A synthesis of the carbon skeleton of maoecrystal V

Ambimodal Addition-Coupled Electron Transfer Mechanism in a Pb(IV)-Promoted Oxidative Dearomatization Reaction

The mechanism of the Pb(IV)-promoted phenol oxidative dearomatization reaction has been traditionally attributed to a carbocation mechanism. In 2011, Pettus reported an oxidative dearomatization reaction leading to a mixture of a formal [5 + 2] and a C-O bond formation product. By employing density functional theory and quasi-molecular dynamics calculations, it was demonstrated that the reaction does not occur through a carbocation intermediate but instead proceeds through an addition-coupled electron transfer (ACET) mechanism. Moreover, the ACET exhibits ambimodality, wherein a transition state results in 4-6 distinct outcomes through post-TS bifurcation. The reported selectivity can be effectively rationalized by a newly proposed mechanism.

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp3-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

Unprecedented diterpene skeletons isolated from vascular plants in the last twenty years (2001-2021)

Every year there are hundreds of reports about the isolation of undescribed terpenoids based on novel functionalizations of known carbocyclic skeletons series. However, on some occasions the compounds obtained have a carbocyclic skeleton that does not correspond with the series established, in these peculiar opportunities, in addition to finding an undescribed natural product, is obtained an unprecedented carbocyclic skeleton, whose biogenesis must necessarily involve other additional steps that explain its formation. This review accounts for the reports of seventy-nine unprecedented diterpene skeletons (corresponding to one-hundred-three undescribed diterpenoids) isolated from vascular plants in the last two decades. According to the genus, Euphorbia and Salvia are the most prolific in reports of unprecedented diterpene skeletons with a total of twenty, and nine skeletons, respectively. If the findings are expressed in terms of the family, Euphorbiaceae and Lamiaceae have the highest number of reports of undescribed diterpene skeletons, with twenty-seven and twenty-two, respectively. Finally, fifty-three skeletons are derived from higher diterpenoids (2-12, 68, 69, 86, 104-109, 158-161, 186, 189, 222, 250-255, 285-298, 403-404, 415, 416, and 436), twenty are derived from lower diterpenoids (135, 136, 192-194, 225-229, 363-370, 397, and 425), and six (96, 97, 147, 148, 205, and 206) are derived from skeletons whose biogenesis has not yet been established, or at least, cannot be formally included within the groups mentioned above. This article comprehensively highlights the hypothetical biosynthetic pathway for each of the one-hundred-three undescribed compounds with unprecedented diterpene skeletons and summarizes their most significant biological activities.

Studies toward synthesis of the core skeleton of spiroaspertrione A

Bioassay-guided isolation of spiroaspertrione A from cultures of Aspergillus sp. TJ23 in 2017 demonstrated potent resensitization of oxacillin against methicillin-resistant Staphylococcus aureus by lowering the oxacillin minimal inhibitory concentration up to 32-fold. To construct this unique spiro[bicyclo[3.2.2]nonane-2,1′-cyclohexane] system, a protocol for ceric ammonium nitrate-induced intramolecular cross-coupling of silyl enolate is disclosed.

Review of synthetic approaches toward maoecrystal V

Synthetic approaches toward the complex natural product diterpenoid maoecrystal V are reviewed, including successful total syntheses, published synthetic efforts, and efforts compiled from dissertations.

Brønsted Acid Catalyzed Synthesis of Functionalized 1,4- and 1,6-Dicarbonyl Monosilyl Enol Ethers under Operationally Practical Conditions

Herein, we report an improved protocol for the concise synthesis of functionalized 1,4- and 1,6-dicarbonyl-derived monosilyl enol ethers via ionization of α'-hydroxy silyl enol ethers to generate unsymmetrical silyloxyallyl cations that were subsequently captured by TBS silyl enolates. These transformations were efficiently performed in acetonitrile at room temperature by employing pyridinium triflate as a catalyst. Our new reaction conditions are operationally more practical and broaden the accessibility of various 1,4- and 1,6-dicarbonyl groups, which include diketone, ketoester, and ketothioester functionalities.

Diterpenoids from Isodon species: an update

Covering: December 2005 to June 2016. Previous review: Nat. Prod. Rep., 2006, 23, 673-698Over the last decade, great efforts have been made to conduct phytochemistry research on the genus Isodon, which have led to the isolation and identification of a number of diterpenoids. At the same time, these newly reported diterpenoids with diverse structures have led to new findings on their biological functions and chemical synthesis research. In this update, we review more than 600 new diterpenoids, including their structures, classifications, biogenetic pathways, bioactivities, and chemical synthesis.

Cycloaddition of Spiroepoxycyclohexa-2,4-dienones, Radical Cyclization and 1,3-Acyl Shift in Excited State: Aromatics to Sterpuren-4-one

A stereoselective route to sterpuren-4-one from a simple aromatic precursor is presented. Oxidative dearomatization, π⁴s + π²s cycloaddition of 6,6-spiroepoxycyclohexa-2,4-dienones with ethyl acrylate, radical cyclization and 1,3-acyl shift in excited state are the important aspects of our approach. An interesting effect of a remote substituent on radical cyclization has also been presented.

11-Step Total Synthesis of (-)-Maoecrystal V

An expedient, practical, and enantioselective route to the highly congested ent-kaurane diterpene maoecrystal V is presented. This route, which has been several years in the making, is loosely modeled after a key pinacol shift in the proposed biosynthesis. Only 11 steps, many of which are strategic in that they build key skeletal bonds and incorporate critical functionalities, are required to access (−)-maoecrystal V. Several unique and unexpected maneuvers are featured in this potentially scalable pathway. Reevaluation of the biological activity calls into question the initial exuberance surrounding this natural product.

Synthesis of Highly Functionalized Triarylbismuthines by Functional Group Manipulation and Use in Palladium- and Copper-Catalyzed Arylation Reactions

Organobismuthines are an attractive class of organometallic reagents that can be accessed from inexpensive and nontoxic bismuth salts. Triarylbismuthines are particularly interesting due to their air and moisture stability and high functional group tolerance. We report herein a detailed study on the preparation of highly functionalized triarylbismuth reagents by triple functional group manipulation and their use in palladium- and copper-catalyzed C-, N-, and O-arylation reactions.

A Failed Late-Stage Epimerization Thwarts an Approach to Ineleganolide

Significant efforts were made to complete a synthesis of the complex norcembranoid ineleganolide via a seemingly attractive strategy involving late-stage creation of the central seven-membered ring. While the two key enantioenriched building blocks were made via high-yielding sequences and their convergent union was efficient, the critical C4-C5 bond of this sterically congested natural product could never be forged. Several interesting examples of unexpected acid-base behavior and unanticipated proximity-induced reactivity accounted for most of the problems in the execution of the synthesis plan.

Efficient approach to 2-hydroxy-2,3-dihydrofuran derivatives and its application for the synthesis of novel 4-(1H-pyrazol-4-yl)pyridazines

A highly efficient method for the synthesis of 2-hydroxy-2,3-dihydrofuran derivatives from 1,4-enediones and phenacyl pyridinium halides via a domino reaction has been developed. At the same time, we also synthesize the novel 4-(1H-pyrazol-4-yl)pyridazine skeleton.

Enantioselective synthesis of (-)-maoecrystal V by enantiodetermining C-H functionalization

The evolution of a program directed at the enantioselective total synthesis of maoecrystal V, a highly modified ent-kauranoid, is described. An early stage chiral auxiliary-directed asymmetric C–H functionalization for the construction of a key benzofuran intermediate enabled the first asymmetric synthesis of the natural enantiomer of maoecrystal V, confirming the assigned stereochemistry. A divergent course of the central intramolecular Diels–Alder reaction, which is dependent on the nature of the dienophile, initially led to the development of an unanticipated and previously unknown isomer of maoecrystal V, which we named maoecrystal ZG. In light of the reported selective and potent cytotoxic activity of maoecrystal V, the cytotoxic properties of maoecrystal ZG were also investigated.

Total synthesis of maoecrystal V: early-stage C-H functionalization and lactone assembly by radical cyclization

A total synthesis of the unusual ent-kaurane maoecrystal V is described. The synthesis strategy features a counterintuitive early disconnection of the lactone subunit to a polycyclic enol ether intermediate in order to preserve the central tetrahydrofuran ring until the beginning stages of the synthesis. This strategy enables an application of C-H functionalization at the early phase of the synthesis during the construction of a dihydrobenzofuran intermediate.

Evaluation of 'east-to-west' ether-forming strategies for the total synthesis of maoecrystal V

Model systems that evaluated different approaches to construct the central ether ring of maoecrystal V are described. Our first model systems attempted the ether formation using C-H functionalization reactions, which led to interesting rearrangements but none of the desired ether product. An intramolecular conjugate addition strategy was then explored that successfully formed the targeted C-O bond but resulted in the undesired stereochemistry.