Do Anti-Bredt Natural Products Exist? Olefin Strain Energy as a Predictor of Isolability

Total Synthesis of abeo-Steroids via Cycloaddition Strategy

ConspectusSteroids continue to play a significant role in organic chemistry, medicinal chemistry, and drug discovery due to their important biological activities and diverse intriguing structures. Although synthetic organic chemists have successfully constructed and elaborated the classical [6-6-6-5] tetracyclic steroid skeleton for nearly a century, synthesis of the unusual rearranged steroids, particularly abeo-steroids with a medium-sized ring, remains a challenge in the synthetic community. Furthermore, the structures of abeo-steroids are complex and diverse, containing a seven-membered ring embedded in the fused or bridged A/B ring system and possessing numerous stereogenic centers. Besides their structural complexity, various abeo-steroids have shown remarkable biological activities. However, the relative scarcity of abeo-steroids in natural sources has impeded the systematic evaluation of their biological activities. In addition, direct strategies to build the core structures of abeo-steroids are very rare, partially because of the high ring-strain energies of their rearranged A/B ring systems. Therefore, the development of direct and efficient synthetic approaches to these complex molecules is highly desired.Our long-standing interest in the total synthesis of abeo-steroids and the development of new cycloaddition reactions for streamlining complex molecule synthesis have led us to develop a series of unique and powerful intramolecular cycloaddition strategies to access a diverse array of highly strained abeo-steroids. These strategies include Ru-catalyzed [5 + 2] cycloaddition, acid-promoted type I [5 + 2] cycloaddition, Rh-catalyzed [2 + 2 + 1] cycloaddition, and type II [5 + 2] cycloaddition. Since 2018, we have accomplished the first total syntheses of five synthetically challenging abeo-steroids, i.e., bufogargarizins A and B, phomarol, bufospirostenin A, and cyclocitrinol, thus facilitating the evaluation of their pharmacological potentials. In this Account, we summarize our laboratory's systematic efforts in the total synthesis of these abeo-steroids via cycloaddition strategies. We highlight the efficiency and versatility of each cycloaddition strategy for constructing structurally complex abeo-steroid cores by forming the A/B ring system. The evolution of each strategy and key lessons learned from the synthetic journey are also discussed. We believe that our unique perspective in this field will promote advances in the total synthesis of abeo- and related steroids.

The curious yellow colouring matter of the Iceland poppy

The pigment responsible for the yellow colour of the Iceland poppy (Papaver nudicaule) has fascinated chemists for decades. Named nudicaulin by Sir Robert Robinson in 1939, this flavoalkaloid has been the subject of several structural elucidation, biosynthesis, total synthesis and chemical ecology investigations. This account provides a chronological overview of the fascinating history of this natural product.

Regioselective activation of benzocyclobutenones and dienamides lead to anti-Bredt bridged-ring systems by a [4+4] cycloaddition

To the best of our knowledge, bridgehead carbon benzofused-bridged ring systems have previously not been accessible to the synthetic community. Here, we describe a formal type-II [4 + 4] cycloaddition approach that provides fully sp2-carbon embedded anti-Bredt bicyclo[5.3.1] skeletons through the Rh-catalyzed C1-C8 activation of benzocyclobutenones (BCBs) and their coupling with pedant dienamides. Variously substituted dienamides have been coupled with BCBs to provide a range of complex bicyclo[5.3.1] scaffolds (>20 examples, up to 89% yield). The bridged rings were further converted to polyfused hydroquinoline-containing tetracycles via a serendipitously discovered transannular 1,5-hydride shift/Prins-like cyclization/Schmidt rearrangement cascade.

EBC-232 and 323: A Structural Conundrum Necessitating Unification of Five In Silico Prediction and Elucidation Methods

Structurally unique halimanes EBC-232 and EBC-323, isolated from the Australian rainforest plant Croton insularis, proved considerably difficult to elucidate. The two diastereomers, which consist an unusual oxo-6,7-spiro ring system fused to a dihydrofuran, were solved by unification and consultation of five in silico NMR elucidation and prediction methods [i.e., ACDLabs, olefin strain energy (OSE), DP4, DU8+ and TD DFT CD]. Structure elucidation challenges of this nature are prime test case examples for empowering future AI learning in structure elucidation.

Total Synthesis of Natural Products with Bridged Bicyclo[m.n.1] Ring Systems via Type II [5 + 2] Cycloaddition

Natural products containing bridged ring systems are widely identified and show significant biological activity. The development of efficient synthesis reactions and strategies to construct bridged ring systems is a long-standing but very significant challenge in organic chemistry. In 2014, our group developed a unique type II [5 + 2] cycloaddition reaction that provides a facile and direct methodology for constructing highly functionalized bridged bicyclo[4.3.1], bicyclo[4.4.1], bicyclo[5.4.1], bicyclo[6.4.1], and other bicyclo[m.n.1] systems containing a strained bridgehead double bond. In this Account, we summarize the methodology development and report the results of application of our unique strategy for the total synthesis of several natural products with bridged ring systems (i.e., cyclocitrinol, cerorubenic acid-III, and vinigrol) during the past 5 years in our laboratory. In the first part, we introduce the logic behind the design and discovery of type II [5 + 2] cycloadditions. The substrates can be easily synthesized by a modular approach, followed by base-promoted group elimination under heat to form an oxidopyrylium ylide, which can undergo cycloaddition under relatively mild conditions with a variety of double bonds to generate bridged bicyclo[m.n.1] frameworks in high yield. The diastereocontrol and unique endo selectivity of this methodology are favorable for further application to the synthesis of complex natural products. In the second part, we highlight our endeavors in the total synthesis of several different types of molecules bearing bridged ring systems using our methodology. The bridged bicyclo[4.4.1] system is the core structure of two different types of natural products, cyclocitrinol and cerorubenic acid-III, that can be efficiently constructed by type II [5 + 2] cycloadditions. The development of suitable strategies and methods for site-selective cleavage of the C-O bond of the oxa-[3.2.1] ring system in the products of type II [5 + 2] cycloadditions is also discussed and highlighted during the syntheses. Moreover, the bridged bicyclo[5.3.1] system is the core structure of vinigrol, which can be constructed through a novel ring contraction sequence of the bicyclo[5.4.1] system formed by a type II [5 + 2] cycloaddition. By combining with a ring contraction cascade, we believe that type II [5 + 2] cycloadditions have the potential to be used as a unified approach to constructing natural products containing bridged bicyclo[m.n.1] frameworks.

Constrained sialic acid donors enable selective synthesis of α-glycosides

Sialic acid is a sugar residue present in many biologically significant glycans of mammals, commonly as a terminal α-glycoside. The chemical structure of sialic acid, which features an anomeric center with carboxyl and methylene substituents, poses a challenge for synthesis of the α-glycoside, thus impeding biological and therapeutic studies on sialic acid–containing glycans. We present a robust method for the selective α-glycosidation of sialic acid using macrobicyclized sialic acid donors as synthetic equivalents of structurally constrained oxocarbenium ions to impart stereoselectivity. We demonstrate the power of our method by showcasing broad substrate scope and applicability in the preparation of diverse sialic acid–containing architectures.

The value of universally available raw NMR data for transparency, reproducibility, and integrity in natural product research

Covering: up to 2018With contributions from the global natural product (NP) research community, and continuing the Raw Data Initiative, this review collects a comprehensive demonstration of the immense scientific value of disseminating raw nuclear magnetic resonance (NMR) data, independently of, and in parallel with, classical publishing outlets. A comprehensive compilation of historic to present-day cases as well as contemporary and future applications show that addressing the urgent need for a repository of publicly accessible raw NMR data has the potential to transform natural products (NPs) and associated fields of chemical and biomedical research. The call for advancing open sharing mechanisms for raw data is intended to enhance the transparency of experimental protocols, augment the reproducibility of reported outcomes, including biological studies, become a regular component of responsible research, and thereby enrich the integrity of NP research and related fields.

Meroterpenoids isolated from Arnebia euchroma (Royle) Johnst. and their cytotoxic activity in human hepatocellular carcinoma cells

Six previously undescribed naturally occurring meroterpenoids (2, 5-9) together with seven known meroterpenoids (1, 3, 4, 10-13) were isolated from the root plant of Arnebia euchroma. Their structures and absolute configurations were determined by extensive 1D (¹H NMR, ¹³C NMR) and 2D NMR (¹H¹H COSY, DEPT, HMQC, HMBC, NOESY) spectroscopic methods, spectroscopy high resolution mass spectrometry, as well as DFT and MM2 force-field calculations. Meroterpenoids 1-13 were evaluated for their cytotoxicities against human liver cancer cell lines SMMC-7721, HepG2, QGY-7703 and HepG2/ADM. Meroterpenoid 5 exhibited the most potent activity with IC₅₀ values of 6.40 ± 0.51, 3.86 ± 0.28, 3.43 ± 0.27 and 11.31 ± 0.67 μM, respectively. Meroterpenoid 4 exhibited significant growth inhibitory effects against HepG2/ADM with IC₅₀ at 18.77 ± 1.23 μM, and meroterpenoid 8 with IC₅₀ at 5.41 ± 0.51 and 6.18 ± 0.47 μM against HepG2 and QGY-7703, respectively. These were more potent than the positive drug, Cisplatin.

Carbocyclic Amino Ketones by Bredt's Rule-Arrested Kulinkovich-de Meijere Reaction

The Ti(II) -mediated formation of cyclopropylamines from alkenes and amides, the Kulinkovich-de Meijere reaction, involves two carbon-carbon bond-forming steps. Strategic use of a tricyclic intermediate can arrest the process if the second step requires formation of a bridgehead double bond. Use of this Bredt's rule constraint results in the production of carbocyclic amino ketones, key alkaloid building blocks.

EBC-318 and 339: bicyclo[10.2.1]alkanes from Croton insularis

EBC-318 and EBC-339 were isolated from Croton insularis, an Australian rain forest plant. EBC-318 is characterised by a bridgehead double bond.

Ingenane Diterpenoids

Despite a more recent isolation and chemical characterization when compared to phorbol, along with its chemical instability, limited distribution in Nature, and scarce availability, ingenol is the only Euphorbia diterpenoid that has undergone successful pharmaceutical development, with ingenol 3-angelate (ingenol mebutate, Picato(®)) entering the pharmaceutical market in 2012 for the treatment of actinic keratosis. The phytochemical, chemical, and biological literature on members of the ingenane class of diterpenoids is reviewed from their first isolation in 1968 through 2015, highlighting unresolved issues both common to phorboids (biogenesis, relationship between molecular targets, and in vivo activity) and specific to ingenol derivatives (two-dimensional representation, in-out stereoisomerism, versatility of binding mode to PKC, and inconsistencies in the structural elucidation of some classes of derivatives). The biogenesis of ingenol is discussed in the light of the Jakupovic proposal of a dissection between the formation of the macrocyclic Euphorbia diterpenoids and the phorboids, and the clinical development of ingenol mebutate is chronicled in the light of its "reverse-pharmacology" focus.