Total Synthesis of the Marine Diterpenoid Blumiolide C

Total Synthesis of Isoxeniolide A

Isoxeniolide A is a highly strained xenicane diterpenoid of marine origin. This natural product is representative for a subfamily of xenicanes incorporating an allylic hydroxy group in the nine-membered ring; members of this xenicane subfamily so far have not been targeted by total synthesis. Herein, we describe the first asymmetric total synthesis of isoxeniolide A. Key to forming the challenging E-configured cyclononene ring was a diastereoselective intramolecular Nozaki-Hiyama-Kishi reaction. Other important transformations include an enzymatic desymmetrization for absolute stereocontrol, a diastereoselective cuprate addition and the use of a bifunctional vinyl silane building block. Our strategy also permits access to the enantiomer of the natural product and holds potential to access a multitude of xenicane natural products and analogs for structure-activity relationship studies.

Synthesis of Waixenicin A: Exploring Strategies for Nine-Membered Ring Formation

We present a comprehensive account on our efforts behind the recently published synthesis of waixenicin A. Our approach for constructing the dihydropyran ring relied on an Achmatowicz rearrangement. For the assembly of the nine-membered ring, four distinct strategies were investigated. Our initial attempts using radical-based addition/fragmentation reactions targeting the C7-C11 bond proved unsuitable for accessing the 6/9-bicycle. By employing anionic fragmentation conditions at the furfuryl alcohol stage, we successfully reached a 5/9-bicycle. However, subsequent ring-expansion was unsuccessful. Alternative approaches, such as Nozaki-Hiyama-Kishi or Heck reactions to connect the C6-C7 bond, also encountered difficulties, with no nine-membered ring formation observed. Our first breakthrough came from our attempts to install the C5-C6 bond via an intramolecular alkylation. Surprisingly, subsequent functional group modifications proved unexpectedly challenging, necessitating a redesign of our synthetic route. Drawing from all our investigations, we concluded that construction of the C9-C10 bond would enable efficient nine-membered ring alkylation and would facilitate the installation of the desired substitution pattern along the southern periphery. Exploration of this strategy yielded further surprises but ultimately led to the successful synthesis of waixenicin A and 9-deacetoxy-14,15-deepoxyxeniculin. For the latter compound, a bioinspired one-step rearrangement to xeniafauranol A was achieved.

Total Syntheses of (+)-Waixenicin A, (+)-9-Deacetoxy-14,15-deepoxyxeniculin, and (-)-Xeniafaraunol A

The first asymmetric total synthesis of the Xenia diterpenoid waixenicin A, a potent and highly selective TRPM7 inhibitor, is reported. The characteristic trans-fused oxabicyclo[7.4.0]tridecane ring system was constructed via a diastereoselective conjugate addition/trapping sequence, followed by an intramolecular alkylation to forge the 9-membered ring. While a β-keto sulfone motif enabled efficient ring-closure, the subsequent radical desulfonylation suffered from (E)/(Z)-isomerization of the C7/C8-alkene. Conducting the sequence with a trimethylsilylethyl ester allowed for a fluoride-mediated decarboxylation that proceeded without detectable isomerization. The acid-labile enol acetal of the delicate dihydropyran core was introduced at an early stage and temporarily deactivated by a triflate function. The latter was critical for the introduction of the side chain. Diverting from a common late-stage intermediate provided access to waixenicin A and 9-deacetoxy-14,15-deepoxyxeniculin. A high-yielding base-mediated dihydropyran-cyclohexene rearrangement of 9-deacetoxy-14,15-deepoxyxeniculin led to xeniafaraunol A in one step.

Cytotoxic analogues of marine diterpenoid plumisclerin A by shifting the lipophilic branch on the characteristic tricyclic core

Plumisclerin A is one of the most complex cytotoxic xenicane diterpenes from marine sources, featuring a unique congested and rigid tricyclo[4.3.1.0^1,5]decane core and a lipophilic acyl chain. This work explored a number of new analogues of plumisclerin A through modifying the characteristic tricyclo[4.3.1.0^1,5]decane core with lipophilic chains starting from a common lactone intermediate. Bioactivity examination of all the synthetic analogues shows that new analogues 2a, 18 and 21 exhibited comparable inhibitory potencies to that of the natural product against the proliferation of cancer cells. Structural comparison of these bioactive natural and unnatural compounds reveals that the location of lipophilic substituent(s) on the tricyclo[4.3.1.0^1,5]decane core is spatially flexible, and this work thus offers a new channel to diverse bioactive analogues of plumisclerin A.

Enantioselective anti- and syn-(Borylmethyl)allylation of Aldehydes via Brønsted Acid Catalysis

The enantioselective anti- and syn-(borylmethyl)allylation of aldehydes via phosphoric acid catalysis is reported. Both (E)- and (Z)-γ-borylmethyl allylboronate reagents were prepared via the Cu-catalyzed highly stereoselective protoboration of 1,3-dienylboronate. Chiral phosphoric acid-catalyzed aldehyde allylation with either the (E)- or (Z)-allylboron reagent provided 1,2-anti- or 1,2-syn-adducts in good yields with high enantioselectivities. The application to the synthesis of morinol D was accomplished.

Role of symbiosis in the discovery of novel antibiotics

Antibiotic resistance has been an ongoing challenge that has emerged almost immediately after the initial discovery of antibiotics and requires the development of innovative new antibiotics and antibiotic combinations that can effectively mitigate the development of resistance. More than 35,000 people die each year from antibiotic resistant infections in just the United States. This signifies the importance of identifying other alternatives to antibiotics for which resistance has developed. Virtually, all currently used antibiotics can trace their genesis to soil derived bacteria and fungi. The bacteria and fungi involved in symbiosis is an area that still remains widely unexplored for the discovery and development of new antibiotics. This brief review focuses on the challenges and opportunities in the application of symbiotic microbes and also provides an interesting platform that links natural product chemistry with evolutionary biology and ecology.

9-Membered Carbocycles: Strategies and Tactics for their Synthesis

Many natural products comprising a nine-membered carbocyclic core structure exhibit interesting biological effects. However, only a minority have succumbed to their synthesis in the past. The synthesis of functionalized nine-membered carbocycles still remains a challenging goal for synthetic chemists, mainly due to their high ring strain. Different strategies to overcome the unfavorable enthalpic and entropic factors associated with their formation are highlighted in this Concept article. The presented methods are classified into two different categories: (1) the ring-expansion of smaller rings or the ring-contraction of larger rings and (2) the direct cyclization of acyclic precursors.

Synthesis of Xenia diterpenoids and related metabolites isolated from marine organisms

This review describes strategies for the chemical synthesis of xenicane diterpenoids and structurally related metabolites. Selected members from the four different subclasses of the Xenia diterpenoid family, the xenicins, xeniolides, xeniaphyllanes and xeniaethers, are presented. The synthetic strategies are discussed with an emphasis on the individual key reactions for the construction of the uncommon nine-membered carbocycle which is the characteristic structural feature of these natural products. Additionally, the putative biosynthetic pathway of xenicanes is illustrated.

A two-step, stereoselective synthesis of nine- and ten-membered carbocycles from phthalates

A two-step, stereoselective procedure for the synthesis of nine- and ten-membered carbocycles from readily available phthalates is described. A variety of dialkyl phthalates have been transformed into [6,n]-fused bicyclo systems (n = 5, 6, 7) by a dearomatization/cyclization process and then converted into cyclonona- and cyclodecadienes through a bond cleavage reaction, whereby both processes are promoted by alkaline metals in THF.

9-Membered carbocycle formation: development of distinct Friedel-Crafts cyclizations and application to a scalable total synthesis of (±)-caraphenol A

Explorations into a series of different approaches for 9-membered carbocycle formation have afforded the first reported example of a 9-exo-dig ring closure via a Au(III)-promoted reaction between an alkyne and an aryl ring as well as several additional, unique Friedel-Crafts-type cyclizations. Analyses of the factors leading to the success of these transformations are provided, with the application of one of the developed 9-membered ring closures affording an efficient and scalable synthesis of the bioactive resveratrol trimer caraphenol A. That synthesis proceeded with an average yield of 89% per step (7.8% overall yield) and has provided access to more than 600 mg of the target molecule.

Enantio- and diastereoselective synthesis of syn-β-hydroxy-α-vinyl carboxylic esters via reductive aldol reactions of ethyl allenecarboxylate with 10-TMS-9-Borabicyclo[3.3.2]decane and DFT analysis of the hydroboration pathway

An enantio- and diastereoselective synthesis of syn-β-hydroxy-α-vinyl carboxylate esters 3 via the reductive aldol reaction of ethyl allenecarboxylate (2) with 10-trimethylsilyl-9-borabicyclo[3.3.2]decane (1R) has been developed. Density functional theory calculations suggest that the allene hydroboration involves the 1,4-reduction of 2 with the 1R, leading directly to dienolborinate Z-(O)-8a.

Progress in Microwave-Aided Chemical Synthesis

The continuing use of microwave (µwave) energy in chemical synthesis has been impressive over the past decade, with many reports incorporating µwave-based reactions. Two of the major benefits of using µwave heating are the remarkable decrease in reaction times and often high yield of products in comparison with classical heating, an ideal technology for synthetic chemists. Herein, we highlight some exciting examples of its recent utility in organic, medicinal, and natural product synthetic endeavours.