Diastereoselectivity in Diels-Alder Cycloadditions of Erythrose Benzylidene-acetal 1,3-Butadienes with Maleimides (original) (raw)
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Organic & Biomolecular Chemistry, 2011
In this study, the origins of diastereoselectivity in the hydrogen bonding assisted Diels-Alder reactions of chiral dienes with achiral dienophiles have been investigated with density functional methods. The distortion/interaction model has been applied to shed light on the origins of selectivity. C9-Substituted chiral anthracene templates (R = (CH 3 )(OCH 3 )(H), R = (CH 3 )(OH)(H), R = (CH 3 )(CH 2 CH 3 )(H) and R = (-CH 2 -C(CH 3 )(OCH 3 )(H)) are used to rationalize the role of a stereogenic center and H-bonding on the product distribution ratio. Even though hydrogen bonding increases the reactivity of the diene, the stereoselectivity is reduced because of the hydrogen bonding capacity of both diastereomeric transition states. The interaction energies of the studied anthracene templates with N-methyl maleimide at the transition state correlate linearly with an increase in reactivity. The selectivity is determined by both favorable distortion and interaction energies. The p-facial selectivity induced by the presence of a chiral auxiliary in 1-substituted 1,3-pentadienes (R1 = (CH 3 )(OCH 3 )(H) and R1 = (CH 3 )(OH)(H)) has also been modeled in order to rationalize the role of the stereogenic center and H-bonding on the stereoselectivity of an aliphatic diene. In both parts, the product distribution ratios calculated from Boltzmann distributions based on Gibbs free energies are in reasonable agreement with the experimental results. Finally the role of OH-substituted five-membered pyrrolidine on C9 of anthracene is investigated since the successful usage of the conformationally rigid pyrrolidines in asymmetric synthesis is well known. Overall, both in the acyclic system and in anthracene, the facilitation due to H-bonding is reflected in the interaction energies: the higher the difference in interaction energies in the transition structures of the two diastereomers, the more selective the H-bonding assisted Diels-Alder reaction is. opens up the possibility of stereoselectively functionalizing achiral dienophiles by means of a Diels-Alder/retro-Diels-Alder sequence. 2-4 The Diels-Alder/retro-Diels-Alder strategy involves a binding-transforming-releasing sequence, which can be regarded as a basic mimic of enzymes, nature's complex homochi-
Diastereoselective Diels-Alder reactions. The role of the catalyst
Arkivoc, 2001
The Diels-Alder reaction between (R)-(−)-methyl (Z)-3-(4,5-dihydro-2-phenyl-4-oxazolyl)-2-propenoate (1) and cyclopentadiene in the presence of one equivalent of Et 2 AlCl gave stereochemical results opposite to those obtained with one equivalent of EtAlCl 2 . Energy minimizations of proposed complexes of these Lewis acids with the chiral dienophile at the RHF/3-21G level suggest that the aluminum is tetrahedrally complexed with Et 2 AlCl, but bound in a trigonal bipyramid with EtAlCl 2 . These complexes expose the diastereotopic faces of the dienophile to reaction with diene.
New carbohydrate-based chiral auxiliaries in Diels–Alder reaction
Tetrahedron-asymmetry, 1998
The carbohydrate derivatives 1–5 were evaluated as chiral auxiliaries in the Diels–Alder reaction of its acrylate derivatives 6a–e with cyclopentadiene promoted by Lewis acids. Although excellent endo:exo ratios (98:2) were obtained in many cases, the π-facial selectivities were from low to moderate (up to 60% d.e.). An effect of non-coordinating solvents reversing the stereoselectivities of adducts obtained from acrylates 6a,b
Highly Diastereoselective Diels−Alder Reaction Using a Chiral Auxiliary Derived from Levoglucosenone
Organic Letters, 2005
A new chiral auxiliary derived from levoglucosenone is reported. The compound is obtained by a cycloaddition reaction with 9-methoxy methylanthracene followed by a diastereoselective reduction of the C-2 keto function. The auxiliary has been used as a chiral template in an asymmetric Diels−Alder reaction of the corresponding acrylic ester derivative with cyclopentadiene. The results showed excellent diastereomeric excess even at room temperature when the reaction was promoted by Et 2 AlCl as the Lewis acid.
Tetrahedron: Asymmetry, 1991
Diels-Alder reaction of the achiral I-silyloxybutadiene la with the chiral acylnitroso dienophile 2a gave cycloadduct 4 in high diastereomeric excess (de. > 98 %), a result which is undoubtedly due to the C-2 symmetrical chiial dimethylpyrrolidine inductor. Excellent d.e. was also observed when the (R)-prolinol dienophile 2d was reacted with the chiral diene lb (d.e. = 96 a), whereas cycloaddition of the (S) enantiomer 2e with lb gave only poor asymmetric induction (d.e. = 4 a). These two latter examples nicely illustrate the influence of "matched pair" (lb/2d) versus "mismatched pair" (lbne) upon double asymmetric induction. All herein sported Diels-A~ ~~lo~ti~s were ~~~~if~.