Parallel kinetic resolution of racemic aldehydes by use of asymmetric Horner-Wadsworth-Emmons reactions

[reaction: see text] A racemic aldehyde can undergo parallel kinetic resolution (PKR) by simultaneous reaction with two different chiral phosphonates, differing either in the structure of the chiral auxiliary or in the structure of the phosphoryl group (i.e., one (E)- and one (Z)-selective reagent). This strategy allows conversion of a racemic aldehyde to two different, synthetically useful chiral products with essentially doubled material throughput and similar or improved selectivities as compared to conventional kinetic resolution.     

Quasiracemic synthesis: concepts and implementation with a fluorous tagging strategy to make both enantiomers of pyridovericin and mappicine

The concept of quasiracemic synthesis is introduced and illustrated with syntheses of both enantiomers of pyridovericin (whose absolute configuration is assigned as R) and mappicine. Like racemic synthesis, quasiracemic synthesis provides both enantiomers in a single synthetic sequence; however, separation tagging is used to ensure that quasiracemic mixtures can be analyzed, separated, and identified on demand. Fluorous tags of differing chain lengths are used to tag two enantiomeric starting materials. The resulting quasienantiomers are mixed to make a quasiracemate, which is then treated like a true racemate in successive steps of the synthesis. Fluorous chromatography is used to separate, or demix, the final quasiracemate into its two components, which are then detagged to provide (true) enantiomeric products. Quasiracemic synthesis is portrayed as the first and simplest of a series of mixture synthesis techniques based on separation tagging, and the prospects for using other types of separation tags are briefly evaluated.     

Asymmetric Pauson-Khand-type reaction mediated by Rh(I) catalyst at ambient temperature

An efficient asymmetric PKR mediated by Rh(I) catalyst at ambient temperature was developed. The reaction utilizing a Rh(I) catalyst bearing a (R)-3,5-diMeC4H4-BINAP ligand at 18-20 degrees C under a reduced partial pressure of CO (0.1 atm) provided PKR products in high chemical yield as well as high enantioselectivity.     

Quasienantiomeric levoglucosenone and isolevoglucosenone allow the parallel kinetic resolution of a racemic nitrone

Cycloadditions of chiral pyrroline N-oxides to levoglucosenone (1) and isolevoglucosenone (2) proceed with a high level of double asymmetric induction. Partial kinetic resolutions (KR) of both enantiomers of a racemic nitrone 3 were achieved, and a parallel kinetic resolution (PKR) experiment allowed the stereoselective divergent synthesis of two quasienantiomeric imino-C-disaccharide precursors.     

Catalytic intermolecular Pauson-Khand-type reaction: strong directing effect of pyridylsilyl and pyrimidylsilyl groups and isolation of Ru complexes relevant to catalytic reaction

Some circumstantial evidence for the directing effect of the 2-pyridylsilyl group in the Ru-catalyzed intermolecular Pauson-Khand-type reaction (PKR) of alkenyl(2-pyridyl)silane, alkyne, and carbon monoxide has been provided. Most importantly, we have succeeded in isolating several monometallic Ru complexes relevant to the catalytic reaction: Ru(vinylsilane)(CO)(3) complexes and ruthenacyclopentene. While the stoichimetric reaction of the Ru(vinylsilane)(CO)(3) complex with an alkyne led to the formation of the corresponding cyclopentenone (PKR product) at 100 degrees C, the ruthenacyclopentene intermediate was quantitatively produced at 50 degrees C. This complex was also converted to a cyclopentenone upon heating at 100 degrees C. Moreover, it was also found that the Ru(vinylsilane)(CO)(3) complex and ruthenacyclopentene serve as catalysts in intermolecular PKR.     

Synthesis and application of bimetallic chiral [Co(salen)]‐type complexes: a new catalytic approach to synthesis of optically pure β‐blockers via kinetic resolution of epichlorohydrin

A series of bimetallic chiral [Co(salen)]‐type complexes were successfully applied for the synthesis of optically pure β‐blockers via phenolic kinetic resolution (PKR) of racemic epichlorohydrin [2‐(chloromethyl)oxirane; ( ± )ECH]. The reaction proceeded readily at room temperature and consequently provided enantiomerically enriched corresponding α‐aryloxy alcohols with excellent enantioselectivities of up to 98% ee. The PKR method described in this work is highly efficient and straightforward strategy for the synthesis of chiral building blocks. Copyright © 2008 John Wiley & Sons, Ltd.     

Regiochemistry of S-alkyl-substituted alkynes in Pauson-Khand reactions. Is a correlation with X-ray data and charge distribution calculations of the Co(2)(CO)(6)-alkyne complexes possible?

[reaction: see text] Pauson-Khand reactions (PKR) of RSCtbd1;CR' (6-10) yielded in all cases as the main product the regioisomer with the alkyl-S group disposed alpha to the CO group (16a-22a). Correlation of these results with X-ray data and charge distribution calculations of the corresponding dicobalthexacarbonyl complexes proves that a recently postulated "trans effect" in these complexes is not suitable for predicting the regiochemical outcome of the PKR unambiguously.     

Quasienantiomers and quasiracemates: new tools for identification, analysis, separation, and synthesis of enantiomers

The old adage “never mix pure organic compounds” holds in spades for enantiomers. After going to all the trouble to make enantiopure molecules, who in their right mind would ever mix them to make a racemate? Quasienantiomers are almost enantiomers, but not quite. Yet unlike enantiomers, the interest is not so much in separating them but in mixing them to make quasiracemates. This backwards thinking opens new possibilities for identification, analysis, separation and synthesis of enantiomers. A short history is provided, the terms are defined and illustrated, and recent applications of quasienantiomers, quasiracemates and related species are reviewed.     

The Pauson-Khand reaction as a new entry to the synthesis of bridged bicyclic heterocycles: application to the enantioselective total synthesis of (−)-alstonerine

The first application of the Pauson-Khand reaction (PKR) to the synthesis of azabridged bicyclic structures is described. Compounds containing azabicyclo[3.3.1]nonane and azabicyclo[3.2.1]octane rings fused to cyclopentenones were efficiently constructed via the PKR of cis-2,6-disubstituted N-acyl piperidine enyne substrates, many of which can be readily prepared from 4-methoxypyridine in a few steps. Moreover, the PKR of cis-2,6-disubstituted piperazine enynes allowed the preparation of diazabicyclo[3.3.1]nonanes fused to cyclopentenones. This new strategy for the synthesis of azabridged bicyclic frameworks was exploited as a key step in a concise, enantioselective total synthesis of the macroline alkaloid (−)-alstonerine.     

Dynamic kinetic resolution (DKR) using immobilized amine nucleophiles

The first example of a dynamic kinetic resolution (DKR) using immobilized amine nucleophiles is described. This approach utilizes a nucleophilic amine attached to a solid phase resin via an organic spacer. The optical purities of the N-substituted α-amino ester products are superior to the solution phase DKR with diastereomeric ratios ranging from 11:1 to 18:1 and chemical yields between 66% and 98%.     

A sub-stoichiometric tungsten-mediated Pauson-Khand reaction: Scope and limitations

Terminal alkynes do not partake in sequential [2+2+1]/benzoannulation reactions with Fischer carbene complexes and highly reactive and sterically hindered olefins; instead, they undergo a Fischer carbene complex-catalyzed Pauson-Khand reaction (PKR). This result has allowed the development, for the first time, of a tungsten-catalyzed PKR, although of limited scope.     

The Pauson-Khand reaction, a powerful synthetic tool for the synthesis of complex molecules

There are still some synthetic chemists who hesitate to use metal-mediated or -catalysed reactions. The Pauson-Khand reaction (PKR) is a powerful transformation that has now been sufficiently well developed to be routinely considered when planning a synthesis, especially of polycyclic complex molecules. This tutorial review aims to encourage the use of this process explaining the best ways of performing a PKR both in the stoichiometric and the catalytic version, showing the scope of the process and its limitations. Additionally, asymmetry can be introduced in the reaction using several strategies, which will be discussed. The most recent examples of the synthetic applications of the PKR in natural product synthesis will give the reader an idea of the great usefulness of this reaction.     

Palladium‐Catalyzed Enantioselective C−H Olefination of Diaryl Sulfoxides through Parallel Kinetic Resolution and Desymmetrization

The first example of Pd^II‐catalyzed enantioselective C?H olefination with non‐chiral or racemic sulfoxides as directing groups was developed. A variety of chiral diaryl sulfoxides were synthesized with high enantioselectivity (up to 99 %) through both desymmetrization and parallel kinetic resolution (PKR). This is the first report of Pd^II‐catalyzed enantioselective C(sp²)?H functionalization through PKR, and it represents a novel strategy to construct sulfur chiral centers.     

Efficient construction of the bicyclo[5.3.0]decenone skeleton based on the Rh(I)-catalyzed allenic Pauson-Khand reaction

A reliable procedure for constructing a bicyclo[5.3.0]deca-1,7-dien-9-one ring system by rhodium-catalyzed Pauson-Khand reaction (PKR) of allenynes with a sulfonyl group has been developed. Investigation of the rhodium-catalyzed PKR on 19 examples of 1,2-nonadien-8-yne derivatives demonstrated that (i) acceptable yields could be consistently achieved through the proper choice of the rhodium catalyst ([RhCl(CO)(2)](2) or [RhCl(CO)dppp](2)) depending on the starting allenyne and that (ii) an ester functionality as well as hydroxy and siloxy groups could be tolerated in this rhodium-catalyzed PKR.     

Deuterium isotope effects observed during competitive binding chiral recognition electrospray ionization--mass spectrometry of cinchona alkaloid-based systems

Deuterium isotope effects are reported for binding between tert-butylcarbamoyl-quinine/quinidine chiral selectors and isotopomeric quasienantiomers of N-(3,5-dinitrobenzoyl)leucine measured using electrospray ionization-mass spectrometry (ESI-MS) and competitive binding. Evaluation of mixtures of each selector with one labeled and one unlabeled enantiomeric selectand of identical configuration showed a significant difference in measured ion abundances of diastereomeric complexes between the selector and each selectand. It was found that in some cases, the complex containing the nondeuterated selectand was 15% more abundant than its deuterated counterpart. On the basis of an assessment of solution- and gas-phase isotope effects reported in the literature, a series of control experiments were performed to study the origin of the effects. On the basis of these measurements, our preliminary conclusion is that the differing gas-phase physicochemical nature of the deuterated versus nondeuterated selectand represents the strongest contribution to the observed effect in this chiral molecular recognition system.     

Asymmetric Total Synthesis of Propindilactone G,Part 1: Initial Attempts towards the Synthesis of Schiartanes

Our first‐generation synthetic study towards the total synthesis of propindilactone G ( 1 ) and its analogues is reported. The key synthetic steps were an intramolecular Pauson–Khand reaction (PKR) and a vinylogous Mukaiyama reaction (VMAR). The stereoselective synthesis of the CDE ring moiety with an all‐carbon quaternary center through a PKR was difficult, whilst a VMAR afforded a product with the opposite stereochemistry at the C20 position on the side chain. These results led us to redesign our synthetic strategy for the total synthesis of compound 1 .     

Development of intermolecular additive free Pauson-Khand reactions for estrone E-ring extension using microwaves

Intermolecular additive free microwave (MW) promoted and cobalt octacarbonyl mediated Pauson-Khand reaction (PKR) performance was improved for estrone ring extension. The reaction development with norbornene and cyclopentene produced cyclopentenones in yields of comparable levels with those previously obtained with the aid of chemical additives as promoters. The PKRs with norbornene demonstrated that a low cobalt complex concentration increases yields, especially for the aliphatic alkynes. Furthermore, a boost for the MW PKR could be obtained by a gradual cobalt complex addition. These expedients combined with the use of an excess of free and cobalt complexed alkyne led to successful estrone ring system extension with various alkynes.     

Enantioselective Rhodium-Catalyzed Pauson–Khand Reactions of 1,6-Chloroenynes with 1,1-Disubstituted Olefins

An enantioselective rhodium(I)-catalyzed Pauson–Khand reaction (PKR) using 1,6-chloroenynes that contain challenging 1,1-disubstituted olefins is described. In contrast to the previous studies with these types of substrates, which are only suitable for a single type of tether and alkyne substituent, the new approach results in a more expansive substrate scope, including carbon and heteroatom tethers with polar and non-polar substituents on the alkene. DFT calculations provide critical insight into the role of the halide, which pre-polarizes the alkyne to lower the barrier for metallacycle formation and provides the proper steric profile to promote a favorable enantiodetermining interaction between substrate and chiral diphosphine ligand. Hence, the chloroalkyne enables the efficient and enantioselective PKR with 1,6-enynes that contain challenging 1,1-disubstituted olefins, thereby representing a new paradigm for enantioselective reactions involving 1,6-enynes.     

An organocascade kinetic resolution

Products of a novel iminium-catalyzed oxa-Michael addition undergo a kinetic resolution by a subsequent enamine-catalyzed intermolecular reaction. This is a rare example of kinetic resolution by enamine catalysis and the first organocascade kinetic resolution. This resolution produces enantioenriched 2,6-cis-tetrahydropyrans and, notably, cascade products with absolute and relative configurations normally not observed using this diphenyl prolinol silyl ether. This resolution thus provides new insight into asymmetric induction in reactions employing this catalyst.     

Rh(I)-catalyzed Pauson-Khand reaction and cycloisomerization of allenynes: selective preparation of monocyclic, bicyclo[m.3.0], and bicyclo[5.2.0] ring systems

[reaction: see text] Rhodium(I)-catalyzed PKR of allenynes was found to be applicable for constructing azabicyclo[5.3.0]decadienone as well as oxabicyclo[5.3.0]decadienone frameworks. In addition, a reliable procedure for constructing a 10-monosubstituted bicyclo[5.3.0]deca-1,7-dien-9-one ring system by the rhodium(I)-catalyzed PKR of allenynes was developed under the condition of 10 atm of CO. Investigation of the rhodium(I)-catalyzed cycloisomerization of 4-phenylsulfonylnona-2,3-dien-8-ynes under nitrogen atmosphere gave the corresponding cyclohexene derivatives, whereas the C1-homologated allenynes produced cycloheptene derivatives and/or bicyclo[5.2.0]nonene skeletons depending on the substitution pattern at the allenic terminus. Thus, proper choice of the starting allenynes and reaction conditions led to the selective formation of 2-phenylsulfonylbicyclo[5.3.0]deca-1,7-dien-9-ones (Pauson-Khand-type product), 3-alkylidene-1-phenylsulfonyl-2-vinylcycloheptene derivatives, and bicyclo[5.2.0]nonene frameworks.