A novel, selective, and efficient route to carotenoids and related natural products via Zr-catalyzed carboalumination and Pd- and Zn-catalyzed cross coupling

Direct synthesis of conjugated tetraenes from 1,3-enynes with 1,3-dienes

New direct access to conjugated tetraenes has been achieved. A Ru(0)-catalysed reaction of 1,3-enynes with 1,3-dienes gives 1,3,5,7-octatetraene derivatives by formal regioselective insertion of the alkynyl group of 1,3-enynes into the terminal C-H bond in 1,3-dienes. With a silyl substituent on the alkynyl side in 1,3-enynes, the reaction regioselectively proceeds to give the linear cross-dimerisation product having the silyl group at the internal position. Stoichiometric and DFT calculations support the oxidative coupling mechanism for the linear cross-dimerisation. Methyl (2E,4E,6E,8E)-10-hydroxy-2,4,6,8-decatetraenoate, a versatile polyene intermediate, is accessed by this method as a formal synthesis of biologically active compounds.

Synthesis of Polyene Bioactive Natural Products: FR252921 and Vitamin A

A formal synthesis of FR252921, a potent macrocyclic immunosuppressive agent, and a six-step synthesis of vitamin A have been demonstrated. The application of a ruthenium-catalyzed step-economic and environmentally benign strategy for the highly stereo- and chemoselective construction of valuable polyene motifs of FR252921 and vitamin A highlights the syntheses. The key features for the synthesis FR252921 include preparation of the triene moiety followed by two consecutive peptide couplings of the three fragments.

Synthesis of Conjugated Dienes in Natural Compounds

This review describes the various synthetic methods commonly used to obtain molecules possessing conjugated dienes. We focus on methods involving cross-coupling reactions using various metals such as nickel, palladium, ruthenium, cobalt, cobalt/zinc, manganese, zirconium, or iron, mainly through examples that aimed to access natural molecules or their analogues. Among the natural molecules covered in this review, we discuss the total synthesis of a phytohormone, Acid Abscisic (ABA), carried out by our team involving the development of a conjugated diene chain.

Ruthenium Catalyzed Stereo- and Chemoselective Oxidative Coupling of Vinyl Ketones: An Efficient Access to (E,E)-1,6-dioxo-2,4-dienes

A Ru-catalyzed direct oxidative coupling reaction of vinyl ketones was developed. It offers a straightforward and atom-economical protocol for synthesis of functionalized (E,E)-1,6-dioxo-2,4-diene derivatives in moderate to good yields with...

Iron-Catalyzed Vinylzincation of Terminal Alkynes

Organozinc reagents are among the most commonly used organometallic reagents in modern synthetic chemistry, and multifunctionalized organozinc reagents can be synthesized from structurally simple, readily available ones by means of alkyne carbozincation. However, this method suffers from poor tolerance for terminal alkynes, and transformation of the newly introduced organic groups is difficult, which limits its applications. Herein, we report a method for vinylzincation of terminal alkynes catalyzed by newly developed iron catalysts bearing 1,10-phenanthroline-imine ligands. This method provides efficient access to novel organozinc reagents with a diverse array of structures and functional groups from readily available vinylzinc reagents and terminal alkynes. The method features excellent functional group tolerance (tolerated functional groups include amino, amide, cyano, ester, hydroxyl, sulfonyl, acetal, phosphono, pyridyl), a good substrate scope (suitable terminal alkynes include aryl, alkenyl, and alkyl acetylenes bearing various functional groups), and high chemoselectivity, regioselectivity, and stereoselectivity. The method could significantly improve the synthetic efficiency of various important bioactive molecules, including vitamin A. Mechanistic studies indicate that the new iron-1,10-phenanthroline-imine catalysts developed in this study have an extremely crowded reaction pocket, which promotes efficient transfer of the vinyl group to the alkynes, disfavors substitution reactions between the zinc reagent and the terminal C-H bond of the alkynes, and prevents the further reactions of the products. Our findings show that iron catalysts can be superior to other metal catalysts in terms of activity, chemoselectivity, regioselectivity, and stereoselectivity when suitable ligands are used.

General, Straightforward, and Atom-Economical Synthesis of Vinyl Triflimides

Vinyl triflimides were only accessible recently and their chemistry is yet to be discovered. Herein, we describe a general, straightforward and atom-economical synthesis of these materials from alkynes and triflimide. A vast array of terminal and internal alkynes with broad spectrum of functionalities could be employed to generate various di- and trisubstituted vinyl triflimides regiospecifically with high to specific stereoselectivity. Moreover, the protocol could be conducted on gram scale using terminal and internal alkynes. Preliminarily attempts to probe the unprecedented reactivity of vinyl triflimides revealed part of its chemical properties.

Convergent Synthesis of Fluoroalkenes Using a Dual-Reactive Unit

Fluoroalkenes have shown importance as a metabolically stable isostere of amide compounds. To expedite the synthesis of diverse fluoroalkenes, we have developed a dual-reactive C2-unit, (Z)-1-boryl-1-fluoro-2-tosyloxyethene, containing nucleophilic and electrophilic moieties. Consecutive palladium-catalyzed cross-coupling reactions of this unit with aryl bromides and aryl boronic acids allow for the convergent synthesis of diverse trans-1,2-diaryl-substituted fluoroethenes in a chemoselective and stereoretentive manner.

Carboalumination of Seven-Membered-Ring trans-Alkenes

Seven-membered-ring trans-alkenes undergo rapid hydro- and carboalumination reactions in the absence of a catalyst with complete regio- and diastereoselectivity. Control experiments, including deuterium labeling, adding radical inhibitors, and using a radical clock, suggest that these reactions proceed by a concerted mechanism. The products of the reaction possess a new carbon-aluminum bond that can then undergo subsequent transformations, particularly oxidation, providing functionalized products as single stereoisomers.

Benzothiadiazole oligoene fatty acids: fluorescent dyes with large Stokes shifts

Herein, we report on the synthesis and characterization of novel fluorescent fatty acids with large Stokes shifts. Three examples consisting of the same number of carbon atoms and thus of similar chain length are presented differing in their degree of unsaturation. As major fluorogenic contributor at the terminus benzo[c][1,2,5]thiadiazole was used. Respective syntheses based on Wittig reactions followed by iodine-mediated isomerization are presented. The absorption properties are modulated by the number of conjugated C=C double bonds of the oligoene chain ranging from one to three. Large Stokes shifts of about 4900-5700 cm-1 and fluorescence quantum yields of up to 0.44 were observed.

Catalytic enantioselective synthesis of chiral organic compounds of ultra-high purity of >99% ee

Shortly after the discovery of Zr-catalyzed carboalumination of alkynes in 1978, we sought expansion of the scope of this reaction so as to develop its alkene version for catalytic asymmetric C-C bond formation, namely the ZACA (Zr-catalyzed asymmetric carboalumination of alkenes). However, this seemingly easy task proved to be quite challenging. The ZACA reaction was finally discovered in 1995 by suppressing three competitive side reactions, i.e., (i) cyclic carbometalation, (ii) β-H transfer hydrometalation, and (iii) alkene polymerization. The ZACA reaction has been used to significantly modernize and improve syntheses of various natural products including deoxypolypropionates and isoprenoids. This review focuses on our recent progress on the development of ZACA-lipase-catalyzed acetylation-transition metal-catalyzed cross-coupling processes for highly efficient and enantioselective syntheses of a wide range of chiral organic compounds with ultra-high enantiomeric purities.

An alkyne hydrosilylation–Hiyama coupling approach to highly functionalised 1,3-dienes

A high yielding and completely stereoselective hydrosilylation–Hiyama protocol has been established for the synthesis of highly functionalised E,E-dienes.

Regio- and stereoselective multisubstituted olefin synthesis via hydro/carboalumination of alkynes and subsequent iron-catalysed cross-coupling reaction with alkyl halides

A new synthetic route towards multisubstituted olefins was developed based on the direct cross coupling of alkenyl aluminium reagents, prepared by hydro- and carboalumination, with alkyl halides in the presence of an iron catalyst.

Linchpin dienes: key building-blocks in the synthesis of polyenic frameworks

This review focuses on the preparation of dienic linchpin reagents and on their use in the synthesis of polyenic frameworks.

Fluorescent penta- and hexaene fatty acids by a Wittig-Horner/elimination strategy

Molecular fluorescent probes have revolutionized biochemical and biophysical studies in the last decades, but with regard to lipids there has been a lack of combining the slim shape of saturated acyl chains with fluorescent properties. Our strategy to pentaene and hexaene fatty acids builds upon commercially available 4-(E)-decenal, which is subjected to a Wittig-Horner reaction after chlorination in α-position. DBU-mediated β-elimination of HCl proceeding the olefination establishes a highly conjugated system to which a salt-free Wittig reaction adds a final double bond leading to a good (Z)-selectivity of 83-86%. The double bond geometry can be optionally isomerized with I(2) to furnish the all-(E)-species. The five conjugated alkene moieties result in a longest-wavelength absorption maximum of about 350 nm. A red-shift to 380 nm was realized by addition of another double bond employing a common Wittig-Horner prolongation sequence. Stokes shifts of about 7300 and 7800 cm(-1), respectively, were observed.

(+)-Sorangicin A: evolution of a viable synthetic strategy

An effective, asymmetric total synthesis of the antibiotic (+)-sorangicin A (1) has been achieved. Central to this venture was the development of first and second generation syntheses of the signature dioxabicyclo[3.2.1]octane core, the first featuring chemo- and stereoselective epoxide ring openings facilitated by a Co(2)(CO)(6)-alkyne complex, the second involving a KHMDS-promoted epoxide ring formation/opening cascade. Additional highlights include effective construction of the dihydro- and tetrahydropyran ring systems, respectively via a stereoselective conjugate addition/α-oxygenation protocol and a thioketalization/hydrostannane reduction sequence. Late-stage achievements entailed two Julia-Kociénski olefinations to unite three advanced fragments with high E-stereoselectivity, followed by a modified Stille protocol to introduce the Z,Z,E trienoate moiety, thereby completing the carbon skeleton. Mukaiyama macrolactonization, followed by carefully orchestrated Lewis and protic acid-promoted deprotections that suppressed isomerization and/or destruction of the sensitive (Z,Z,E)-trienoate linkage completed the first, and to date only, total synthesis of (+)-sorangicin A (1).

Convenient one-pot synthesis of (E)-beta-aryl vinyl halides from benzyl bromides and dihalomethanes

(E)-beta-aryl vinyl iodides are synthesized in high yield with excellent stereoselectivity from benzyl bromides by a one-pot homologation/stereoselective elimination procedure. Convenient conditions involving the anion of diiodomethane and an excess of base provide complete consumption of the benzyl bromide and elimination from a diiodoalkane intermediate. Similar conditions provide (E)-beta-aryl vinyl chlorides and bromides by employing the anions of ICH(2)Cl or CH(2)Br(2). The functional group tolerance and facile purification allows rapid access to a wide range of functionalized vinyl halides.

Alkyne elementometalation-Pd-catalyzed cross-coupling. Toward synthesis of all conceivable types of acyclic alkenes in high yields, efficiently, selectively, economically, and safely: "green" way

Palladium-catalyzed cross-coupling reactions, especially those involving Zn, Al, Zr (Negishi coupling), and B (Suzuki coupling), collectively have brought about "revolutionary" changes in organic synthesis. Thus, two regio- and stereodefined carbon groups generated as R(1)M (M = Zn, Al, B, Cu, Zr, etc.) and R(2)X (X = I, Br, OTs, etc.) may now be cross-coupled to give R(1)-R(2) with essentially full retention of all structural features. For alkene syntheses, alkyne elementometalation reactions including hydrometalation (B, Al, Zr, etc.), carbometalation (Cu, Al-Zr, etc.), and haloboration (BX(3) where X is Cl, Br, and I) have proven to be critically important. Some representative examples of highly efficient and selective (>or=98%) syntheses of di-, tri-, and oligoenes containing regio- and stereodefined di- and trisubstituted alkenes of all conceivable types will be discussed with emphasis on those of natural products. Some interesting but undesirable cases involving loss of the initial structural identities of the alkenyl groups are attributable to the formation of allylpalladium species, which must be either tamed or avoided. Some such examples involving the synthesis of 1,3-, 1,4-, and 1,5-dienes will also be discussed.

Recent advances in efficient and selective synthesis of di-, tri-, and tetrasubstituted alkenes via Pd-catalyzed alkenylation-carbonyl olefination synergy

Although generally considered competitive, the alkenylation and carbonyl olefination routes to alkenes are also complementary. In this Account, we focus on these approaches for the synthesis of regio- and stereodefined di- and trisubstituted alkenes and a few examples of tetrasubstituted alkenes. We also discuss the subset of regio- and stereodefined dienes and oligoenes that are conjugated. Pd-catalyzed cross-coupling using alkenyl metals containing Zn, Al, Zr, and B (Negishi coupling and Suzuki coupling) or alkenyl halides and related alkenyl electrophiles provides a method of alkenylation with the widest applicability and predictability, with high stereo- and regioselectivity. The requisite alkenyl metals or alkenyl electrophiles are most commonly prepared through highly selective alkyne addition reactions including (i) conventional polar additions, (ii) hydrometalation, (iii) carbometalation, (iv) halometalation, and (v) other heteroatom-metal additions. Although much more limited in applicability, the Heck alkenylation offers an operationally simpler, viable alternative when it is highly selective and satisfactory. A wide variety of carbonyl olefination reactions, especially the Wittig olefination and its modifications represented by the E-selective HWE olefination and the Z-selective Still-Gennari olefination, collectively offer the major alternative to the Pd-catalyzed alkenylation. However, the carbonyl olefination method fundamentally suffers from more limited stereochemical options and generally lower stereoselectivity levels than the Pd-catalyzed alkenylation. In a number of cases, however, very high (>98%) stereoselectivity levels have been attained in the syntheses of both E and Z isomers. The complementarity of the alkenylation and carbonyl olefination routes provide synthetic chemists with valuable options. While the alkenylation involves formation of a C-C single bond to a CC bond, the carbonyl olefination converts a CO bond to a CC bond. When a precursor to the desired alkene is readily available as an aldehyde, the carbonyl olefination is generally the more convenient of the two. This is a particularly important factor in many cases where the desired alkene contains an allylic asymmetric carbon center, since alpha-chiral aldehydes can be prepared by a variety of known asymmetric methods and readily converted to allylically chiral alkenes via carbonyl olefination. On the other hand, a homoallylically carbon-branched asymmetric center can be readily installed by either Pd-catalyzed isoalkyl-alkenyl coupling or Zr-catalyzed asymmetric carboalumination (ZACA reaction) of 1,4-dienes. In short, it takes all kinds to make alkenes, just as it takes all kinds to make the world.

Total Synthesis of Peridinin and Related C37-Norcarotenoid Butenolides

As an extension of our synthesis of symmetrical carotenoids, the preparation of the highly functionalized C37-norcarotenoid butenolide peridinin (1), its 6'-epi- and 11'Z stereoisomers has been completed. Featuring a central dihalogenated C8 linchpin unit 6, two synthetic routes, differing in the ordering of the last three steps were explored by using the C3,C3'-bisdehydroxylated target as the model system. The first route uses the combination of a modified Z-selective Julia reaction and two sequential Stille couplings, the last one producing the isomerisation of the polyene Z double bond. The second route inverts these steps and makes the isolation of the 11'Z stereoisomers as major products possible. An efficient Z to E isomerisation of the final carotenoid skeleton simply uses the Stille reaction conditions at ambient temperature. As the reaction of bromoallene 12 with alkenylstannane 11 occurs with inversion of configuration, 6'-epi-peridinin could also be prepared by route A. The advantages and limitations of the sequential Stille cross-coupling approach to carotenoids are highlighted.

Palladium-catalyzed cross-coupling reactions in total synthesis

In studying the evolution of organic chemistry and grasping its essence, one comes quickly to the conclusion that no other type of reaction plays as large a role in shaping this domain of science than carbon-carbon bond-forming reactions. The Grignard, Diels-Alder, and Wittig reactions are but three prominent examples of such processes, and are among those which have undeniably exercised decisive roles in the last century in the emergence of chemical synthesis as we know it today. In the last quarter of the 20th century, a new family of carbon-carbon bond-forming reactions based on transition-metal catalysts evolved as powerful tools in synthesis. Among them, the palladium-catalyzed cross-coupling reactions are the most prominent. In this Review, highlights of a number of selected syntheses are discussed. The examples chosen demonstrate the enormous power of these processes in the art of total synthesis and underscore their future potential in chemical synthesis.

The Stille Reaction in the Synthesis of the C37-Norcarotenoid Butenolide Pyrrhoxanthin. Scope and Limitations

The sequential Stille cross-coupling reactions of the dihalogenated gamma-alkylidenebutenolide 7 with stannanes 9 and 6 afforded the carbon skeleton of pyrrhoxanthin, a highly functionalized C7'-C8' acetylenic C37-norcarotenoid butenolide. Although the first halogen-selective Stille coupling takes place in 90% yield at ambient temperature, double isomerization of the Z,E- to the E,Z-C7'-C10' enyne, likely induced by the catalyst, accompanyied the bond formation, leading to 9'Z-20 and, ultimately, to 9'Z-pyrrhoxanthin 9'Z-1.

Palladium-Catalyzed Cross-Coupling Reactions of 2-Iodo-4-(phenylchalcogenyl)-1-butenes

[reaction: see text] 2-Iodo-4-(phenylchalcogenyl)-1-butenes 2 or 3, which are derived from the ring-opening reaction of methylenecyclopropanes (MCPs) 1 by iodine, can be applied to some palladium-catalyzed cross-coupling reactions such as the Sonogashira, Heck, Kumada, Suzuki, and Negishi reactions under mild conditions to give the corresponding coupling products in good yields. These reactions proceeded smoothly at room temperature (20 degrees C) in most cases without any phosphine ligand and additive. The phenylchalcogenyl group plays an important role in the reactions and a plausible reaction mechanism has been proposed.

The Stille Reaction in the Synthesis of Carotenoid Butenolides: Synthesis of 6‘-epi-Peridinin

A new strategy for carotenoid butenolides has been developed that is based in part in halogen-selective Stille cross-coupling of dihalogenated ylidenebutenolide segment 2 and highly functionalized alkenylstannanes. [reaction: see text]

A quarter of a century of explorations in organozirconium chemistry

Systematic explorations of organozirconium chemistry over the past quarter of a century have led to the discoveries and development as well as structural and mechanistic clarifications of novel Zr-catalyzed and -promoted carbon-carbon bond-forming reactions including (i) Ni- or Pd-catalyzed cross-coupling reaction of organozirconiums, (ii) Zr-catalyzed carboalumination of alkynes, (iii) Zr-catalyzed asymmetric carboalumination of alkenes, (iv) generation and carbometallative ring expansion of zirconacyclopropanes and zirconacyclopropenes and a myriad of their transformations and (v) various organozirconium migratory insertion reactions.

Stereocontrolled Total Synthesis of a Polyfunctional Carotenoid, Peridinin

Peridinin, which was isolated from the planktonic algae dinoflagellates causing red tides, is a highly oxidized carotenoid containing an allene and a characteristic (Z)-gamma-ylidenebutenolide function in the main conjugated polyene chain in addition to functionalized cyclohexane rings at both ends of the molecule. We achieved a stereocontrolled total synthesis of peridinin by featuring the Sharpless asymmetric epoxidation under precise reaction conditions, Wittig reaction with silylfuranmethylide followed by photosensitized oxygenation, stereocontrolled Pd-catalyzed one-pot (Z)-gamma-ylidenebutenolide synthesis, and modified Julia-Kocienski olefination. This synthesis is the first example of controlling the stereochemistry of polyfunctional allenic carotenoids.

Efficient and Selective Synthesis of 6,7-Dehydrostipiamide via Zr-Catalyzed Asymmetric Carboalumination and Pd-Catalyzed Cross-Coupling of Organozincs

[structure: see text] 6,7-Dehydrostipiamide has been synthesized in 23% yield in 15 steps in the longest linear sequence through the application of the Zr-catalyzed asymmetric carboalumination and the Pd-catalyzed organozinc cross-coupling in addition to the Brown crotylboration, the Corey-Peterson olefination, and the Corey-Fuchs reaction for carbon-carbon bond formation.