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The authors acknowledge support in the form of NSERC (Canada) Discovery Grants to EF, WSH, and TBM and an EPSRC (UK) responsive mode grant to DO’H and NSK is gratefully acknowledged. Experiments have been carried out in which electrospray ionization has been used to generate ionic complexes of all-cis 1,2,3,4,5,6 hexafluorocyclohexane. These complexes were subsequently mass isolated in a quadrupole ion trap mass spectrometer and then irradiated by the tunable infrared output of a free electron laser in the 800-1600 cm−1 range. From the frequency dependence of the fragmentation of the complexes, vibrational signatures of the complexes were obtained. Computational work carried out in parallel reveals that the complexes formed are very strongly bound and are among the most strongly bound complexes of Na+ and Cl− ever observed with molecular species. The dipole moment calculated for the heaxafluorocyclohexane is very large (~7 D) and it appears that the bonding in each of the complexes has a significant electrostatic contribution. Postprint

Under Sc(OTf)3 catalysis, benzylidene Meldrum’s acids bearing a tethered p-methoxyphenethyl group were observed to undergo a [1,5]-hydride shift/cyclization at room temperature, representing a mild Csp3–H bond functionalization. The resulting spiro Meldrum’s acid intermediates then underwent intramolecular Friedel–Crafts acylation, completing the one-pot, domino reaction. The reported protocol generates the 6-6-5-6 tetracyclic core of tetrahydrobenzo[b]fluoren-11-ones.

The intramolecular Friedel–Crafts acylation of arenes with Meldrum's acid derivatives catalyzed by Sc(OTf)3 was reported as a mild and general entry into functionalized 1-indanones. Mechanistic investigations were undertaken to determine the rate-determining step in the acylation sequence using Meldrum's acid, as well as to examine the role of the Lewis acid catalyst. Enolizable Meldrum's acid derivatives react via an acyl ketene intermediate under thermal conditions, while quaternized Meldrum's acid derivatives are thermally stable and only act as effective Friedel–Crafts acylating agents in the presence of a Lewis acid catalyst. The acylation was postulated to proceed through direct acylation of a Lewis acid-activated carbonyl. In the catalytic Friedel–Crafts acylation of Meldrum's acids, triflic acid appeared to be the active catalytic species, with Sc(OTf)3 serving as a very mild and convenient reagent for its delivery.

Alkylidene Meldrum's acids are allylated in a conjugate fashion by allyltin nucleophiles under mild Sc(OTf)(3)-catalyzed conditions. The addition is functional group tolerant and reactions with nonracemic alkylidenes are highly diastereoselective. Allylation of alkylidenes derived from alpha-ketoesters yield all-carbon quaternary stereocenters.

The rhodium-catalyzed conjugate addition of functionalized vinyltin reagents to alkylidene Meldrum's acids, followed by Pd-catalyzed intramolecular allylation, is a direct entry into vinyl-substituted gamma-lactones via O-alkylation and vinylcyclopropanes via C-alkylation.

The conjugate alkynylation of alkylidene Meldrum’s acids with alkynylalanes and alkynyl Grignards allows for the formation of propargylic all-carbon quaternary stereocenters in high yields. Ag2CO3-catalyzed intramolecular cyclization of propargylic Meldrum’s acid derivatives offers a two-step entry into complex γ-alkylidene butyrolactones containing an all-carbon quaternary center at the C-4 position.

Although many methods have been reported for the Knoevenagel condensation of aldehydes and Meldrum’s acid, most are not general or use unconventional reagents and conditions. We have found that alkylidene Meldrum’s acids form readily in benzene solution under mild pyrrolidinium acetate catalysis, and that this reaction is general, highly functional group compatible, and can be scaled-up easily.

Unsymmetrical 8-oxa-3-thiabicyclo[3.2.1]octane and 9-oxa-3-thiabicyclo[3.3.1]nonane underwent highly regioselective base-induced elimination but low selectivity was observed for the sulfonyl and the aza-oxa systems. A correlation between the regioselectivity of the reaction and the proton undergoing abstraction (i.e. equatorial vs. the axial proton) was made.

The design as well as the synthesis, resolution, and biological evaluation of the neutral endopeptidase 24.11 inhibitors, (1S,2R,5S)-(−)-[(2-mercapto-5-phenyl-cyclopentanecarbonyl)-amino]-acetic acid and (1R,2S,5R)-(+)-[(2-mercapto-5-phenyl-cyclopentanecarbonyl)-amino]-acetic acid are described.

Lewis acid promoted substitution reactions employing Meldrum's acid and 5-methyl Meldrum's acid as carbon-based leaving groups are described which transform unstrained quaternary and tertiary benzylic C(sp(3))-C(sp(3)) bonds into C(sp(3))-X bonds (X = C, H, N). Importantly, this reaction has a broad scope in terms of both suitable substrates and nucleophiles with good to excellent yields obtained (typically90%).

Herein, we describe enantioselective addition of alkenyltin reagents possessing a reactive and sensitive allylic functionality not readily available to other classes of alkenyl metals. This method is enabled by the use of highly electrophilic alkylidene Meldrum's acids as acceptors and a cationic Rh(I)-diene complex as catalyst.

The enantioselective alkynylation of benzylidene Meldrum’s acids has been successfully achieved through rhodium-catalyzed addition of TMS-acetylene in the presence of bisphosphine ligand 3,5-Xylyl-MeOBIPHEP. The resulting Meldrum’s acids were obtained in good yields and up to 99% enantiomeric excess. The alkynylation method is carried out under mild reactions conditions and is compatible with an array of functional groups.

The modification of sp(3)-hybridized carbon centers through Pd-catalyzed reductive cleavage of unstrained carbon-carbon sigma bonds is described. From the hydrogenolysis of benzyl Meldrum's acids bearing an all-carbon benzylic quaternary center, Meldrum's acid and aromatics substituted with a tertiary benzylic stereocenter were obtained in good to excellent yields. Mechanistic studies showed that the reductive cleavage of enantioenriched benzylic quaternary centers proceeded with inversion of configuration, supporting a "loose" S(N)2 pathway.

The asymmetric synthesis of carboxylic acid derivatives having an all-carbon alpha-quaternary center has been achieved via copper-catalyzed 1,4-addition of dialkylzinc reagents to aryl acetate derivatives in the presence of phosphoramidite ligand. High isolated yields and enantioselectivities were obtained. It was demonstrated that the Meldrum's acid and ester moieties present on the all-carbon quaternary center allow for a wide variety of subsequent transformations, leading to the expedient preparation of succinimides, succinate esters and succinic acids, gamma-butyrolactones, and beta-amino acid derivatives.

The asymmetric synthesis of all-carbon benzylic quaternary stereocenters has been successfully achieved through copper-catalyzed addition of dialkylzinc reagents to 5-(1-arylalkylidene) and 5-(dihydroindenylidene) Meldrum's acids in the presence of phosphoramidite ligand. The resulting benzyl-substituted Meldrum's acids and 1,1-disubstituted indanes were obtained in good yields and up to 99% ee. The significance of substituting the position para, meta, and ortho to the electrophilic benzylic center was highlighted. A benzyl Meldrum's acid product was further transformed to a 3,3-disubstituted 1-indanone and a beta,beta-disubstituted pentanoic acid.

2-Substituted 1,2-dihydro-1-naphthols underwent regioselective oxidative dimerization, when treated with Jones reagent, to furnish 3,3′-disubstituted 1,1′-binaphthyl-4,4′-diols. A series of symmetrical binaphthols were prepared and it was shown that the coupling reaction proceeds via the sequential oxidation of 2-substituted 1,2-dihydro-1-naphthols to 2-substituted 1-naphthols, which oxidatively dehydrodimerized.

[reaction: see text] The intramolecular Friedel-Crafts acylation of benzyl Meldrum's acids is catalyzed by Sc(OTf)(3) under mild reaction conditions. Several polysubstituted 1-indanones have been prepared.

Two complementary routes to sp3-gem-organodimetallic iodopalladio-trialkylstannanylalkanes are presented. Such intermediates have been proposed as Pd-carbenoid precursors in the Busacca-Farina cine-substitution mechanism in the Stille coupling. The decomposition of iodomethyltrialkylstannanes by Pd(0) catalysts was monitored by 1H, 2D, and 119Sn NMR. The formation of ethylene, trace formaldehyde, and iodotrialkylstannanes was detected. When the reaction was carried out in the presence of norbornene, the corresponding cyclopropane was produced in good yield. These observations are consistent with the intermediacy of a Pd-carbenoid species. sp3-gem-Organodimetallic iodopalladio-trialkylstannanylalkane complexes were also prepared under stoichiometric conditions via transmetalation from tin to Pd(II). Me3SnCH2Sn(CH2CH2CH2)3 reacted with [(D-t-BPF)PdI]+I-, yielding the (D-t-BPF)Pd(II)ICH2SnMe3 complex that dimerized to form ethylene and cyclopropanated norbornene. The carbenoid reactivity of iodopalladio-trialkylstannanylalkanes complexes validates the Busacca-Farina mechanism of the cine-substitution in the Stille coupling.

The first synthesis of taiwaniaquinol B, a 6-nor-5(6-->7)abeoabietane-type diterpenoid exhibiting the uncommon fused 6-5-6 tricyclic carbon skeleton, was accomplished in 15 steps. A Lewis acid-promoted tandem intramolecular Friedel-Crafts/carbonyl alpha-tert-alkylation reaction was exploited as the core strategy for the synthesis of the sterically congested 1-indanone-containing tricyclic structure. This multiple carbon-carbon bond forming reaction exploits the unique reactivity of Meldrum's acid. The facile precursor synthesis makes this a useful methodology for the expedient modification and assembly of sterically congested 1-indanone-containing ring systems.

A one-pot, two-step process that transforms terminal alkynes into ethyl methyl-substituted benzylic quaternary carbon centers is described. (E)-2,2-Disubstituted-1-alkenyldimethylalanes have been shown to participate in 1,2-alkyl migration from aluminum to carbon with concomitant arylation at the 2-position to furnish ethyl methyl-substituted benzylic quaternary carbon centers, when reacted intramolecularly with aryl halides and triflates in the presence of a Pd(0) catalyst. The protocol is initiated with Cp2ZrCl2-catalyzed methylalumination of terminal alkynes followed by palladium-catalyzed intramolecular arylation of the resulting (E)-2,2-disubstituted-1-alkenyldimethylalanes, leading to 1,2-methyl shift from aluminum to carbon. In that sequence, a total of three new C-C single bonds are made, and two of the three alkyl groups on Me3Al transferred to the substrate on vicinal carbons. This method was applied to a variety of substrates, and the mechanism was investigated by deuterium-labeling experiments, which revealed that protodealumination of the final dialkylaluminum triflate or halide intermediates by CH3CN results in the formation of the fourth bond in the course of the transformation.