Syn−Anti Isomerization of Aldols by Enolization

Ward, Dale E.; Sales, Marcelo Magalhães; Sasmal, Pradip K.

doi:10.1021/ol016581u

Cited by 19 publications

(16 citation statements)

References 32 publications

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“…Selected examples are shown in Table 1. Relatively high enantioselectivities were obtained by using Al III , Sc III , Fe II , Cu II , Cd II , Sn II , Yb III , Lu III , Hg II , Pb II , and Bi III 29. However, the yields were low by using Al III and Cu II in both systems A and B.…”

Section: Resultsmentioning

confidence: 97%

Iron‐ and Bismuth‐Catalyzed Asymmetric Mukaiyama Aldol Reactions in Aqueous Media

Kitanosono

Ollevier

Kobayashi

2013

Chemistry An Asian Journal

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Abstract:We have developed asymmetric Mukaiyama aldol reactions of silicon enolates with aldehydes catalyzed by chiral Fe(II) and Bi(III) complexes.Although previous reactions often required relatively harsh conditions, such as strictly anhydrous conditions, very low temperature (-78 °C), etc., the present reactions proceeded in the presence of water at 0 °C. To find appropriate chiral watercompatible Lewis acids for the Mukaiyama aldol reactions, many Lewis acids combined with chiral bipyridine L1, which was already found to be a suitable chiral ligand in aqueous media, were screened. Three types of chiral catalysts consisting of Fe(II) or Bi(III), L1, and an additive have been discovered, and a wide variety of substrates (silicon enolates and aldehydes) reacted to afford the desired aldols in high yields with high diastereo-and enantioselectivities by appropriate selection of one of the three sets of catalytic systems. Mechanistic studies have elucidated the coordination environments around Fe(II) and Bi(III), and the effect of additives in the chiral catalysts. It is noted that both Brønsted acids and bases worked as efficient additives in Fe(II)-catalyzed reactions. The assumed catalytic cycle and transition states have shown the important roles of water for efficient asymmetric Mukaiyama aldol reactions in aqueous media on the basis of the broadly applicable and versatile catalytic systems.

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Section: Resultsmentioning

confidence: 97%

Iron‐ and Bismuth‐Catalyzed Asymmetric Mukaiyama Aldol Reactions in Aqueous Media

Kitanosono

Ollevier

Kobayashi

2013

Chemistry An Asian Journal

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“…As mentioned previously, all twenty possible diastereomeric forms of hexapropionates 10 are known and methodology studies established access to all possible diastereomers. 4,5,7,13,17 In order to understand the diastereoselectivity in the aldol reaction of ketones 9a-d and aldehyde 6, some terms should be identified. Seebach and Prelog proposed the terms like and unlike to differentiate the reactions of two chiral reactants, 18 in this case ketones 9a-d and aldehyde 6.…”

Section: Preparation Of Hexapropionate Synthons 10mentioning

confidence: 99%

Total Synthesis of Muamvatin

Ward

Zahedi

2012

Org. Lett.

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Enantioselective total synthesis of muamvatin was achieved by ring-chain tautomerization of an acyclic derivative assembled by sequential substrate-controlled stereoselective aldol reactions of a chiral ketone with two achiral aldehydes. Although the trioxaadamantane tautomer was shown to be thermodynamically more stable than alternative forms, the kinetic barrier to cyclization was significant. This observation raises doubts about the proposed formation of muamvatin as an artifact of isolation.

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“…As imidazole has been demonstrated in other systems also to catalyze rearrangements of hydrogen bonding, 40 we do think that its role may be similar in its effect on the rate of ground-state recovery in AppA. Together with unpublished information from structural studies, this leads to models in which Y21 is indirectly linked (e.g.…”

Section: The Transcriptional Anti-repressor Appamentioning

confidence: 84%

From primary photochemistry to biological function in the blue-light photoreceptors PYP and AppA

Horst

Laan

Yeremenko

et al. 2005

Photochem Photobiol Sci

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To properly respond to changes in fluency conditions, Nature has developed a variety of photosensors that modulate gene expression, enzyme activity and/or motility. Dedicated types have evolved, which can be classified in six families: rhodopsins, phytochromes, xanthopsins, cryptochromes, phototropins and BLUF-proteins. The photochemistry of the first three families is based on cis/trans isomerization of an ethylene bond. Surprisingly, the latter three all use flavin as their chromophore, but each with very different photochemistry. In this contribution we will discuss the molecular basis of signal generation in a xanthopsin (Photoactive Yellow Protein (PYP) from Halorhodospira halophila), a photoreceptor for negative phototaxis, and in a BLUF protein (AppA from Rhodobacter sphaeroides), a transcriptional anti-repressor. PYP is activated through trans/cis isomerization of the 7,8-vinyl bond of its 4-hydroxycinnamic acid chromophore. This initiates a photocycle with multiple intermediates, like pB, which is formed after intramolecular proton transfer. The negative charge thus formed in the interior of the protein triggers formation of a partially unfolded signaling state. For AppA much less is known about the underlying photochemistry. Available evidence suggests that it is based on a light-induced change in the hydrogen-bonding of its flavin chromophore and/or a change in hydrophobic stacking between the flavin and/or nearby aromatic amino acids like Y 21. A signaling state is formed within microseconds, which recovers with a rate of approximately 10(-3) s(-1). The change in conformation between receptor- and signaling-state in AppA, however, appear to be minute as compared to those in PYP. Here we review the underlying chemistry in the various steps of the photocycle of these two photoreceptor proteins and provide new data on their mechanism and function.

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Syn−Anti Isomerization of Aldols by Enolization

Abstract: [reaction--see text] A variety of aldol adducts (i.e., 3-hydroxy ketones) are shown to undergo syn-anti isomerization in the presence of imidazole by an enolization mechanism with negligible retroaldol or elimination products.

Cited by 19 publications

References 32 publications

Iron‐ and Bismuth‐Catalyzed Asymmetric Mukaiyama Aldol Reactions in Aqueous Media

Iron‐ and Bismuth‐Catalyzed Asymmetric Mukaiyama Aldol Reactions in Aqueous Media

Total Synthesis of Muamvatin

From primary photochemistry to biological function in the blue-light photoreceptors PYP and AppA

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