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K A Woerpel - One of the best experts on this subject based on the ideXlab platform.
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stereoelectronic model to explain highly stereoselective reactions of seven membered ring Oxocarbenium ion intermediates
Angewandte Chemie, 2016Co-Authors: Matthew G Beaver, Trixia M Buscagan, Olga Lavinda, K A WoerpelAbstract:Nucleophilic attack on seven-membered-ring Oxocarbenium ions is generally highly stereoselective. The preferred mode of nucleophilic attack forms the product in a conformation that minimizes transannular interactions, thus leading to different stereoselectivity as compared to that of reactions involving six-membered-ring Oxocarbenium ions.
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effect of conformational rigidity on the stereoselectivity of nucleophilic additions to five membered ring bicyclic Oxocarbenium ion intermediates
Organic and Biomolecular Chemistry, 2014Co-Authors: Olga Lavinda, Vi Tuong Tran, K A WoerpelAbstract:Nucleophilic substitution reactions of five-membered ring acetals bearing fused rings reveal that subtle changes in the structure of the fused ring can exert dramatic influences on selectivity. If the fused ring did not constrain the five-membered ring undergoing substitution, selectivity was comparable to what was observed for an unconstrained system (≥92% diastereoselectivity, favoring the product of inside attack on the Oxocarbenium ion). If the ring were more constrained by including at least one oxygen atom in the ring, selectivity dropped considerably (to 60% diastereoselectivity in one case). Transition states of the nucleophilic addition of allyltrimethylsilane to selected Oxocarbenium ions were calculated using DFT methods. These computational models reproduced the correlation between additional conformational rigidity and selectivity.
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nucleophilic substitution reactions of 2 phenylthio substituted carbohydrate acetals and related systems episulfonium ions vs Oxocarbenium ions as reactive intermediates
ChemInform, 2008Co-Authors: Matthew G Beaver, Susan B Billings, K A WoerpelAbstract:A powerful approach for controlling the stereochemistry of glycosylation reactions involves using a directing group at an adjacent carbon to influence the stereochemistry of nucleophilic attack. The use of substituents such as I, PhS, and PhSe has proven to be especially valuable because it enables the preparation of 2-deoxysugars after reductive removal of the directing group. The mechanisms of these reactions are generally considered to involve nucleophilic opening of bridged onium ions and to proceed with inversion. Onium ions, however, equilibrate with open cations, and these open forms can be favored when the cation is highly stabilized. Although the selectivities of reactions of carbohydrate-derived cations are consistent with the intermediacy of onium ions, the reactions of acyclic cations are not. Instead, consideration of the preferred conformations of the Oxocarbenium ions and stereoelectronically favored additions to these intermediates provides a predictive model to explain the diastereoselectivities observed in reactions of Oxocarbenium ions bearing proximal heteroatoms.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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electrostatic effects on the reactions of cyclohexanone Oxocarbenium ions
Journal of Organic Chemistry, 2006Co-Authors: Glen Baghdasarian, K A WoerpelAbstract:Nucleophilic substitution reactions of 4-substituted cyclohexanone acetals display different selectivities depending upon the electronic nature of the substituent. Alkyl groups favor equatorial positions in the Oxocarbenium ions, but alkoxy groups prefer axial conformers. The reactions of acetals with alkoxy groups constrained to either equatorial or axial positions suggest that the presence of an axial alkoxy group distorts the Oxocarbenium ion, changing its inherent preferences for facial attack.
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nucleophilic additions of trimethylsilyl cyanide to cyclic Oxocarbenium ions evidence for the loss of stereoselectivity at the limits of diffusion control
Journal of the American Chemical Society, 2006Co-Authors: Siddhartha R Shenoy, Deborah M Smith, K A WoerpelAbstract:The limitations of stereoelectronic models in assessing the stereoselective nucleophilic substitution reactions of cyclic Oxocarbenium ions at high reaction rates are discussed. Evidence is provided suggesting that the diastereoselectivity of nucleophilic substitution reactions is attenuated at the limits of diffusion control. The low diastereoselectivities observed in the reactions of trimethylsilyl cyanide with five- and six-membered ring Oxocarbenium ions are attributed to the high reactivity of the nucleophile and its reactions with these electrophiles at diffusion control rates.
Michael P Jennings - One of the best experts on this subject based on the ideXlab platform.
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syntheses of cryptocaryolone and cryptocaryolone diacetate via a diastereoselective oxy michael addition and Oxocarbenium allylation
Journal of Organic Chemistry, 2012Co-Authors: Aymara M M Albury, Michael P JenningsAbstract:The total syntheses of both (−)-cryptocaryolone and (−)-cryptocaryolone diacetate is presented herein. The usage of a diastereoselective oxy-Michael addition/benzylidene acetal formation coupled with a selective axial Oxocarbenium allylation allowed for the preparation of the α-C-glycoside moiety present in the bicyclic bridged structure. In addition, the syn-1,3-diol of the linear portion was installed via a Wacker oxidation followed by a subsequent directed reduction of the appropriate homoallylic alcohol precursor.
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a formal synthesis of cyanolide a featuring a stereoselective mukaiyama aldol reaction and Oxocarbenium reduction
Journal of Organic Chemistry, 2011Co-Authors: Robert J Sharpe, Michael P JenningsAbstract:The formal synthesis of the marine natural product (−)-cyanolide A is presented. The synthetic strategy is centered on two acyclic diastereoselective reactions and a single cyclic reaction with modest to excellent dr based on an initial stereocenter. Most notable is a highly stereoselective Oxocarbenium reduction based on a “mismatched” reactive conformer to afford the β-C-glycoside subunit leading to an efficient synthesis of the diolide aglycon in 12 overall steps.
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synthesis of the purported ent pochonin j structure featuring a stereoselective Oxocarbenium allylation
Journal of Organic Chemistry, 2011Co-Authors: Dionicio Martinezsolorio, Kenneth A Belmore, Michael P JenningsAbstract:The synthesis of the alleged natural product pochonin J is presented. Key steps of this convergent synthesis include a chemoselective Wacker oxidation, and an Evans’ anti-reduction of the resulting ketone. Upon ozonolysis, this intermediate undergoes a 6-exo-trig cyclization to give a hemiketal intermediate, the key Oxocarbenium precursor. The construction of the α-C-glycoside subunit is highlighted by a mismatched Oxocarbenium cation formation/allylation sequence. An olefin metathesis afforded the 14-membered macrolactone, and final oxidation provided the “desired” compound that does not spectroscopically correlate to the initially described natural product.
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formal synthesis of neopeltolide featuring a highly stereoselective Oxocarbenium formation reduction sequence
ChemInform, 2010Co-Authors: Dionicio Martinezsolorio, Michael P JenningsAbstract:The synthesis of macrolactone (IV) as precursor of unnatural neopeltolide is achieved via allylation of lactone (I) and subsequent stereoselective reduction of an Oxocarbenium compound, generated in situ.
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convergent synthesis of aspergillide b via a highly diastereoselective Oxocarbenium allylation
Tetrahedron Letters, 2010Co-Authors: Amanda Mueller J Hendrix, Michael P JenningsAbstract:The synthesis of (+)-aspergillide B is described by means of a highly diastereoselective Oxocarbenium allylation followed by a cross-metathesis and final Yamaguchi macrolactonization.
Paul E Floreancig - One of the best experts on this subject based on the ideXlab platform.
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stereocontrolled cyanohydrin ether synthesis through chiral bronsted acid mediated vinyl ether hydrocyanation
Journal of Organic Chemistry, 2013Co-Authors: Chunliang Lu, Xiaoge Su, Paul E FloreancigAbstract:Vinyl ethers can be protonated to generate Oxocarbenium ions that react with Me3SiCN to form cyanohydrin alkyl ethers. Reactions that form racemic products proceed efficiently upon conversion of the vinyl ether to an α-chloro ether prior to cyanide addition in a pathway that proceeds through Bronsted acid-mediated chloride ionization. Enantiomerically enriched products can be accessed by directly protonating the vinyl ether with a chiral Bronsted acid to form a chiral ion pair. Me3SiCN acts as the nucleophile and PhOH serves as a stoichiometric proton source in a rare example of asymmetric bimolecular nucleophilic addition into an Oxocarbenium ion. Computational studies have provided a model for the interaction between the catalyst and the Oxocarbenium ion.
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prins type cyclization reactions in natural product synthesis
European Journal of Organic Chemistry, 2013Co-Authors: Paul E FloreancigAbstract:Prins-type cyclization reactions proceed through the intramolecular addition of a π-nucleophile to an Oxocarbenium ion or related species to generate a ring through carbon–carbon bond formation, often with excellent and predictable stereocontrol. The widespread presence of naturally occurring oxygen-containing heterocycles has made these reactions exceedingly valuable for total synthesis efforts. This microreview covers several common variations of Prins-type cyclizations and highlights their applicability to the total synthesis of natural products and analogs.
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cyclopropane compatibility with oxidative carbocation formation total synthesis of clavosolide a
Organic Letters, 2012Co-Authors: Guangrong Peh, Paul E FloreancigAbstract:Cyclopropane-substituted allylic ethers react with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to form Oxocarbenium ions with no competitive ring cleavage. This reaction can be used for the preparation of cyclopropane-substituted tetrahydropyrans. The protocol was used as a key step in the total synthesis of the sponge-derived macrolide clavosolide A.
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Synthesis of Bridged Inside–Outside Bicyclic Ethers through Oxidative Transannular Cyclization Reactions
2012Co-Authors: Xun Han, Paul E FloreancigAbstract:The classical geometry of the 6-endo transition state for nucleophilic additions into Oxocarbenium ions can be perturbed by incorporating the reactive groups into medium-sized rings, leading to the formation of 2,6-trans-dialkyl tetrahydropyrans. The bicyclic products exhibit inside–outside stereoisomerism, as seen in numerous macrolide natural products
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Aromatic Cations from Oxidative Carbon–Hydrogen Bond Cleavage in Bimolecular Carbon–Carbon Bond Forming Reactions
2012Co-Authors: Dane J. Clausen, Paul E FloreancigAbstract:Chromenes and isochromenes react quickly with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to form persistent aromatic Oxocarbenium ions through oxidative carbon–hydrogen cleavage. This process is tolerant of electron-donating and electron-withdrawing groups on the benzene ring and additional substitution on the pyran ring. A variety of nucleophiles can be added to these cations to generate a diverse set of structures
Xing-wang Wang - One of the best experts on this subject based on the ideXlab platform.
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organo catalyzed asymmetric michael hemiketalization oxa pictet spengler cyclization for bridged and spiro heterocyclic skeletons Oxocarbenium ion as a key intermediate
Organic Letters, 2017Co-Authors: Wei-tai Fan, Chunhua Qiao, Xing-wang WangAbstract:A Michael–hemiketalization–oxa-Pictet–Spengler cyclization has been developed for the construction of chiral bridged and spiro heterocyclic skeletons with one spiro stereogenic carbon center and two bridgehead carbon centers, utilizing cooperative catalysts of a Takemoto thiourea catalyst and a triflimide. In particular, an Oxocarbenium ion acts as a key intermediate for this cyclization reaction. Additionally, biological evaluation of this type of novel structure has revealed obvious antiproliferative activity against some cancer cell lines.
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Organo-Catalyzed Asymmetric Michael–Hemiketalization–Oxa-Pictet–Spengler Cyclization for Bridged and Spiro Heterocyclic Skeletons: Oxocarbenium Ion as a Key Intermediate
2017Co-Authors: Wei-tai Fan, Chunhua Qiao, Xing-wang WangAbstract:A Michael–hemiketalization–oxa-Pictet–Spengler cyclization has been developed for the construction of chiral bridged and spiro heterocyclic skeletons with one spiro stereogenic carbon center and two bridgehead carbon centers, utilizing cooperative catalysts of a Takemoto thiourea catalyst and a triflimide. In particular, an Oxocarbenium ion acts as a key intermediate for this cyclization reaction. Additionally, biological evaluation of this type of novel structure has revealed obvious antiproliferative activity against some cancer cell lines
Daniel Seidel - One of the best experts on this subject based on the ideXlab platform.
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highly acidic conjugate base stabilized carboxylic acids catalyze enantioselective oxa pictet spengler reactions with ketals
Angewandte Chemie, 2020Co-Authors: Zhengbo Zhu, Chenfei Zhao, Minami Odagi, Khalil A Abboud, Helmi Ulrika Kirm, Jaan Saame, Mart Lokov, Ivo Leito, Daniel SeidelAbstract:Acyclic ketone-derived Oxocarbenium ions are involved as intermediates in numerous reactions that provide valuable products, however, they have thus far eluded efforts aimed at asymmetric catalysis. We report that a readily accessible chiral carboxylic acid catalyst exerts control over asymmetric cyclizations of acyclic ketone-derived trisubstituted Oxocarbenium ions, thereby providing access to highly enantioenriched dihydropyran products containing a tetrasubstituted stereogenic center. The high acidity of the carboxylic acid catalyst, which exceeds that of the well-known chiral phosphoric acid catalyst TRIP, is largely derived from stabilization of the carboxylate conjugate base through intramolecular anion-binding to a thiourea site.
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direct formation of Oxocarbenium ions under weakly acidic conditions catalytic enantioselective oxa pictet spengler reactions
Journal of the American Chemical Society, 2016Co-Authors: Chenfei Zhao, Shawn B Chen, Daniel SeidelAbstract:Two catalysts, an amine HCl salt and a bisthiourea, work in concert to enable the generation of Oxocarbenium ions under mild conditions. The amine catalyst generates an iminium ion of sufficient electrophilicity to enable 1,2-attack by an alcohol. Catalyst turnover is achieved by amine elimination with concomitant formation of an Oxocarbenium intermediate. The bisthiourea catalyst accelerates all of the steps of the reaction and controls the stereoselectivity via anion binding/ion pair formation. This new concept was applied to direct catalytic enantioselective oxa-Pictet–Spengler reactions of tryptophol with aldehydes.
