- The origin of stereoselectivity in cycloaddition reactions promoted by stereoisomers of 8-phenylmenthyl glyoxylate oxime
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A structural study of three synthesized stereoisomeric oximes, (-)-8-phenylmenthyl glyoxylate oxime (8-PMGO), (+)-8-phenylneomenthyl glyoxylate oxime (8-PnMGO), and (-)-8-phenylisoneomenthyl glyoxylate oxime (8-PinMGO), was performed by means of variable temperature 1H NMR spectroscopy, X-ray crystallography, and ab initio calculations. It was found that in 8-PMGO a conformation where the phenyl and oxime moieties are stacked is significantly favored, whereas in the other stereoisomers this preference was not so evident. The conformational differences found between the isomers were used to rationalize the outcome of the reaction (simultaneous 1,3-cycloaddition and aza-Diels-Alder reaction) between the referred oximes and cyclopentadiene, in which the stereoselectivity was evaluated and found to be nicely reproduced by a simple conformational analysis. The global results indicate that the stereoselectivity of the studied oximes, a bit higher for 8-PMGO, originates from their particular conformational distribution, in which the phenyloxime aromatic interaction plays a decisive role.
- Sousa, Carlos A.D.,Lima, Carlos F.R.A.C.,Andrade, Mariana,García-Mera, Xerardo,Rodriguez-Borges, José E.
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p. 5048 - 5057
(2013/07/04)
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- The use of (-)-8-phenylisoneomenthol and (-)-8-phenylmenthol in the enantioselective synthesis of 3-functionalized 2-azabicyclo[2.2.1]heptane derivatives via aza-Diels-Alder reaction
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The asymmetric aza-Diels-Alder reaction of the (1R)-8-phenylmenthyl or (1R)-8-phenylisoneomenthyl glyoxylate-derived N-benzylimine with cyclopentadiene resulted in the enantioselective synthesis of the corresponding pure [(1S,3-exo)-2-benzyl-2-azabicyclo[2.2.1]hept-5-ene]-3-carboxylates (80 or 69% yield, respectively). Reduction of these cycloadducts with LiAlH4 afforded pure (-)-[(1S,3-exo)-2-benzyl-2-azabicyclo[2.2.1]hept-5-en-3-yl]methanol. Furthermore, a reaction sequence based on Barbier-Wieland degradation of both (1S,3-exo)-adducts afforded pure (+)-(1R)-2-benzoyl-2-azabicyclo[2.2.1]heptan-3-one. In the course of the two transformation sequences referred, the chiral auxiliaries were recovered in a virtually quantitative way.
- Cardoso do Vale, Maria Luísa,Rodríguez-Borges, José Enrique,Caama?o, Olga,Fernández, Franco,García-Mera, Xerardo
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p. 9475 - 9482
(2007/10/03)
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- An efficient method for preparation of chiral arylmenthol glyoxylates
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Glyoxylates of three chiral alcohols were conveniently prepared by reaction of the alcohol with oxalyl chloride followed by reduction of the resulting alkoxy oxalyl chloride with tributyl- tin hydride. The products were isolated in 75-80% yield by straightforward flash chromatography.
- Blanco, Jose M.,Caamano, Olga,Fernandez, Franco,Garcia-Mera, Xerardo,Lopez, Carmen,Rodriguez-Borges, Jose E.,Hergueta, Antonio R.
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p. 1590 - 1592
(2007/10/03)
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- PREPARATION OF THREE DIASTEREOISOMERS OF 2-(1-METHYL-1-PHENYLETHYL)-5-METHYLCYCLOHEXAN-1-OL FROM (R)-(+)-PULEGONE
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New isolation of (1R,2S,5R)-(-)-2-(1-methyl-1-phenylethyl)-5-methylcyclohexan-1-ol ((-)-8-phenylmenthol, Ia), prepared from (R)-(+)-pulegone, is described.The method consists in the preparation of phenylcarbamate of Ia and its transesterification with ethanol.Further two diastereoisomers of (-)-8-phenylmenthol were isolated: the (1S,2R,5R)-isomer IIa and the (1R,2R,5R)-isomer IIIa.Compounds Ia, IIa and IIIa were converted into their respective glyoxylates Ic, IIc and IIIc.
- Cervinka, Otakar,Svatos, Ales,Masojidkova, Milena
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p. 491 - 498
(2007/10/02)
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