Olefinic-ester cyclizations using Takai–Utimoto reduced titanium alkylidenes

Carbonyl-Olefin Metathesis

This Review describes the development of strategies for carbonyl-olefin metathesis reactions relying on stepwise, stoichiometric, or catalytic approaches. A comprehensive overview of currently available methods is provided starting with Paternò-Büchi cycloadditions between carbonyls and alkenes, followed by fragmentation of the resulting oxetanes, metal alkylidene-mediated strategies, [3 + 2]-cycloaddition approaches with strained hydrazines as organocatalysts, Lewis acid-mediated and Lewis acid-catalyzed strategies relying on the formation of intermediate oxetanes, and protocols based on initial carbon-carbon bond formation between carbonyls and alkenes and subsequent Grob-fragmentations. The Review concludes with an overview of applications of these currently available methods for carbonyl-olefin metathesis in complex molecule synthesis. Over the past eight years, the field of carbonyl-olefin metathesis has grown significantly and expanded from stoichiometric reaction protocols to efficient catalytic strategies for ring-closing, ring-opening, and cross carbonyl-olefin metathesis. The aim of this Review is to capture the status quo of the field and is expected to contribute to further advancements in carbonyl-olefin metathesis in the coming years.

Brønsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Reactions: Interrupted Metathesis vs Carbonyl-Ene Reaction

Lewis acid catalysts have been shown to promote carbonyl-olefin metathesis through a critical four-membered-ring oxetane intermediate. Recently, Brønsted-acid catalysis of related substrates was similarly proposed to result in a transient oxetane, which fragments within a single elementary step via a postulated oxygen-atom transfer mechanism. Herein, careful quantum chemical investigations show that Brønsted acid (triflic acid, TfOH) instead invokes a mechanistic switch to a carbonyl-ene reaction, and oxygen-atom transfer is uncompetitive. TfOH's conjugate base is also found to rearrange H atoms and allow isomerization of the carbocations that appear after the carbonyl-ene reaction. The mechanism explains available experimental information, including the skipped diene species that appear transiently before product formation. The present study clarifies the mechanism for activation of intramolecular carbonyl-olefin substrates by Brønsted acids and provides important insights that will help develop this exciting class of catalysts.

Structural elucidation of a methylenation reagent of esters: synthesis and reactivity of a dinuclear titanium(iii) methylene complex

Transmetallation of a zinc methylene complex [ZnI(tmeda)]₂(μ-CH₂) with a titanium(iii) chloride [TiCl₃(tmeda)(thf)] produced a titanium methylene complex. The X-ray diffraction study displayed a dinuclear methylene structure [TiCl(tmeda)]₂(μ-CH₂)(μ-Cl)₂. Treatment of an ester with the titanium methylene complex resulted in methylenation of the ester carbonyl to form a vinyl ether. The titanium methylene complex also reacted with a terminal olefin, resulting in olefin-metathesis and olefin-homologation. Cyclopropanation by methylene transfer from the titanium methylene proceeded by use of a 1,3-diene. The mechanistic study of the cyclopropanation reaction by the density functional theory calculations was also reported.

Transmetallation of a zinc methylene complex [ZnI(tmeda)]₂(μ-CH₂) with a titanium(iii) chloride [TiCl₃(tmeda)(thf)] produced a titanium methylene complex.

Tropylium-promoted carbonyl-olefin metathesis reactions

The non-benzenoid aromatic tropylium ion acts as an efficient promoter for carbonyl–olefin metathesis reactions.

The carbonyl–olefin metathesis (COM) reaction is a highly valuable chemical transformation in a broad range of applications. However, its scope is much less explored compared to analogous olefin–olefin metathesis reactions. Herein we demonstrate the use of tropylium ion as a new effective organic Lewis acid catalyst for both intramolecular and intermolecular COM and new ring-opening metathesis reactions. This represents a significant improvement in substrate scope from recently reported developments in this field.

Synthetic efforts towards the stereoselective synthesis of NF00659B1

NF00659B₁ is a novel α-pyrone diterpenoid natural product with potent anti-colon cancer activity. A stereoselective approach to the 2,2-dimethyl oxepanol core of NF00659B₁ is described enlisting a sequence of olefinic ester ring-closing metathesis, epoxidation, and Grignard addition. This strategy paves the way to a total synthesis of NF00659B₁ for further biological studies.

Carbonyl–olefin metathesis: a key review

In organic chemistry, olefin–olefin metathesis of two unsaturated substrates for the formation of a new carbon–carbon bond has been widely explored and applied.

Recent applications in natural product synthesis of dihydrofuran and -pyran formation by ring-closing alkene metathesis

Examples from 2003 to 2015 of ring-closing alkene metathesis (RCM) in the generation of dihydro-furans or -pyrans for natural product synthesis, are reviewed.

Synthesis and biological evaluation of QRSTUVWXYZA' domains of maitotoxin

The synthesis of QRSTUVWXYZA' domains 7, 8, and 9 of the highly potent marine neurotoxin maitotoxin (1), the largest secondary metabolite isolated to date, is described. The devised synthetic strategy entailed a cascade Takai-Utimoto ester olefination/ring closing metathesis to construct ring Y, a hydroxydithioketal cyclization/methylation sequence to cast ring X, a Horner-Wadsworth-Emmons coupling of WXYZA' ketophosphonate 11 with QRSTU aldehyde 12 to form enone 10, and a reductive hydroxyketone ring closure to forge ring V. 2D NMR spectroscopic analysis and comparison of (13)C chemical shifts with those of the corresponding carbons of maitotoxin revealed close similarities supporting the originally assigned structure of this region of the natural product. Biological evaluations of various synthesized domains of maitotoxin in this and previous studies from these laboratories led to fragment structure-activity relationships regarding their ability to inhibit maitotoxin-elicited Ca(2+) influx in rat C6 glioma cells.

Maitotoxin: An Inspiration for Synthesis

Maitotoxin holds a special place in the annals of natural products chemistry as the largest and most toxic secondary metabolite known to date. Its fascinating, ladder-like, polyether molecular structure and diverse spectrum of biological activities elicited keen interest from chemists and biologists who recognized its uniqueness and potential as a probe and inspiration for research in chemistry and biology. Synthetic studies in the area benefited from methodologies and strategies that were developed as part of chemical synthesis programs directed toward the total synthesis of some of the less complex members of the polyether marine biotoxin class, of which maitotoxin is the flagship. This account focuses on progress made in the authors' laboratories in the synthesis of large maitotoxin domains with emphasis on methodology development, strategy design, and structural comparisons of the synthesized molecules with the corresponding regions of the natural product. The article concludes with an overview of maitotoxin's biological profile and future perspectives.

Total synthesis of brevenal

This Article describes the total synthesis of the marine ladder toxin brevenal utilizing a convergent synthetic strategy. Critical to the success of this work was the use of olefinic-ester cyclization reactions and the utilization of glycal epoxides as precursors to C-C and C-H bonds.

Synthesis of the WXYZA' domain of maitotoxin

A synthesis of the WXYZA' domain (7) of the marine neurotoxin maitotoxin (1) is reported. The convergent synthetic strategy involves construction of key building blocks 11 and 12, their coupling, and the elaboration of the resulting ester (10) to the target molecule through a ring-closing metathesis and a hydroxy dithioketal cyclization as the key steps. For the construction of fragment 11, the Noyori reduction/Achmatowicz rearrangement and hydroxy epoxide opening technologies were applied (starting from furfuryl alcohol (13)), whereas for the synthesis of fragment 12, a carbohydrate-based approach was adopted (starting from 2-deoxy-D-ribose (14)). The synthesized WXYZA' domain (7) of maitotoxin (1) exhibited the expected (13)C NMR chemical shifts, supporting the originally assigned structure of the corresponding region of the natural product.

Synthesis of the QRSTU domain of maitotoxin and its 85-epi- and 86-epi-diastereoisomers

A devised synthetic strategy toward the QRSTU ring system 4 of the marine-derived biotoxin maitotoxin (1) delivered, in addition to 4, its diastereoisomers 85-epi-QRSTU and 86-epi-QRSTU ring systems 5 and 6. The convergent route to these maitotoxin fragments involved coupling of UT and Q building blocks 9 (obtained from 2-deoxy-D-ribose) and 10 (obtained from D-ribose) followed by ring-closing metathesis to afford enol ether 8, whose elaboration to the targeted QRSTU ring system 4 required its conversion to hydroxy ketone 7. The latter compound (7) was transformed to the final product through a hydroxy dithioketal cyclization, followed by oxidation/methylation of the resulting O,S-mixed ketal to install the last of the five methyl groups contained within the target molecule (4). (13)C NMR spectroscopic analysis of synthesized fragments 4, 5, and 6 and comparisons with maitotoxin provided strong support for the originally assigned structure of the QRSTU domain of the natural product.

Two-directional olefinic-ester ring-closing metathesis using reduced Ti alkylidenes. A rapid entry into polycyclic ether skeletons

The use of a reduced titanium ethylidene reagent in an efficient two-directional approach to polycyclic ether skeletons is described.

Synthesis of an A-E gambieric acid subunit with use of a C-glycoside centered strategy

This paper describes our synthesis of the A-E subunit of gambieric acid (GA) in addition to the synthesis of the A-ring and the C-E tricycle. The use of an enol ether-olefin RCM strategy to couple the A and C-E subunits and, in the process, generate the B-ring is noteworthy.

Sequential N-Acylamide Methylenation−Enamide Ring-Closing Metathesis: Construction of Benzo-Fused Nitrogen Heterocycles

The dimethyltitanocene methylenation of N-acylamides derived from ortho-vinylanilines, ortho-allylaniline, and ortho-vinylbenzylamine provides the corresponding enamides, which upon exposure to the second generation Grubbs ruthenium catalyst give access to indoles, 1,4-dihydroquinolines, and 1,2-dihydroisoquinolines, respectively. This sequential protocol also allows the synthesis of dihydrobenzoazepines, although the ring-closing metathesis (RCM) step is complicated by the alkene isomerization processes. From certain substrates, the direct annulation is observed in the titanium-mediated step, which is likely to occur through an olefin metathesis-intramolecular olefination sequence.

Total Synthesis of Gambierol: Subunit Coupling and Completion

The preceding manuscript detailed our synthesis of the gambierol A-C and F-H ring precursors. Reported herein is a description of the coupling of the two precursors and the conversion of the coupled material into gambierol. Coupling of the subunits involved ester formation, enol ether RCM, and mixed thioketal formation and reduction. By employing this strategy we were able to bring highly advanced subunits into the coupling and, as a result, we were able to minimize the number of post-coupling transformations required to complete gambierol. At the completion of the synthesis, we generated 7.5 mg (1.5 % overall yield) of (-)-gambierol in 44 steps (longest linear sequence).

Total Synthesis of Gambierol: The Generation of the A–C and F–H Subunits by Using a C-Glycoside Centered Strategy

Gambierol, a representative of the marine ladder toxin family, consists of eight ether rings, 18 stereocenters, and two challenging pyranyl rings having methyl groups that are in a 1,3-diaxial orientation to one another. Herein we describe the generation of gambierol's A-C and F-H ring systems and demonstrate the versatility of the glycosyl anhydride, enol ether-olefin RCM strategy to fused polycyclic ethers. This work has both enabled us to generate sufficient quantities of the gambierol precursors and has enabled us to better understand the chemical transformations that were key to these efforts. Fundamental work included efforts to C-glycosides and C-ketosides, Claisen rearrangements, and enol ether-olefin RCM reactions.