cis-3,5-Cyclohexadiene-1,2-diol derivatives: facial selectivity in their Diels-Alder reactions with ethylenic, acetylenic and azo dienophiles

The Diels-Alder reactions of maleimide with the acetonide derivative (6a) of cis-3,5-cyclohexadiene-1,2-diol (1a) in various solvents showed facial selectivities ranging from 1 : 1 to 1 : 9. The same derivative 6a reacted in benzene with ethylenic dienophiles with generally modest facial selectivity, but acetylenic dienophiles added exclusively anti to the oxygen functions of 6a. Dimerization of cyclic acetals 6a and 7 was mainly, but for 6a not exclusively, by anti addition with respect to both the diene and the dienophile partners. Reactions of azo dienophiles with derivatives of 1a were predominantly by anti addition, but the diol itself (1a) gave the syn adduct as the major product.     

Complete facial selectivity in the Diels-Alder reaction of a 5-amino-5-carboxycyclopentadiene derivative

5-tert-Butoxycarbonylamino-5-carbethoxy-2-tert-butyldimethylsilyloxy-cyclopentadiene undergoes a Diels-Alder reaction exclusively from the face syn to the nitrogen functionality. Complete reversal of facial bias may be achieved, but at the cost of diminished reactivity, through steric shielding of the N-syn face.     

Endo-exo and facial stereoselectivity in the Diels-Alder reactions of 3-substituted cyclopropenes with butadiene

A computational examination of the four modes of addition in the Diels-Alder reactions of 3-substituted cyclopropene derivatives (substituents: BH(2), CH(3), SiH(3), NH(2), PH(2), OH, SH, F, and Cl) with butadiene have been carried out at the B3LYP/6-31++G(d)//HF/6-31++G(d) level. The degree of stabilization of these derivatives at the ground state correlates with the electronegativity of the substituent. This attenuation of reactivity and differences in steric interactions are the only factors needed to explain both the high facial selectivity and the differences in the endo-exo selectivity seen in these reactions. Furthermore, evidence is presented that indicates that stabilization by an interaction involving the syn C-3 hydrogen of cyclopropene and butadiene is small or irrelevant in controlling the endo-exo selectivity of the Diels-Alder reaction.     

Diels-Alder reactions with dienes bearing a remote stereogenic center. Conformational model for diastereofacial selectivity

In Diels-Alder reactions of 1-acyloxydienes, the 1-O-methylmandeloxy group has the distinct advantage of serving dual functions. Not only is it an effective chiral auxiliary for Diels-Alder stereocontrol, but also it provides rather reliable determination of the absolute stereochemistry of the Diels-Alder adduct, via the Dale-Mosher NMR model, subject only to the ability to resolve the proper NMR resonances. Study of the Diels-Alder reactions of such chiral 1-acyloxydienes has led us to a transition structure model which uniquely explains the origin of the observed stereoselectivities and is supported by experimental evidence, including X-ray structures showing the conformations of three Diels-Alder adducts. The model may also have wider applicability in conformational analysis and control of selectivity in other reactions.     

Diastereofacial selectivity in Diels-Alder cycloadditions involving vinyl sulfoxides

Diastereofacial selectivities observed for Diels-Alder cycloadditions of achiral dienes with chiral vinyl sulfoxides are rationalized in terms of electrostatics. The “nucleophilic” diene adds to the electron-poor face of the “electrophilic” dienophile.     

Catalytic, asymmetric trans-selective hetero Diels-Alder reactions using a chiral zirconium complex

[reaction: see text] The first catalytic, asymmetric 2,3-trans-selective hetero Diels-Alder reaction has been developed. The reactions of aldehydes with Danishefsky's dienes proceeded smoothly to afford the pyranone derivatives in high yields with high trans-selectivities and enantioselectivities in the presence of a chiral zirconium complex, which was prepared from zirconium tert-butoxide and (R)-3,3'-diiodobinaphthol or its derivatives, primary alcohol, and a small amount of water. This reaction was applied to the concise synthesis of (+)-prelactone C.     

对映选择催化 Diels-Alder 反应

本文综述了对映选择催化 Diels-Alder 反应的进展。     

Asymmetric Diels-Alder reactions in ionic liquids

Recent interest in ionic liquids has developed various uses for them, including some applications by synthetic chemists. Ionic liquids have joined the potential list of non-traditional solvents for Diels-Alder reactions. We report here our own efforts to examine the rates and selectivities of carbon Diels-Alder reactions. Our investigations show that excellent diastereoselective and enantioselective carbon Diels-Alder reactions can be achieved in imidazolium ionic solvents at room temperature.     

Iminonitroso Diels-Alder reactions for efficient derivatization and functionalization of complex diene-containing natural products

A remarkably efficient method for derivatization of complex diene-containing natural products by using stabilized iminonitroso Diels-Alder reactions is described. Turimycin H3, ergosterol, reductiomycin, isoforocidin, colchicine and thebaine were found to react with nitrosopyridines in a highly efficient regio- and stereoselective fashion. Preliminary bioactivity evaluations of turimycin cycloadducts are reported.     

Asymmetric induction in Diels-Alder reactions of o-quinodimethanes

Asymmetric inductive effects have been measured on the Diels-Alder reaction of dimethyl fumarate with o-quinodimethane bearing a chiral α-alkoxy group. The chiral substituents used were 1-phenylethoxy, 2-(1-phenyl)propoxy, 1-(2-phenyl)propoxy, 2-(4-phenyl)-butoxy and 1-cyclohexylethoxy. The greatest asymmetric induction was found with the first of these chiral substituents (47% ee). A π-stacking effect, previously suggested as the rationale for asymmetric induction in a similar system, is shown to be inconsistent with the results from this study.     

Enantioselective Diels-Alder reactions with N-hydroxy-N-phenylacrylamide

[reaction: see text] The use of hydroxamic acids as templates for Lewis acid catalyzed enantioselective Diels-Alder reactions has been examined. A very simple chiral Lewis acid, prepared by mixing optically pure binaphthol with 3 equiv of trimethylaluminum, catalyzes the [4 + 2] cycloaddition of N-hydroxy-N-phenylacrylamide with cyclopentadiene at 0 degrees C in high yield (>96%) and with a fairly good level of enantioselectivity (91% ee). Facile conversion of the products to the corresponding alcohols or aldehydes makes the hydroxamic acid intermediates particularly useful.     

Storable, powdered chiral zirconium complex for asymmetric aldol and hetero Diels-Alder reactions

A storable, powdered chiral zirconium catalyst for asymmetric aldol and hetero Diels-Alder reactions has been developed. The catalyst has the same activity as that prepared in situ even after being stored for 3 months. Moreover, this chiral Zr catalyst worked efficiently not only for asymmetric aldol reactions but also for asymmetric hetero Diels-Alder reactions affording the corresponding addition products with high enantioselectivities.     

Tandem Diels-Alder/ene reactions

[reaction: see text] The reactions of unsaturated aldehydes and triene 3 afford adducts via a tandem Diels-Alder/ene reaction.     

A DFT study of Diels-Alder reactions of o-quinone methides and various substituted ethenes: selectivity and reaction mechanism

The Diels-Alder (DA) reactions of various substituted ethenes (methyl vinyl ether (MVE), styrene, and methyl vinyl ketone (MVK)) with o-quinone methides (o-QM) are studied by means of density functional theory (DFT) at the B3LYP/6-31G(d,p) level. On the basis of analysis for frontier molecular orbital and comparison of the activation energies for different reaction pathways, the ortho attack modes present transition structures more stable than the meta ones. The reactivity, ortho selectivity, and asynchronicity are enhanced with the increase of the electron-releasing character of the substitute on ethene fragment. The discussions for the charge distribution and charge transfer on different transition states indicate that there are different molecular mechanisms for the different substituted ethenes. The calculations show that the effect of solvent decreases the activation energy and increases the asynchronicity. The results also indicate that the hydrogen-bond formation between chloroform and the carbonyl oxygen of the o-QM lowers the activation energies and increases the asynchronicity.     

Synergistic catalysis of anthrone Diels-Alder reactions

[reaction: see text] The combination of an amine base and Lewis acid (Li+) leads to synergistic catalysis of the Diels-Alder reaction of anthrone with methyl crotonate and some other less reactive dienophiles. These cycloadditions either do not occur with the individual catalysts used separately or they are greatly accelerated by the combined catalysts. DMSO solvent allows the use of LiCl as the Lewis acid source and can provide greater control of subsequent conversion to Michael adduct.     

Intramolecular Diels-Alder reactions of N-substituted oxazolones

The first intramolecular Diels-Alder reactions of simple trienes featuring an N-substituted oxazolone as the dienophilic component have been investigated and are reported herein.     

Regioselective Diels-Alder reactions of 3-indolylquinones

6-(3-Indolyl)quinolinequinone derivatives gave regioselective Diels-Alder reactions with a variety of dienophiles, yielding polycyclic carbazole derivatives. One-pot reactions, proceeding through a cascade of reactions including regioselective Michael and Diels-Alder steps, gave heptacyclic derivatives starting from indoles and 2,5,8(1H)-quinolinetriones. Double Diels-Alder reactions of 6-(3-indolyl)quinolinequinones and dihalobenzoquinones gave eleven-cycle products in one step.