Carbocycles by Stereoselective Coupling of Alkynes to 1,3- and 1,4-Dicarbonyl Systems

Densely substituted and highly oxygenated carbocycles are challenging targets for synthesis. In particular, those possessing numerous contiguous, fully substituted carbon atoms (i.e., tertiary alcohols and quaternary centers) are often not accessible in a direct fashion, necessitating the strategic decoupling of ring-formation from the establishment of functionality about the system. Here, we describe an approach to the construction of highly oxygenated mono-, di-, and polycyclic carbocycles from the reaction of disubstituted alkynes with β- or γ-dicarbonyl systems. These processes embrace a variant of metallacycle-mediated annulation chemistry where initial alkyne-carbonyl coupling is followed by a second, now intramolecular, stereoselective C-C bond-forming event. In addition to revealing the basic reactivity pattern in intermolecular settings, we demonstrate that this class of reactivity is quite powerful in a fully intramolecular context and, when terminated by a stereoselective oxidation process, can be used to generate polycyclic systems containing a fully substituted and highly oxygenated five-membered ring.

Synthesis of (-)-11-O-Debenzoyltashironin: Neurotrophic Sesquiterpenes Cause Hyperexcitation

11-O-Debenzoyltashironin (1) is a member of the neurotrophic sesquiterpenes, trace plant metabolites that enhance neurite outgrowth in cultured neurons. We report its synthesis in six steps from a butenolide heterodimer via its likely biosynthetic precursor, 3,6-dideoxy-10-hydroxypseudoanisatin, here identified as the chain tautomer of 1. Access to the tashironin chemotype fills a gap in a comparison set of convulsive and neurotrophic sesquiterpenes, which we hypothesized to share a common target. Here we show that both classes mutually hyperexcite rat cortical neurons, consistent with antagonism of inhibitory channels and a mechanism of depolarization-induced neurite outgrowth.

The Development of Alkoxide-Directed Metallacycle-Mediated Annulative Cross-Coupling Chemistry

Alkoxide-directed metallacycle-mediated cross-coupling is a rapidly growing area of reaction methodology in organic chemistry. Over the last decade, developments have resulted in > thirty new and highly selective intermolecular (or "convergent") C-C bond-forming reactions that have established powerful retrosynthetic relationships in stereoselective synthesis. While early studies were focused on developing transformations that forge a single C-C bond by way of a functionalized and unsaturated metallacyclopentane intermediate, recent advances mark the ability to employ this organometallic intermediate in additional stereoselective transformations. Among these more advanced coupling processes, those that embrace the metallacycle in subsequent [4+2] chemistry have resulted in the realization of a number of highly selective annulative cross-coupling reactions that deliver densely functionalized and angularly substituted carbocycles. This review discusses the early development of this chemistry, recent advances in reaction methodology, and shares a glimpse of the power of these processes in natural product synthesis.

Oxidative Entry into the Illicium Sesquiterpenes: Enantiospecific Synthesis of (+)-Pseudoanisatin

Illicium sesquiterpenes have been the subject of numerous synthetic efforts due to their ornate and highly oxidized structures as well as significant biological activities. Herein we report the first chemical synthesis of (+)-pseudoanisatin from the abundant feedstock chemical cedrol (∼$50 USD/kg) in 12 steps using extensive site-selective C(sp³)-H bond functionalization. Significantly, this work represents a novel oxidative strategic template for future approaches to these natural products and their analogs.

Formal Total Synthesis of (-)-Jiadifenolide and Synthetic Studies toward seco-Prezizaane-Type Sesquiterpenes

Synthetic studies toward highly oxygenated seco-prezizaane sesquiterpenes are reported, which culminated in a formal total synthesis of the neurotrophic agent (-)-jiadifenolide. For the construction of the tricyclic core structure, an unusual intramolecular and diastereoselective Nozaki-Hiyama-Kishi reaction involving a ketone as electrophilic coupling partner was developed. In addition, synthetic approaches toward the related natural product (2R)-hydroxy-norneomajucin, featuring a Mn-mediated radical cyclization for the tricycle assembly and a regioselective OH-directed C-H activation are presented.

Synthesis of Angularly Substituted trans-Fused Decalins through a Metallacycle-Mediated Annulative Cross-Coupling Cascade

A convergent coupling reaction is described that enables the stereoselective construction of angularly substituted trans-fused decalins from acyclic precursors. The process builds on our alkoxide-directed titanium-mediated alkyne-alkyne coupling and employs a 1,7-enyne coupling partner. Overall, the reaction is thought to proceed through initial formation of a tetrasusbstituted metallacyclopentadiene, stereoselective intramolecular [4+2] cycloaddition, elimination, isomerization, and regio- and stereoselective protonation. Distinct from our early studies directed at the synthesis of trans-fused hydrindanes, the current annulative coupling reveals an important effect of TMSCl in controlling the final protonation-the event that establishes the stereochemistry of the ring fusion.