Phosphine-Catalyzed Asymmetric Synthesis of β-Lactones from Arylketoketenes and Aromatic Aldehydes

Probing Regio- and Enantioselectivity in the Formal [2 + 2] Cycloaddition of C(1)-Alkyl Ammonium Enolates with β- and α,β-Substituted Trifluoromethylenones

The isothiourea-catalyzed regio- and enantioselective formal [2 + 2] cycloaddition of C(1)-alkyl and C(1)-unsubstituted ammonium enolates with β- and α,β-substituted trifluoromethylenones has been developed. In all cases, preferential [2 + 2]-cycloaddition over the alternative [4 + 2]-cycloaddition is observed, giving β-lactones with excellent diastereo- and enantioselectivity (34 examples, up to >95:5 dr, >99:1 er). The regioselectivity of the process was dictated by the nature of the substituents on both reaction components. Solely [2 + 2] cycloaddition products are observed when using α,β-substituted trifluoromethylenones or α-trialkylsilyl acetic acid derivatives; both [2 + 2] and [4 + 2] cycloaddition products are observed when using β-substituted trifluoromethylenones and α-alkyl-α-trialkylsilyl acetic acids as reactants, with the [2 + 2] cycloaddition as the major reaction product. The beneficial role of the α-silyl substituent within the acid component in this protocol has been demonstrated by control experiments.

Diastereoselective and Enantioselective Synthesis of α-p-Methoxyphenoxy-β-Lactones: Dependence on the Stereoelectronic Properties of the β-Hydroxy-α-p-Methoxyphenoxycarboxylic Acid Precursors

Treatment of β-hydroxy-α-p-methoxyphenoxy carboxylic acids derived from the asymmetric glycolate aldol addition reaction with p-nitrobenzenesulfonyl chloride yielded divergent results depending on the nature of the β-substituent of the carboxylic acid. Substrates bearing either alkyl substituents (R = -n-butyl, -n-octyl, -benzyl, isopropyl, -tert-butyl) or aryl systems bearing electron-withdrawing substituents (R = -p-C₆H₄Cl, -p-C₆H₄Br, -p-C₆H₄NO₂) yielded β-lactones. In contrast, α-p-methoxyphenoxy-β-hydroxycarboxylic acids bearing electron-donating aryl groups or the sterically demanding 2-naphthyl group formed (Z)-alkenes.

Synthesis of heterocyclic compounds through nucleophilic phosphine catalysis

Nucleophilic phosphine catalysis is a practical and powerful tool for the synthesis of various heterocyclic compounds with the advantages of environmentally friendly, metal-free, and mild reaction conditions. The present report summarizes the construction of four to eight-membered heterocyclic compounds containing nitrogen, oxygen and sulphur atoms through phosphine-catalyzed intramolecular annulations and intermolecular [2+2], [3+2], [4+1], [3+1+1], [5+1], [4+2], [2+2+2], [3+3], [4+3] and [3+2+3] annulations of electron-deficient alkenes, allenes, alkynes and Morita-Baylis-Hillman carbonates.

Catalytic enantioselective synthesis of perfluoroalkyl-substituted β-lactones via a concerted asynchronous [2 + 2] cycloaddition: a synthetic and computational study

The enantioselective preparation of a range of perfluoroalkyl-substituted β-lactones through an isothiourea (HyperBTM) catalysed reaction using symmetric anhydrides as ammonium enolate precursors and perfluoroalkylketones (R_F = CF₃, C₂F₅, C₄F₉) is reported. Following optimisation, high diastereo- and enantioselectivity was observed for β-lactone formation using C₂F₅- and C₄F₉-substituted ketones at room temperature (26 examples, up to >95 : 5 dr and >99 : 1 er), whilst -78 °C was necessary for optimal dr and er with CF₃-substituted ketones (11 examples, up to >95 : 5 dr and >99 : 1 er). Derivatisation of the β-lactones through ring-opening, as well as a two-step conversion to give perfluoroalkyl-substituted oxetanes, is demonstrated without loss of stereochemical integrity. Density functional theory computations, alongside ¹³C natural abundance KIE studies, have been used to probe the reaction mechanism with a concerted asynchronous [2 + 2]-cycloaddition pathway favoured over a stepwise aldol-lactonisation process.

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.

Phosphine-catalyzed Friedel–Crafts reaction of naphthols with para-quinone methides: expedient access to triarylmethanes

A novel phosphine-catalyzed Friedel–Crafts reaction of naphthols with para-quinone methides has been developed.

Catalytic Asymmetric Synthesis of Ketene Heterodimer β-Lactones: Scope and Limitations

In this article we describe extensive studies of the catalytic asymmetric heterodimerization of ketenes to give ketene heterodimer β-lactones. The optimal catalytic system was determined to be a cinchona alkaloid derivative (TMS-quinine or Me-quinidine). The desired ketene heterodimer β-lactones were obtained in good to excellent yields (up to 90%), with excellent levels of enantioselectivity (≥90% ee for 33 Z and E isomer examples), good to excellent (Z)-olefin isomer selectivity (≥90:10 for 20 examples), and excellent regioselectivity (only one regioisomer formed). Full details of catalyst development studies, catalyst loading investigations, substrate scope exploration, protocol innovations (including double in situ ketene generation for 7 examples), and an application to a cinnabaramide A intermediate are described. The addition of lithium perchlorate (1-2 equiv) as an additive to the alkaloid catalyst system was found to favor formation of the E isomer of the ketene heterodimer. Ten examples were formed with moderate to excellent (E)-olefin isomer selectivity (74:25 to 97:3) and with excellent enantioselectivity (84-98% ee).

Asymmetric α-amination of 3-substituted oxindoles using chiral bifunctional phosphine catalysts

A highly enantioselective α-amination of 3-substituted oxindoles with azodicarboxylates catalyzed by amino acids-derived chiral phosphine catalysts is reported. The corresponding products containing a tetrasubstituted carbon center attached to a nitrogen atom at the C-3 position of the oxindole were obtained in high yields and with up to 98% ee.

Chiral Lewis acid catalyzed asymmetric cycloadditions of disubstituted ketenes for the synthesis of β-lactones and δ-lactones

Highly diastereo- and enantioselective [2 + 2]- and [4 + 2]-cycloadditions of disubstituted ketenes were realized by chiral Lewis acid catalysis. A series of arylalkylketenes underwent the reaction smoothly with isatins and β,γ-unsaturated α-ketoesters, providing optically active β-lactones and δ-lactones with vicinal chiral centers in excellent yields (up to 99%) and enantioselectivities (up to 99% ee), as well as exclusively high diastereoselectivities under 0.2-2 mol % catalyst loading.

The first asymmetric ring-expansion carbonylation of meso-epoxides

The first asymmetric ring-expansion carbonylation of meso-epoxides to β-lactones is reported. Two structurally diverse chiral Cr(III) chloro complexes in conjunction with Co(2)(CO)(8) were shown to be competent catalytic systems for this transformation, displaying significant levels of asymmetric induction of up to 56% ee.

Cooperative Al(salen)-pyridinium catalysts for the asymmetric synthesis of trans-configured β-lactones by [2+2]-cyclocondensation of acylbromides and aldehydes: investigation of pyridinium substituent effects

The trans-selective catalytic asymmetric formation of β-lactones constitutes an attractive surrogate for anti-aldol additions. Recently, we have reported the first catalyst which is capable of forming trans-β-lactones with high enantioselectivity from aliphatic (and aromatic) aldehyde substrates by cyclocondensation with acyl bromides. In that previous study the concepts of Lewis acid and organic aprotic ion pair catalysis were combined in a salen-type catalyst molecule. Since a pyridinium residue on the salen periphery is essential for high trans- and enantioselectivity, we were interested in the question of whether substituents on the pyridinium rings could be used to further improve the catalyst efficiency, as they might have a significant impact on the effective charges within the heterocycles. In the present study we have thus compared a small library of aluminum salen/bispyridinium catalysts mainly differing in the substituents on the pyridinium residues. As one result of these studies a new catalyst was identified which offers slightly superior stereoselectivity as compared to the previously reported best catalyst. NBO calculations have revealed that the higher stereoselectivity can arguably not be explained by the variation of the effective charge.