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The aim of this review is to present the latest developments in the stereoselective synthesis of conjugated dienes, covering the period 2005-2010. Since the use of this class of compounds is linked to the nature of their appendages (aryls, alkyls, electron-withdrawing, and heterosubstituted groups), the review has been categorized accordingly and illustrates the most representative strategies and mechanisms to access these targets.

Synthesis of substituted 1,3-dienes was achieved via gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers. The N-heterocyclic carbene gold chloride catalyst (IPrAuCl) was superior in terms of activity and selectivity and afforded the 3,3-product in excellent yields. A proposed cation-π inter-action played a significant role in affecting the reaction rate.

A simple and practical procedure for the synthesis of aza- and oxacycles, which possess an array of stereogenic functionalities, is described. This protocol relies on tandem Cu-catalyzed coupling of suitably functionalized terminal alkyne with diazoester followed by isomerization and subsequent aza or oxa-Michael reaction, thus generating the required scaffold with high diastereoselectivity.

The general zinc-catalyzed allenylation of aldehydes and ketones with an allenyl boronate is presented. Preliminary mechanistic studies support a kinetically controlled process wherein, after a site-selective B/Zn exchange to generate a propargyl zinc intermediate, the addition to the electrophile effectively competes with propargyl-allenyl zinc equilibration. The utility of the methodology was demonstrated by application to a rhodium-catalyzed [4+2] cycloaddition., (© 2011 American Chemical Society)

Simple and mild methods for the synthesis of allenes, employing indium- and zinc-mediated dehalogenation reactions of vicinal dihalides in an aqueous solvent, are described. By using these procedures, various allenylmethyl aryl ethers and monosubstituted allenes have been prepared in good to excellent yields.

Three novel ruthenium-catalyzed cyclizations of enynes were developed. In each cyclization, a ruthenacyclopentene derived from enyne and Cp*RuCl(cod) is a common intermediate. When an enyne having an alkyl, an ester, or a formyl group on an alkyne was reacted with Cp*RuCl(cod) under ethylene gas, ethylene was inserted into the ruthenium-sp(2) carbon bond of ruthenacyclopentene to afford ruthenacycloheptene, and β-hydrogen elimination followed by reductive elimination occurred to give a cyclic compound having a 1,3-diene moiety. When an acyl group was placed on the alkyne, the carbonyl oxygen coordinated to the ruthenium metal of ruthenacyclopentene to produce a ruthenium carbene complex, which reacted with ethylene to give a cyclic compound having a cyclopropane ring on the substituent. On the other hand, when the substituent on the alkyne was pent-4-enyl, insertion of an alkene part into ruthenacyclopentene followed by reductive elimination gave a tricyclic compound by a ruthenium-catalyzed [2+2+2] cyclization of diene and an alkyne., (Copyright © 2011 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.)

Background: Curcumin is known to possess many anti-tumor properties such as inhibition of tumor growth and induction of apotosis. However, limited bioavailability of curcumin prevents its clinical application. A synthesized curcumin analog, 1,5-diaryl-3-oxo-1,4-pentadiene such as GO-Y030, has the improved anti-tumor potential in vitro as well as in mouse model of colorectal carcinogenesis., Results: These compounds were divided into two groups; one is the higher anti-proliferative group, in which 79.7% of 1,5-diaryl-3-oxo-1,4-pentadienes were clustered. One of the 1,5-diaryl-3-oxo-1,4-pentadiene analogs, GO-Y078 has the most enhanced growth inhibition, and its solubility was improved, compared with curcumin. GO-Y078 inhibits NF-κB transactivation, as well as expression of TP53 and DR5 more effectively than curcumin. In a mouse model, GO-Y078 presented 1.4 fold more survival elongation that was not achieved by curcumin and GO-Y030., Conclusions: The 1,5-diaryl-3-oxo-1,4-pentadiene analogs can yield good lead compounds for cancer chemotherapy, to overcome low bioavailability of curcumin.

A ruthenium-catalyzed hydrovinylation-type cross-coupling of ynamides and ethylene proceeds via ruthenacyclopentene to give 2-aminobuta-1,3-diene derivatives in a highly regioselective manner. It was also demonstrated that 2-aminobuta-1,3-diene derivatives reacted with various dienophiles or singlet oxygen to give a cyclic enamide derivative.

A Cu-catalyzed method for coupling α-substituted-α-diazoesters with terminal alkynes to give substituted allenoates is described. Key to the development of a selective method was the recognition that an adventitous base catalyzes the isomerization to form the allenoate product. A plausible mechanism is proposed, based in part on evidence against a mechanism that involves a Cu(I)-acetylide as a low-valent intermediate.

A series of α,β-divinyl tetrahydropyrroles, synthesized by asymmetric allylic C-H bond activation/conjugated diene addition reaction of ene-2-dienes, were found to be very efficient chiral chain diene ligands in the rhodium-catalyzed conjugated addition of organoboronic acids to various α,β-unsaturated compounds, achieving the desired chiral adducts with good to excellent yields and ee values.

1,4-Diiodo-1,3-dienes are unique reagents in organic synthesis and have been employed in several well-known and recently developed areas of application. Furthermore, these dienes are easily accessible, starting from the alkynes and iodine, and they have demonstrated high reactivity in cross-coupling reactions, organometallic synthesis, in the preparation of heterocyclic compounds, and several other transformations. The high reactivity of the 1,4-diiodo-1,3-dienes allows for the development of synthetic procedures that use mild conditions (room temperature). The key advantages in assembling complex organic molecules, natural products, and compounds for material science using 1,4-diiodo-1,3-dienes as building blocks include high yields, excellent selectivity, and diverse reactivity in carbon-carbon and carbon-heteroatom bond formation. This Focus Review describes the scope and application of the 1,4-diiodo-1,3-dienes in organic synthesis as well as summarizes the methods for preparation of the dienes., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A mild and direct pathway for the formation of five-membered heterocyclic compounds from hydroxylated enynes has been developed. In this reaction, hydroxylated enynes were selectively transformed into five-membered heterocyclic compounds 2, with an allene moiety at the 3-position, in the presence of F(3) CSO(3) H (0.1 mol %). When R(1) , R(2) =Ph, diphenylvinyl-2,3-dihydro-1H-pyrrole (2 y) was obtained. With HSbF(6) (5 mol %) as the catalyst, polycyclic skeletons 3 and 4 with adjacent stereocenters were obtained. When R(1) =H and R(2) =styrene, 1,3-dienyl-2,5-dihydro-1H-pyrrole (6 as) was formed. This Brønsted acid catalyzed domino process involves the formation of an allene carbocation intermediate, which can be readily trapped by olefins to give various novel five-membered heterocyclic skeletons., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Phosphine-mediated olefination between alpha-substituted allenoates and aldehydes to form 1,2,3,4-tetrasubstituted 1,3-dienes is presented. High levels of chemo- and diastereoselectivity and yield are obtained for a wide scope of substrates with the choice of appropriate phosphines. This reaction evidences the capacity of phosphines in the control of reaction pathways and provides a highly efficient synthetic method for tetrasubstituted conjugated dienes.

Under ruthenium-catalyzed transfer hydrogenation conditions, direct C-C coupling of ethanol and 2-substituted dienes occurs to furnish products of hydro-hydroxyethylation: anti-configured neopentyl homoallylic alcohols. Identical adducts are generated from acetaldehyde under related conditions employing isopropanol as reductant.

The formal syntheses of (+)-prelaureatin (1) and (+)-laurallene (2), halogenated eight-membered-ring ethers, are described. The key step of our strategy relies on diastereoselective construction of a trans-alpha,alpha'-disubstituted oxocene structure through a Brook rearrangement-mediated [3 + 4] annulation with acryloylsilane 9 and 6-oxa-2-cycloheptenone derivative 22'.

Dense and stereo-controlled integrations of C-N bonds on the azulenone scaffold are achieved by sequential (i) ene, (ii) [6 + 2]-cycloaddition, and (iii) hetero-conjugate addition reactions leading to a topologically complex bis-bridged skeleton.

Unsymmetric diynes possessing a terminal alkyne moiety reacted with organoboronic acids both chemo- and stereoselectively to afford arylated or alkenylated exocyclic dienes by catalysis from the [Rh(cod)OCH(3)](2) complex. The use of a polar protic solvent, e.g. CH(3)OH is required for the success of the process under mild conditions.

Novel [{RhCl(CO)dppp}(2)]-catalyzed intramolecular carbonylative [2+2+1] cycloadditions of bis(phenylsulfonylallene) derivatives under CO, leading to the facile formation of bis(phenylsulfonyl)bicyclo[n.3.0] frameworks (n=4-6), have been developed. The terminal double bonds of both allenyl moieties served exclusively as the two pi-components. In particular, this newly developed method was shown to be a powerful tool for the construction of bicyclo[6.3.0]undecadienones, which have hardly been prepared by the known Pauson-Khand (-type) reactions. The bicyclo[7.3.0]dodecadienone homologue (one extra carbon) could be formed in rather low yields. Alternatively, novel cycloisomerizations of bis(phenylsulfonylallene) derivatives with catalysis by the same Rh(I) complex under N(2) readily produced the 3,4-dimethylene-2,5-bis(phenylsulfonyl)cyclononene and the corresponding cyclooctene and cycloheptene frameworks.

At the palladium: Dimeric palladium bromide complexes bearing monodentate N-heterocyclic carbene ligands have been identified as efficient catalysts for the chemoselective racemization of axially chiral allenyl alcohols. In combination with porcine pancreatic lipase as biocatalyst, a dynamic kinetic resolution has been developed, giving access to optically active allenes in good yield and high enantiomeric purity (see scheme)., (Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Monosubstituted acyclic (E)-1,3-dienes undergo efficient hydrovinylation giving (Z)-3-alkylhexa-1,4-dienes upon treatment with catalytic amounts of bidentate phosphine-CoCl(2) complexes {[P~P](CoCl(2))} and Me(3)Al in an atmosphere of ethylene. The regioselectivity of the reaction (i.e., 1,4- or 1,2-addition) depends on the nature of the ligand and temperature at which the reaction is carried out. Complexes derived from (RR)-DIOP and (SS)-BDDP at -45 degrees C give very high enantioselectivities for several prototypical 1,3-dienes. In sharp contrast to the corresponding Ni(II)-catalyzed hydrovinylation, 1-aryl-substituted 1,3-dienes give almost exclusively achiral linear 1,4-addition products, unless the 2-position is also substituted. Solid-state structures of the precatalysts are presented.

The allene-diyne natural product nemotin was synthesized for the first time through an enantioselective route with the stereogenic center at the lactone moiety derived from l-glutamic acid and the allene axis constructed from the corresponding propargylic tosylate, and the absolute configuration was thus established as (4S,5aS).

ZnI(2) has been identified as the catalyst for the one-step synthesis of allenes from terminal alkynes and aldehydes with morpholine as the base in toluene. The reaction is believed to proceed via the intermediacy of propargylic amines, which was converted to allenes by a sequential hydride transfer and beta-elimination process. The reaction is applicable for both aromatic and aliphatic aldehydes. Functionalities such as halide, hydroxyl, or amine may be tolerated.

A new approach for the synthesis of enantioenriched indanones by asymmetric carbonyl-ene/intramolecular Heck cyclization from racemic silyloxyallenes has been developed. The modular procedure affords highly substituted indenes and indanones with excellent chirality transfer from the optically active carbinols. Full transfer of stereochemical information is achieved in the presence of 1,2,2,6,6-pentamethylpiperidine and PdCl(2)(PPh(3))(2) (1 mol %) in DMF under microwave heating. Short reaction times and high yields have been demonstrated on a variety of substrates.

A Pd(0)-catalyzed three-component tandem Michael addition/cross-coupling reaction of electron-deficient enynes with nucleophiles and aryl halides was developed, which provides general, efficient, and regioselective access to multi-functionalized tetra-substituted allenes.

A variety of iodo-substituted isochromenes, dihydroisobenzofurans, and pyranopyridines are readily prepared in good to excellent yields under mild conditions by the iodocyclization of readily available 2-(1-alkynyl)benzylic alcohols or 2-(1-alkynyl)-3-(hydroxymethyl)pyridines. Reactions are carried out in MeCN at 25 degrees C with 3 equiv of I(2) as the iodine source and NaHCO(3) (3 equiv) as the base. The regiochemical outcome of the reaction strongly depends on the substitution pattern of the starting material. In particular, the 5-exo-dig cyclization mode, leading to dihydroisobenzofurans, is observed in the case of substrates bearing a tertiary alcoholic group, owing to the gem-dialkyl effect, while the 6-endo-dig cyclization mode, leading to isochromene or pyranopyridines, is the usually preferred pathway in the case of substrates bearing a primary or secondary alcoholic group.

Silver(I) salts mediated stereospecific transformation of optically active propargylamines to axially chiral allenes with excellent enantioselectivities (17 examples with 96-99% ee; one substrate with 91% ee) without subsequent racemization.

Palladium-catalyzed monofluoromethylation of substituted 2-bromo-1,3-dienes using fluorobis(phenylsulfonyl)methane (FBSM) as a pronucleophile gave previously unknown monofluoromethylated allenes in high yields, which are the isosteres of biologically attractive allenic alcohols.

The utility of propargyl diethanolamine boronates as reagents for the preparation of allenes and homopropargylic alcohols is presented. Protonolysis with TFA and electrophilic substitution with N-halosuccinimides proceeded with inversion to provide the corresponding allene in high yield and regioselectivity. Alternatively, the propargylation of aldehydes was achieved with use of the in situ generated lithiated complex.

1-Alkoxy-2-methyl-3-acyloxy-(E,E)-penta-1,3-dienes have been prepared applying among others a modified Danishefsky's general method, including chiral, racemic, and achiral derivatives.

It is shown that [4 + 3] and [4 + 2] cycloaddition pathways are accessible in the Au(I) catalysis of allene-dienes. Seven-membered ring gold-stabilized carbenes, originating from the [4 + 3] cycloaddition process, are unstable and can rearrange via a 1,2-H or a 1,2-alkyl shift to yield six- and seven-membered products. Both steric and electronic properties of the AuL(+) catalyst affect the electronic structure of the intermediate gold-stabilized carbene and its subsequent reactivity.

The copper hydride-catalyzed S(N)2'-reduction of propargylic carbonates provides an efficient route to functionalized allenes. The method takes advantage of the stabilizing effect of NHC ligands on CuH and combines high reactivity and stereoselectivity with excellent tolerance toward reactive functionalities.

Cu(I)-catalyzed anti-S(N)2'-type reduction of internal propargylic carbonates with hydrosilanes affords various di- and trisubstituted allenes with high regioselectivities; the reactions are compatible with functional groups and work efficiently for the synthesis of optically-active allenes.

Iodination of 1-C-(tetra-O-acetyl-beta-d-galactopyranosyl)allene affords an E/Z-mixture of 1-C-(tetra-O-acetyl-beta-d-galactopyranosyl)-2,3-diiodo-1-propene. S(N)2 displacement of the allylic iodide with primary amines affords an E/Z-mixture of allylic amines under a variety of conditions. Due to its experimental simplicity and low cost of reagents, this procedure may find wide use in the laboratory for functionalization of sugar allenes.

A highly selective convergent coupling reaction is described between alkynes for the synthesis of stereodefined trisubstituted (E,E)-1,3-dienes-structural motifs commonly found embedded in the skeletons of bioactive polyketide-derived natural products. While numerous multistep processes for the synthesis of this stereodefined functional group exist, the current method represents a significant advance as it does not require stereodefined olefinic coupling partners (vinyl halide or vinyl organometallic); it proceeds by a single convergent C-C bond-forming event (avoiding multistep methods based on carbonyl olefination) and is tolerant of a diverse array of functional groups including free hydroxyls. Through a systematic study of titanium-mediated reductive cross-coupling reactions of internal alkynes with terminal alkynes, a fragment coupling reaction of great utility in natural product synthesis has emerged. Here, use of a proximal hydroxy group to control regioselection in the functionalization of a preformed titanacyclopropene has led to the establishment of a highly selective bimolecular coupling process, where C-C bond formation occurs in concert with the establishment of two stereodefined alkenes. Compared to the body of literature known for related metal-mediated coupling reactions, the current work defines a powerful advance, achieving site-selective bimolecular C-C bond formation without the need for using TMS-alkynes or conjugated alkynes. Overall, complex 1,3-dienes relevant for the synthesis of polyketide-derived natural products of varying stereochemistry were prepared with typically >or=20:1 selectivity, defining the important role of an alkoxide directing group located delta to preformed titanacyclopropenes.

A new method of allene synthesis is described. Suitably functionalized vinyl triflates undergo fragmentation to give allenes in high yield. Computational and experimental data provide a mechanistic framework for allene formation and the complementary formation of alkynes. The method is stereospecific.

Two-directional cross-metathesis of a range of alpha,omega dienes with a variety of electron deficient alkenes has been accomplished. It was found that the process is quite general and gives complete selectivity for the E,E-dienes, making this a very useful and high yielding protocol for two-directional chain elongation.

An efficient method for the preparation of a variety of 2-aminomethyl-1,3-dienes was developed through the reaction of imines with organoindium reagent generated in situ from indium and 1,3-dibromo-2-butyne. Three-component reactions of aldehydes, amines, and organoindium reagents gave successful results in a one-pot process.

Palladium-catalyzed cocyclotrimerization of allenes with arynes has been developed for selectively synthesizing phenanthrenes. In the presence of [(allyl)PdCl](2) and P(o-tol)(3), a variety of allenes, including internal and terminal allenes, underwent the cocyclotrimerization with arynes to afford the corresponding phenanthrenes in moderate to good yields. The results showed the selectivity of the reaction based on allenes.

Using epoxy enol triethylsilanes as oxyallyl cation precursors, [4 + 3] cycloadditions with various dienes occur under catalysis by silyl triflates and acids in good yields. The intramolecular [4 + 3] cycloaddition proceeds under mild conditions and generate hydroxylated cycloadducts with high diastereoselectivity and yields. Enantiomerically pure epoxy enol silanes have been shown to give excellent yields of the optically pure cycloadduct bearing multiple stereocenters.

Chiral allenamides are prepared with high levels of enantiomeric purity by [2,3]-sigmatropic rearrangement of propargylic sulfimides. The required branched propargylic sulfides are prepared by an enantioselective organocatalytic aldehyde alpha-sulfenylation followed by Corey-Fuchs alkynylation.

A DABCO-catalyzed highly regio- and stereoselective syn-1,4-bromolactonization of conjugated enynes was discovered. The 1,4-addition occurred selectively over 1,2-addition for most enynes. In contrast to anti 1,2-addition to alkenes, where two adjacent stereogenic centers are created, 1,4-addition across conjugated enynes can produce a stereogenic center and an axially chiral allene simultaneously. Enynes with Z and E configurations yielded products with complementary stereochemistries at the newly generated stereogenic center and axially chiral allene. Di-, tri-, or even tetrasubstituted allenes together with lactones can be efficiently and selectively prepared from this enyne bromolactonization reaction.

A convenient method is described for the dehydration of beta-ketoesters to generate conjugated and deconjugated alkynyl esters and conjugated allenyl esters. This sequential one-pot method involves the formation of a vinyl triflate monoanion intermediate that leads to the selective formation of alkynes or allenes depending on additives and conditions used. Product outcomes appear to be a function of unique mono- and dianion mechanisms which are described.

Functionalized 1,3-diene derivatives have been prepared by regioselective allylation of various nucleophiles with 1,3-dienic carbonates in the presence of a (N,O-carboxylate) allylruthenium precatalyst.

Two very distinct chemical reactions, yet a single catalyst: A gold complex promotes the formation of tertiary enamines from a variety of terminal and internal alkynes. Subsequent addition of a terminal alkyne to the reaction mixture affords allenes (see scheme).

We describe versatile regiocontrolled metal-catalyzed heterocyclization reactions of gamma-allenol derivatives leading to a variety of fused enantiopure tetrahydrofurans, dihydropyrans, and tetrahydrooxepines. Regioselectivity control in the O-C functionalization of gamma-allenols can be achieved through the choice of catalyst: use of AuCl(3) exclusively affords tetrahydrofurans, use of La[N(SiMe(3))(2)](3) usually favors the formation of dihydropyrans, whereas use of PdCl(2) solely gives tetrahydrooxepines. In addition, it has been observed that for the Au-catalyzed cycloisomerization, the presence of a methoxymethyl protecting group not only masks a hydroxy functionality, but also exerts directing effects as a controlling unit in a regioselectivity reversal (7-endo versus 5-exo cyclization). In addition, the regioselectivity of the La-catalyzed cycloetherification can be tuned (5-exo versus 7-endo) simply through a subtle variation in the substitution pattern of the allene component (Ph versus Me). Thus, for the first time the regiocontrolled heterocyclization of gamma-allenol derivatives is both catalyst- and substrate-directable. These metal-catalyzed heterocyclization reactions have been developed experimentally (Part 1, this paper), and their mechanisms have additionally been investigated by a theoretical study (Part 2, accompanying paper).