Mechanistic Insights into the Factors Determining <i>Exo</i><i>−</i><i>Endo</i> Selectivity in the Lewis Acid-Catalyzed Diels−Alder Reaction of 1,3-Dienes with 2-Cycloalkenones
作者:Min Ge、Brian M. Stoltz、E. J. Corey
DOI:10.1021/ol0060026
日期:2000.6.1
We adduce evidence that the asynchronous, Lewis acid-catalyzed Diels-Alderreaction of 2-cycloalkenones with nonsimple alpha,beta-enones can proceed via transition states in which the 1,3-diene subunit is skewed, i.e., nonplanar, with profound effects on the ratio of exo and endo addition products.
Towards the Synthesis of Paulitin: New Insights into the Enyne-Metathesis Mechanism
作者:Laurence Grimaud、Emeline Vedrenne、Frédérique Royer、Julie Oble、Laurent El Kaïm
DOI:10.1055/s-2005-872261
日期:——
In our approach towards Paulitin, a sesquiterpene lactone, we have shown that steric hindrance at the propargylic position is of importance to the outcome of enyne metathesis. Moreover, we have uncovered further evidence to confirm that the alkyne-first pathway should be the favored process in enyne metathesis.
Enantioselective Diels−Alder Cycloadditions with (S<i>S</i>)-2-(<i>p-</i>Tolylsulfinyl)-1,4-naphthoquinone: Efficient Kinetic Resolution of Chiral Racemic Vinylcyclohexenes
作者:M. Carmen Carreño、Antonio Urbano、Claudio Di Vitta
DOI:10.1021/jo9811874
日期:1998.11.1
Chiral racemic vinylcyclohexenes 1a-m, bearing allylic and nonallylic oxygenated substituents on the cyclohexene ring, were submitted to Diels-Alder reactions with enantiomerically pure (SS)(2-p-tolylsulfinyl)-1,4-naphthoquinone. The tandem cycloaddition/pyrolytic sulfoxide elimination led to the formation of enantiomerically enriched angularly tetracyclic quinones (+)-8a and (+)-12, arising from the kinetic resolution of the racemic diene. Dienes la-c, bearing an alkoxy substituent at the allylic position C-3, and 1d-g, with C-3 alkoxy and C-5 methyl groups, gave exclusively quinones (+)-8a-g, resulting from the anti approach to the dienophile. A similar anti selectivity occurred in cycloadditions of dienes 1ij, viith the alkoxy group situated at the allylic position C-6. Nonallylic 4-substituted vinylcyclohexenes 1k-m evolved to ca. 75:25 mixtures of (+)-anti-8k-m and (+)-syn-12k-m derivatives. In all cases, (SS)-(2-p-tolylsulfinyl)-1,4-naphthoquinone reacted from the less hindered face of the more reactive s-cis conformation, giving rise to products in moderate to excellent enantiomeric excesses. Steric effects and torsional interactions in the corresponding approaches account for the observed pi-facial diastereoselectivities at both partners.