Synthesis of Fijiolide A via an Atropselective Paracyclophane Formation

1,3-Butadiynyl sulfide-based compact trialkyne platform molecule for sequential assembly of three azides

A compact trialkyne platform with a silyl-protected 1,3-butadiynyl sulfide moiety and a terminal alkyne group has been developed for sequential regioselective transition metal-catalyzed triazole formation reactions with three azides. This method enabled the facile construction of a low-molecular-weight triazole library and the synthesis of middle-molecular-weight trifunctional probes for protein modification.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Recursive Anion-Triggered Tandem Reactions of ortho-Bis-ynones: Tunable Synthesis of 1-Indenones and Cyclopenta[a]inden-8(2H)-ones

Recursive anion-mediated activation of o-bis-ynones sets off a Michael addition-aldol reaction-dehydrative rearrangement cascade, leading to the one-pot synthesis of 1-indenones via orthogonal interplay between the two ortho-ynone moieties. Repeating the recursive anion engagement with the 1-indenones unfolded access to a functionally embellished cyclopenta[a]inden-8(2H)-one core and its spiroannulated analogues either directly or stepwise through tandem 1,6-Michael-type addition-6π electrocyclization and an in situ oxidation sequence.

Enantioselective Total Synthesis of (+)-Heilonine

Chemical transformations that rapidly and efficiently construct a high level of molecular complexity in a single step are perhaps the most valuable in total synthesis. Among such transformations is the transition metal catalyzed [2 + 2 + 2] cycloisomerization reaction, which forges three new C-C bonds and one or more rings in a single synthetic operation. We report here a strategy that leverages this transformation to open de novo access to the Veratrum family of alkaloids. The highly convergent approach described herein includes (i) the enantioselective synthesis of a diyne fragment containing the steroidal A/B rings, (ii) the asymmetric synthesis of a propargyl-substituted piperidinone (F ring) unit, (iii) the high-yielding union of the above fragments, and (iv) the intramolecular [2 + 2 + 2] cycloisomerization reaction of the resulting carbon framework to construct in a single step the remaining three rings (C/D/E) of the hexacyclic cevanine skeleton. Efficient late-stage maneuvers culminated in the first total synthesis of heilonine (1), achieved in 21 steps starting from ethyl vinyl ketone.

Catalytic Cyclotrimerization Pathway for Synthesis of Selaginpulvilins C and D: Scope and Limitations

A facile and unified approach to the main selaginpulvilin's framework was achieved by catalytic [2 + 2 + 2]-cyclotrimerization of a triyne with monosubtituted alkynes. The reaction proceeded with high "ortho" selectivity by using Wilkinson's catalyst (RhCl(PPh₃)₃) under ambient conditions with reasonable yields. The scope of the reaction with respect to the alkyne as well as the catalytic system was evaluated. The formal total modular syntheses of selaginpulvilin C and D were accomplished by transformation of the cyclotrimerization's products.

Ruthenium-Catalyzed Cycloadditions to Form Five-, Six-, and Seven-Membered Rings

Ruthenium-catalyzed cycloadditions to form five-, six-, and seven-membered rings are summarized, including applications in natural product total synthesis. Content is organized by ring size and reaction type. Coverage is limited to processes that involve formation of at least one C-C bond. Processes that are stoichiometric in ruthenium or exploit ruthenium as a Lewis acid (without intervention of organometallic intermediates), ring formations that occur through dehydrogenative condensation-reduction, σ-bond activation-initiated annulations that do not result in net reduction of bond multiplicity, and photochemically promoted ruthenium-catalyzed cycloadditions are not covered.

Intermolecular cyclotrimerization of haloketoalkynes and internal alkynes: facile access to arenes and phthalides

A highly chemo- and regioselective cyclo(co)trimerization between 3-halopropiolamides and symmetrical internal alkynes is reported. The reaction is catalyzed by CpRuCl(COD) and proceeds under air at ambient temperature in ethanol with no additional precautions. Iodo-, bromo-, and chloropropiolamides, esters, and ketones are viable coupling partners and, in a 2 : 1 stoichiometry relative to internal alkyne, yield fully-substituted arenes in a single step. The highest regioselectivities (96% single isomer) were observed when employing 2° and 3°-halopropiolamides. A mechanistic hypothesis accounting for this selectivity is proposed. Notably, by using 1,4-butynediol as the internal alkyne, in situ lactonization following [2+2+2]-cycloaddition generates therapeutically-relevant phthalide pharmacophores directly.

Recent Advances in Construction of Polycyclic Natural Product Scaffolds via One-Pot Reactions Involving Alkyne Annulation

Polycyclic scaffolds are omnipresent in natural products and drugs, and the synthetic strategies and methods toward construction of these scaffolds are of particular importance. Compared to simple cyclic ring systems, polycyclic scaffolds have higher structure complexity and diversity, making them suitable for charting broader chemical space, yet bringing challenges for the syntheses. In this review, we surveyed progress in the past decade on synthetic methods for polycyclic natural product scaffolds, in which the key steps are one-pot reactions involving intermolecular or intramolecular alkyne annulation. Synthetic strategies of selected polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro rings are discussed with emphasis on the synthetic efficiency and product diversity. Recent examples containing newly developed synthetic concepts or toolkits such as collective and divergent total synthesis, gold catalysis, C–H functionalization, and dearomative cyclization are highlighted. Finally, several “privileged synthetic strategies” for “privileged polycyclic scaffolds” are summarized, with discussion of remained challenges and future perspectives.

Natural product syntheses via carbonylative cyclizations

This review summarizes the application of various transition metal-catalyzed/mediated carbonylative cyclization reactions in natural product total synthesis.

Synthetic Access to Cyclopenta[a]inden-2(1H)-ones from Morita-Baylis-Hillman Products of 2-Alkynyl Benzaldehydes

A new strategy for the synthesis of cyclopenta[a]inden-2(1H)-ones has been developed. An intramolecular Pauson-Khand reaction of the Morita-Baylis-Hillman (MBH) adducts, derived from 2-alkynyl benzaldehydes, provided the consequent novel cyclopenta[a]inden-2(1H)-ones bearing multifunctionalities including an ester group at the fused ring junction (tert-carbon center). The generality of this approach was illustrated by studying the several MBH derivatives containing diverse substitutions/functionalities.

Streamlined Total Synthesis of Shishijimicin A and Its Application to the Design, Synthesis, and Biological Evaluation of Analogues thereof and Practical Syntheses of PhthNSSMe and Related Sulfenylating Reagents

Shishijimicin A is a scarce marine natural product with highly potent cytotoxicities, making it a potential payload or a lead compound for designed antibody-drug conjugates. Herein, we describe an improved total synthesis of shishijimicin A and the design, synthesis, and biological evaluation of a series of analogues. Equipped with appropriate functionalities for linker attachment, a number of these analogues exhibited extremely potent cytotoxicities for the intended purposes. The synthetic strategies and tactics developed and employed in these studies included improved preparation of previously known and new sulfenylating reagents such as PhthNSSMe and related compounds.

Stereoselective Synthesis of the Benzodihydropentalene Core of the Fijiolides

An efficient stereoselective synthesis of the enantiomer of the benzodihydropentalene core of fijiolides A and B has been achieved. The asymmetric conjugate addition of styrylboronic acid to an indenone produced the first stereocenter. Ring C was installed by ring-closing metathesis of a cis disubstituted indanone. Regioselective epoxide opening by NaSePh and subsequent oxidative elimination produced an allylic alcohol. The final introduction of the cyclopentadiene was possible by elimination of an in situ formed triflate.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1340 in 429 papers for 2015), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Recent highlights in biosynthesis research using stable isotopes

The long and successful history of isotopic labeling experiments within natural products research has both changed and deepened our understanding of biosynthesis. As demonstrated in this article, the usage of isotopes is not at all old-fashioned, but continues to give important insights into biosynthetic pathways of secondary metabolites. This review with 85 cited references is structured by separate discussions of compounds from different classes including polyketides, non-ribosomal peptides, their hybrids, terpenoids, and aromatic compounds formed via the shikimate pathway. The text does not aim at a comprehensive overview, but instead a selection of recent important examples of isotope usage within biosynthetic studies is presented, with a special emphasis on mechanistic surprises.