Highly Diastereoselective Synthesis of Bicyclo[3.2.1]octenones through Phosphine-Mediated Condensations of 1,4-Dien-3-ones

Anion Cascade Reactions II: Synthesis of 3-Isoquinuclidone-Based Bridged Polycyclic Lactams

A facile and convenient anion cascade strategy was developed for the synthesis of bridged-ring amides in moderate to excellent yields in one step in the presence of tBuOK in EtOH under mild conditions, starting from various cheap and commercially available 2-cyanoacetamides and precisely designed straight-chain and annular 1,4-dienones. This simple protocol was generally applicable to a wide range of substrates with high chemical conversion and diastereoselectivity.

Disproportionation-Inspired Construction of Highly Functionalized Bicyclo[3.2.1]octanes

The formation of one unavoidable byproduct in traditional disproportionation reactions limits their applications in synthesis. Inspired by convergent disproportionation, we develop an iodine-induced cyclization and oxidation of allylic alcohols to produce highly functionalized bicyclo[3.2.1]octanes through creation of six new bonds. Guided by the mechanism, we elaborated a variety of other bicyclo[3.2.1]octanes bearing distinct groups with presynthesized dienes and enones as the starting materials. This work provides a divergent access to bicyclo[3.2.1]octane frameworks.

Interrupted reactions in chemical synthesis

Interrupted reactions reroute established processes to new and often unanticipated end points. Of particular interest are the cases in which a known reactive intermediate takes on a new reaction pathway, either because this pathway is lower in energy or because the conventional pathway is no longer available. Through analysis of documented cases, we aim to dissect the known interrupted reactions and trace their mechanistic origins. As new chemical processes are being discovered at a seemingly ever-increasing pace, it is likely that new interrupted reactions will continue to emerge. Our hope is that the cases considered in this Review will help identify new classes of these fascinating transformations.

An organophosphorus-mediated cross-Rauhut-Currier/Wittig domino reaction for the efficient synthesis of trisubstituted cyclopentenes

An efficient organophosphorus-mediated cross-Rauhut-Currier/Wittig domino reaction of vinyl ketones with chalcones has been developed for the synthesis of trisubstituted cyclopentenes. The new synthetic method has the advantages of mild reaction conditions, high efficiency, environmental friendliness and satisfactory yields.

Total synthesis of (±)-epi-stemodan-13α,17-diol

Stemodan-13α,17-diol is a natural stemodane-type diterpenoid isolated from Stemodia chilensis. Herein we report the total synthesis of its epimer, stemodan-13β,17-diol, by applying titanium-mediated polyene cyclization and iron-catalyzed [5 + 2] cycloaddition as the key transformations to expeditiously install the molecular scaffold.

Phosphine-catalysed asymmetric dearomative formal [4+2] cycloadditions of 3-benzofuranyl vinyl ketones

An asymmetric dearomative formal [4+2] cycloaddition reaction of 3-benzofuranyl vinyl ketones and 3-olefinic (7-aza)oxindoles is explored which is catalysed by chiral phosphines.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Interrupted Morita-Baylis-Hillman-Type Reaction of α-Substituted Activated Olefins

It was demonstrated that 3-olefinic oxindoles could generate zwitterionic enolate species with tertiary phosphines and undergo C-C bond formation with various electrophiles in an interrupted Morita-Baylis-Hillman-type reaction manner, followed by a dephosphoration process. Although the in situ formation of phosphorus-ylide intermediates was observed, no Wittig reaction was detected, even in the presence of excess formaldehyde. Moreover, excellent enantioselectivity for the construction of quaternary stereogenic centers was induced with chiral phosphines. Deuterium experiments and density functional theory calculations were conducted to elucidate the reaction mechanism.

Bridged [2.2.1] bicyclic phosphine oxide facilitates catalytic γ-umpolung addition-Wittig olefination

A novel bridged [2.2.1] bicyclic phosphine oxide, devised to circumvent the waste generation and burdens of purification that are typical of reactions driven by the generation of phosphine oxides, has been prepared in three steps from commercially available cyclopent-3-ene-1-carboxylic acid. The performance of this novel phosphine oxide was superior to those of current best-in-class counterparts, as verified experimentally through kinetic analysis of its silane-mediated reduction. It has been applied successfully in halide-/base-free catalytic γ-umpolung addition-Wittig olefinations of allenoates and 2-amidobenzaldehydes to produce 1,2-dihydroquinolines with good efficiency. One of the 1,2-dihydroquinoline products was converted to known antitubercular furanoquinolines.

From Mechanistic Study to Chiral Catalyst Optimization: Theoretical Insight into Binaphthophosphepine-catalyzed Asymmetric Intramolecular [3 + 2] Cycloaddition

Density functional M11 was used to study the mechanism and enantioselectivity of a binaphthophosphepine-catalyzed intramolecular [3 + 2] cycloaddition reaction. The computational results revealed that this reaction proceeds through nucleophilic addition of the phosphine catalyst to the allene, which yields a zwitterionic phosphonium intermediate. The subsequent stepwise [3 + 2] annulation process, which starts with the intramolecular nucleophilic addition of the allenoate moiety to the electron-deficient olefin group, determines the enantioselectivity of the reaction. This step is followed by a ring-closing reaction and water-assisted proton-transfer process to afford the final product with concomitant regeneration of the phosphine catalyst. Theoretical predictions of the enantioselectivity for various phosphine catalysts were consistent with experimental observations, and 2D contour maps played an important role in explaining the origin of the enantioselectivity. Moreover, on the basis of our theoretical study, new binaphthophosphepine catalysts were designed and that are expecting to afford higher enantioselectivity in this cycloaddition reaction.

Synthesis and structures of stable phosphorus zwitterions derived from mesoionic 4-trifluoroacetyl-1,3-oxazolium-5-olates

Trialkyl phosphites were evaluated as phosphorus nucleophiles for addition to mesoionic 4-trifluoroacetyl-1,3-oxazolium-5-olates (1), thereby producing tetravalent phosphorus zwitterions (2) in good yields. The structure of 2 was determined to be a tetravalent phosphonium enolate via single crystal X-ray analysis.

Synthesis of silyloxy dienes by silylene transfer to divinyl ketones: application to the asymmetric synthesis of substituted cyclohexanes

Silver-catalyzed silylene transfer to divinyl ketones provided 2-silyloxy-1,3-dienes with control of stereochemistry and regioselectivity. The products participated in Diels-Alder reactions with electron-deficient alkenes and imines to form six-membered-ring products diastereoselectively. Cycloaddition reactions with alkenes bearing chiral auxiliaries provided access to chiral, nonracemic cyclohexenes. The methodology, therefore, represents a synthesis of diastereomerically and enantiomerically pure products in a single flask. The highly substituted cyclohexene products could be functionalized stereoselectively to provide cyclohexanols after oxidation of the carbon-silicon bond.

β-Olefination of 2-alkynoates leading to trisubstituted 1,3-dienes

A phosphine-mediated olefination of 2-alkynoates with aldehydes forming 1,3-dienes with high E-selectivity and up to 88% yield is described. Reaction conditions are optimized and reactions are demonstrated for various aryl, alkyl, and alkenyl aldehydes and for ethyl 2-alkynoates with different substituents in the δ-position. Proof of concept is shown for the generation of a β,γ-unsaturated lactone by intramolecular olefination, and furthermore the use of the generated 1,3-dienes in the Diels-Alder reaction has been demonstrated.

Wittig olefination between phosphine, aldehyde, and allylic carbonate: a general method for stereoselective synthesis of trisubstituted 1,3-dienes with highly variable substituents

A clean and salt-free Wittig olefination between phosphines, aldehydes, and allylic carbonates is described. It represents a general method for convenient and efficient synthesis of 1,2,4-trisubstituted 1,3-dienes from readily available starting materials. This method exhibits high synthetic efficiency, high stereoselectivity, and high variability of substituent.

Phosphine-mediated olefination between aldehydes and allenes: an efficient synthesis of trisubstituted 1,3-dienes with high E-selectivity

The phosphine-mediated olefination of aldehydes with electron-deficient allenes to afford trisubstituted conjugated dienes in fair to excellent yields with high E-selectivity is described. The reaction represents a new reactivity pattern of allenes with aldehydes and also provides a highly stereoselective synthetic method for preparing conjugated dienes. In the reaction, the phosphine acts as a nucleophilic promoter to generate in situ an active phosphorus ylide which mediates the intermolecular olefination.

Efficient 1,4-addition of alpha-substituted fluoro(phenylsulfonyl)methane derivatives to alpha,beta-unsaturated compounds

The 1,4-addition of a monofluoromethyl nucleophile to a variety of α,β-unsaturated compounds has been achieved under mild conditions using either phosphines or potassium carbonate at room temperature. α-Substituted fluoro(phenylsulfonyl)methane easily undergoes Michael addition to α,β-unsaturated ketones, esters, nitriles, sulfones, as well as propynoates at room temperature to yield the corresponding adducts in moderate to excellent yields.

Phosphine-triggered synthesis of functionalized cyclic compounds

Nucleophilic phosphine catalysis has proven to be a powerful tool in organic synthesis, which can provide easy access to cyclic, bicyclic or polycyclic carbocycles and heterocycles. Owing to their comparatively strong and readily tunable nucleophilicity, phosphines can be easily tailored to efficient annulation reactions with good control over reaction selectivity. This has resulted in a tremendous increase in their scope and in a concomitant number of reports where phosphine-triggered annulation reactions occur. This tutorial review summarizes the recent achievements in this area.