Temperature and Viscosity Dependence of the Spin-Directed Stereoselectivity of the Carbonyl-Alkene Photocycloaddition

Griesbeck, Axel G.; Bondock, Samir; Gudipati, Murthy S.

doi:10.1002/1521-3773(20011217)40:24<4684::aid-anie4684>3.0.co;2-9

Cited by 30 publications

(12 citation statements)

References 35 publications

(21 reference statements)

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“…1 In recent publications we have described the effect of substrate concentration on the spin-selectivity of the Paternò-Büchi reactions 2 of aliphatic aldehydes with 2,3-dihydrofuran 3 and allylic alcohols, 4 as well as the temperature dependence of these reactions. 5 In light of recent research activities in the field of enantioselective cyclooctene photoisomerization 6 and diastereoselective photocycloaddition, 7 we became interested in spin-directed effects on both the addition of electronically excited carbonyl compounds and the photoisomerization process of cyclooctene.…”

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confidence: 99%

Spin-imposed stereoselection in the photocycloaddition of (Z)- and (E)-cyclooctene to aliphatic aldehydes

Griesbeck

Bondock

2002

Photochem Photobiol Sci

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confidence: 99%

Spin-imposed stereoselection in the photocycloaddition of (Z)- and (E)-cyclooctene to aliphatic aldehydes

Griesbeck

Bondock

2002

Photochem Photobiol Sci

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“…3 It is also based on the comparison of the intermediate 1,4-triplet biradical conformers as depicted in Fig. 2, however, with emphasis on the additional anomeric stabilization in structure C and B that is not present in A (no lone pair at oxygen antiperiplanar to the ring oxygen).…”

Section: Resultsmentioning

confidence: 99%

Photo aldol reactions with 5-methoxyoxazoles: Highly regio- and diastereoselective synthesis of α-amino β-hydroxy carboxylic acid derivatives

Griesbeck¹,

Bondock²

2003

Can. J. Chem.

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A versatile route to derivatives of α-amino β-hydroxy carboxylic acids, either with tertiary (from a corresponding glycine equivalent) or a quaternary α-carbon center is described. The key reaction is the cycloaddition of electronically excited carbonyl compounds (aromatic and aliphatic aldehydes, respectively) to oxazoles. To make the products hydrolytically more labile, a methoxy substituent at position C-5 was introduced leading to the formation of cycloadducts with an orthoester substructure. The photocycloaddition, either with triplet excited (aromatic) carbonyls or with singlet excited (aliphatic) carbonyls, led to the formation of mixtures of endo-and exo-diastereoisomers with moderate to very high exo-selectivities. The 4-unsubstituted 5-methoxyoxazole 1 (glycine equivalent) gave the [2 + 2]-adducts 3a-3f with aldehydes 2a-2f in high yields and excellent diastereoselectivities. Hydrolysis of these compounds resulted in the α-amino β-hydroxy esters 4a-4f with preferred erythro (S*,S*) configuration. As an extension of this process, 4-alkylated 5-methoxyoxazoles 5a-5f were applied as alkene components and the corresponding cycloadducts with benzaldehyde 6a-6f were obtained. Again, the exo-diastereoisomers were formed as the major products in diastereoselectivities from 73:27 (exo:endo) up to >95:5. Hydrolysis of these adducts resulted in the formation of α-amino β-hydroxy esters 7a-7f with like (S*,S*) configuration of the major diastereoisomers.Résumé : On décrit une voie versatile pour obtenir des acides α-amino-β-hydroxycarboxyliques portant des centres α-carbonés tertiaire (à partir de la glycine équivalente correspondante) ou quaternaire. La réaction clé est une cycloaddition de composés carbonylés électroniquement excités (aldéhydes aromatiques et aliphatiques respectivement) à des oxazoles. Afin de que les produits soient plus facilement hydrolysables, on a introduit un substituant méthoxyle en position C-5 afin d'obtenir des cycloadduits comportant une structure orthoester. La photocycloaddition, avec des carbonyles (aromatiques) excités à l'état triplet ou avec des carbonyles (aliphatiques) excités à l'état singulet, conduit à la formation de mélanges de diastéréomères endo-et exo-dans lesquels la sélectivité en faveur de l'exo-est de modérée à très élevée. La réaction du 5-méthoxyoxazole non substitué en position 4 (1) (équivalent de la glycine) avec les aldéhy-des 2a-2f donne les adduits [2 + 2] 3a-3f avec des rendements élevés et d'excellentes diastéréosélectivités. L'hydrolyse de ces composés conduit aux α-amino-β-hydroxyesters 4a-4f avec la configuration érythro (S*,S*) pré-férée. Comme extension de ce procédé, on a utilisé les 5-méthoxyoxazoles alkylées en position 4 (5a-5f) comme composants alcéniques et, par réaction avec les benzaldéhyde, on a obtenu les cycloadduits correspondants 6a-6f. Une fois de plus, les diastéréoisomères se forment comme produits majeurs, avec des diastéréosélectivités allant de 73:27 (exo:endo) jusqu'à >95:5. L'hydrolyse de ces produits conduit à la formation des α...

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“…Singlet and triplet electronic states are involved, and the exo-endo selectivity can be affected by a number of factors, including temperature, concentration and solvent viscosity. 82 An unusual observation has recently been documented in the case of the reaction of benzophenone with cis-and transcyclooctene. With cis-cyclooctene 63, reactions at higher temperature actually led to an increase in the proportion of the less stable trans-cycloadduct 65 (Scheme 29).…”

Section: Three-membered Ringsmentioning

confidence: 96%

Saturated oxygen heterocycles

Elliott

2002

J. Chem. Soc., Perkin Trans. 1

143

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Temperature and Viscosity Dependence of the Spin-Directed Stereoselectivity of the Carbonyl-Alkene Photocycloaddition

Cited by 30 publications

References 35 publications

Spin-imposed stereoselection in the photocycloaddition of (Z)- and (E)-cyclooctene to aliphatic aldehydes

Spin-imposed stereoselection in the photocycloaddition of (Z)- and (E)-cyclooctene to aliphatic aldehydes

Photo aldol reactions with 5-methoxyoxazoles: Highly regio- and diastereoselective synthesis of α-amino β-hydroxy carboxylic acid derivatives

Saturated oxygen heterocycles

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