Rh(I)-catalyzed allenic Pauson–Khand reaction: first construction of the bicyclo[6.3.0]undecadienone ring system

Asymmetric Total Syntheses of Hypoestin A, Albolic Acid, and Ceroplastol II

The first asymmetric total synthesis of bioactive diterpenoid hypoestin A with an unprecedented [5-8-5-3] tetracyclic skeleton is accomplished in 15 steps from commercially available (R)-limonene. Furthermore, the second asymmetric total syntheses of sesterterpenoids albolic acid and ceroplastol II in 21 steps are also reported. The synthetically challenging and highly functionalized [X-8-5] (X = 5 or 7) tricarbocyclic ring systems found in hypoestin A, albolic acid, ceroplastol II, and schindilactone A, as well as other natural products, are efficiently and directly constructed via a unique intramolecular Pauson-Khand reaction of an allene-yne. This work represents the first reported use of the Pauson-Khand reaction to access synthetically challenging eight-membered-ring systems in natural product synthesis.

Recent developments in the chemistry of allenyl sulfones

Allenyl sulfones are versatile building blocks for the construction of various acyclic, carbocyclic and heterocyclic motifs. Recent developments in the preparation and synthetic applications of allenyl sulfones are summarised.

Construction of Spirofused Tricyclic Frameworks by NHC-Catalyzed Intramolecular Stetter Reaction of a Benzaldehyde Tether with a Cyclic Enone

Various benzaldehyde tethers with a cyclic enone were prepared from commercially available 2-hydroxybenzaldehydes via a three-step sequence involving triflate formation, Sonogashira cross-coupling, and regioselective hydrogenation. These substrates were then exposed to an N-heterocyclic carbene, whereupon intramolecular Stetter reaction proceeded smoothly to give various spirofused tricyclic 1,4-diketones in 30-87% yields. Furaldehyde and nicotinaldehyde derivatives also participated in the reaction under the Stetter conditions.

Creation of Novel Cyclization Methods Using sp-Hybridized Carbon Units and Syntheses of Bioactive Compounds

Some recent results on the development of new and reliable procedures for the construction of diverse ring systems based on the chemistry of sp-hybridized species, especially allene functionality, are described. This review includes: (i) synthesis of the multi-cyclic skeletons by combination of the π-component of allene with suitable other π-components such as alkyne, alkene, or additional allene under Rh-catalyzed conditions; (ii) synthesis of heterocycles as well as carbocycles by reaction of the sp-hybridized center of allene with some nucleophiles in an endo-mode manner; and (iii) total syntheses of natural products and related compounds from the sp-hybridized starting materials.

Reactivity and chemoselectivity of allenes in Rh(I)-catalyzed intermolecular (5 + 2) cycloadditions with vinylcyclopropanes: allene-mediated rhodacycle formation can poison Rh(I)-catalyzed cycloadditions

Allenes are important 2π building blocks in organic synthesis and engage as 2-carbon components in many metal-catalyzed reactions. Wender and co-workers discovered that methyl substituents on the terminal allene double bond counterintuitively change the reactivities of allenes in [Rh(CO)2Cl]2-catalyzed intermolecular (5 + 2) cycloadditions with vinylcyclopropanes (VCPs). More sterically encumbered allenes afford higher cycloadduct yields, and such effects are also observed in other Rh(I)-catalyzed intermolecular cycloadditions. Through density functional theory calculations (B3LYP and M06) and experiment, we explored this enigmatic reactivity and selectivity of allenes in [Rh(CO)2Cl]2-catalyzed intermolecular (5 + 2) cycloadditions with VCPs. The apparent low reactivity of terminally unsubstituted allenes is associated with a competing allene dimerization that irreversibly sequesters rhodium. With terminally substituted allenes, steric repulsion between the terminal substituents significantly increases the barrier of allene dimerization while the barrier of the (5 + 2) cycloaddition is not affected, and thus the cycloaddition prevails. Computation has also revealed the origin of chemoselectivity in (5 + 2) cycloadditions with allene-ynes. Although simple allene and acetylene have similar reaction barriers, intermolecular (5 + 2) cycloadditions of allene-ynes occur exclusively at the terminal allene double bond. The terminal double bond is more reactive due to the enhanced d-π* backdonation. At the same time, insertion of the internal double bond of an allene-yne has a higher barrier as it would break π conjugation. Substituted alkynes are more difficult to insert compared with acetylene, because of the steric repulsion from the additional substituents. This leads to the greater reactivity of the allene double bond relative to the alkynyl group in allene-ynes.

Toward a symphony of reactivity: cascades involving catalysis and sigmatropic rearrangements

Catalysis and synthesis are intimately linked in modern organic chemistry. The synthesis of complex molecules is an ever evolving area of science. In many regards, the inherent beauty associated with a synthetic sequence can be linked to a certain combination of the creativity with which a sequence is designed and the overall efficiency with which the ultimate process is performed. In synthesis, as in other endeavors, beauty is very much in the eyes of the beholder. It is with this in mind that we will attempt to review an area of synthesis that has fascinated us and that we find extraordinarily beautiful, namely the combination of catalysis and sigmatropic rearrangements in consecutive and cascade sequences.

[Intramolecular cyclization reaction of multiple bonds and its application]

The cycloaddition and cycloisomerization of the allene with an alkyne, alkene, or an additional allene for construction of various monocyclic and bicyclic ring systems has been developed. The characteristic features of these methods using allene functionality instead of a simple alkene or alkyne include the reaction mode that originated from the double function as well as the high efficiency for the constructions of medium-sized rings. Furthermore, asymmetric formal synthesis of (+)-nakadomarin A and total synthesis of (+)-fawcettimine and (+)-lycoposerramine-B based on highly stereoselective Pauson-Khand reaction of alkene-alkynes were completed.

Rh(I)-catalyzed intramolecular [2 + 2 + 1] cycloaddition of allenenes: Construction of bicyclo[4.3.0]nonenones with an angular methyl group and tricyclo[6.4.0.0]dodecenone

The [RhCl(CO)dppp]₂-catalyzed intramolecular carbonylative [2 + 2 + 1] cycloaddition of allenenes was developed to prepare bicyclo[4.3.0]nonenones possessing a methyl group at the ring junction, which is difficult to achieve by the Pauson–Khand reaction of the corresponding enynes. This method also provided a new procedure for the construction of the tricyclo[6.4.0.0^1,5]dodecenone framework in a satisfactory yield.

How easy are the syntheses of allenes?

This short review highlights some of the recent important developments on the synthesis of allenes and its applications on the synthesis of some allenic natural products and allenic-based optoelectronic materials.

Synthesis of a core carbon framework of cyanosporasides A and B

Treatment of 3-(2-ethynylphenyl)prop-2-ynyl benzenesulfinate with 2.5 mol % of [RhCl(CO)(2)](2) at 40 degrees C under an atmosphere of CO effected the successive 2,3-sigmatropic rearrangement and carbonylative [2 + 2 + 1] ring-closing reaction to afford the 8-(phenylsulfonyl)-1H-cyclopent[a]-inden-2-one in a high yield. Chemical modification of the ring-closed product via lipase-mediated optical resolution produced the optically active 3-acetoxy-3a-cyclohexyloxy-3,3a-dihydrocyclopent-[a]indene skeleton, the core carbon framework of cyanosporasides A and B.

Enantioselective nickel-catalyzed cross-coupling reactions of trialkynylindium reagents with racemic secondary benzyl bromides

The first enantioselective sp-sp3 cross-coupling reaction between alkynyl organometals and racemic benzyl bromides is reported. The coupling is performed at room temperature by using NiBr2diglyme and (S)-(iPr)-Pybox as the catalytic system and trialkynylindium reagents as nucleophiles. The reaction is stereoconvergent, both enantiomers of the racemic benzyl bromide are converted into one enantiomer of the product, and stereospecific. The reaction takes place efficiently in good yields and with high atom economy, as the triorganoindium reagents transfer the three organic groups attached to indium (only 40 mol % of R3In is used).

Stereoselective total syntheses of three Lycopodium alkaloids, (-)-magellanine, (+)-magellaninone, and (+)-paniculatine, based on two Pauson-Khand reactions

The total syntheses of (-)-magellanine, (+)-magellaninone, and (+)-paniculatine were completed from diethyl l-tartrate via the common intermediate in a stereoselective manner. The crucial steps in these syntheses involved two intramolecular Pauson-Khand reactions of enynes: the first Pauson-Khand reaction constructed the bicyclo[4.3.0] carbon framework, the corresponding A and B rings of these alkaloids in a highly stereoselective manner, whereas the second Pauson-Khand reaction stereoselectively produced the bicyclo[3.3.0]skeleton, which could be converted into the C and D rings of the target natural products.

Synthesis of Hexahydropyrrolo[2,3-b]indole Alkaloids Based on the Aza-Pauson−Khand-Type Reaction of Alkynecarbodiimides

Upon treatment with 30 mol % of Co2(CO)(8) and 30 mol % of TMTU in toluene at 70 degrees C, benzene-bridged alkynecarbodiimides efficiently underwent a ring-closing reaction to give the pyrrolo[2,3-b]indol-2-ones in good yields. These conditions could nearly suppress the formation of the urea derivatives, which were consistently observed when 10 mol % of Co2(CO)(8) and 60 mol % of TMTU in benzene were used. The synthesis of the eight hexahydropyrrolo[2,3-b]indole alkaloids was accomplished from the resulting pyrrolo[2,3-b]indol-2-ones via the introduction of an angular substituent at the C(3a)-position by treatment with NaBH(4)/alkyl bromide as the crucial step.

Intermolecular [4 + 2] Cycloaddition of o-Quinodimethanes Derived from Ene−Bis(sulfinylallenes)

Intermolecular [4 + 2] cycloaddition of o-quinodimethanes, prepared in situ from ene-bis(propargyl alcohols) and benzenesulfenyl chloride via ene-bis(sulfinylallene) formation, was investigated. Benzene-bridged bis(propargyl alcohols) reacted with both electron-deficient and electron-rich olefins to give the corresponding [4 + 2] cycloadducts. Ethylene-bridged bis(propargyl alcohols) underwent similar cycloaddition with electron-deficient olefins. Construction of some heterocycles based on the newly developed sequential reaction is also described.