Eine neue asymmetrische Formylierung von Aldehyden

Kirschning, Andreas; Dräger, Gerald; Jung, Alexander

doi:10.1002/ange.19971090313

Cited by 9 publications

(7 citation statements)

References 26 publications

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“…Aromatic aldehydes 7 and 26 ± 28 gave slightly better yields with the potassium salt of diethoxymethyl diphenylphosphine oxide. In initial studies with enolizable aldehydes, only moderate yields in the reaction with anion 32 were achieved, [15] so that a modified procedure had to be developed. Attempts to improve the yield by adding metal salts like ZnBr 2 , MgBr 2 OEt 2 , or Ti(OiPr) 4 and transmetallating the lithiated diphenylphosphine oxide 31 did not give better yields or were unsuccessful.…”

Section: Resultsmentioning

confidence: 99%

“…Table 2). [15] Indeed, these 1,1-heterodisubstituted alkenes are ideally suited for this concept, because of the excellent nucleophilicity of the alkenic double bond and the small size of the two geminal alkoxy groups. They can conveniently be prepared from aldehydes by the Horner ± Wittig reaction [22] with the dialkoxymethyl diphenyl phosphine oxides 31 and 32 [23] Furthermore, they are ideal substrates in the Sharpless asymmetric dihydroxylation [24] giving intermediate diols that spontaneously collapse to ahydroxy carboxylates (Table 2).…”

Section: Resultsmentioning

confidence: 99%

“…Support for this hypothesis comes from our observation that aromatic a-hydroxy esters tend to racemize rapidly. [15] The use of chiral aldehydes is particularly important when introducing a new acylation method, as numerous applications in natural-product synthesis can be envisaged. Therefore, chiral aldehydes containing an a-aryl-(60), a-alkoxy-(62), and b-siloxy-substituents (64) were transformed into the corresponding a-hydroxy esters 61, 63, and 65 under the standard conditions developed for enolizable aldehydes (Scheme 7).…”

Section: Resultsmentioning

confidence: 99%

“…Recently, we disclosed a new acylation concept [15] that circumvents most of the drawbacks described for the classical approaches via intermediate 2.…”

Section: Introductionmentioning

confidence: 99%

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Asymmetric Nucleophilic Acylation of Aldehydes via 1,1-Heterodisubstituted Alkenes

Monenschein

Dräger

Jung

et al. 1999

Chemistry - A European Journal

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Aldehydes are asymmetrically acylated by a two step sequence that is initiated by a homologation step to 1,1-heterodisubstituted alkenes followed by asymmetric dihydroxylation. Thus, ketene O,S-acetals are efficiently prepared from aldehydes by a Peterson olefination with lithiated methoxy-phenylthiotrimethylsilyl methane 14 as the C-1 source. Although they are dihydroxylated with the Sharpless catalyst with moderate to good enantioselectivity (62 ± 80 % ee), the process is not efficient owing to the low chemical yields of the desired a-hydroxy methyl esters (7 ± 37 %). Use of the corresponding sulfoxide 24 or sulfon 25 led to an improved chemical yield of a-hydroxy methyl ester 19, but the stereoselectivity was diminished. In contrast, intermediate ketene O,O-acetals are prepared by a Horner ± Wittig reaction with phosphine oxide 31 and are dihydroxylated both with good chemical and stereochemical yield. The concept is applicable to aromatic, aliphatic, and chiral aldehydes. For example, this short sequence allows exclusive and independent preparation of both diastereomeric heptoses 69 a and 69 b.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

“…Recently, we disclosed a new acylation concept [15] that circumvents most of the drawbacks described for the classical approaches via intermediate 2.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Asymmetric Nucleophilic Acylation of Aldehydes via 1,1-Heterodisubstituted Alkenes

Monenschein

Dräger

Jung

et al. 1999

Chemistry - A European Journal

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“…[7] Auf Ergebnissen von Sharpless [8] und Kirschning [9] aufbauend, entwickelten wir eine sehr effiziente Synthesesequenz für enantiomerenreine a-Hydroxyketone (Schema 3). Ausgehend von den beiden Aldehyden 8 und 9 liefert diese Sequenz in wenigen, sehr effektiven Schritten die enantiomerenreinen a-Hydroxyketone 10.…”

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1,2‐Chiralitätstransfer bei der Synthese von Cyclopropanen

Wessig

Mühling

2005

Angewandte Chemie

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1,2‐Chirality Transfer in the Synthesis of Cyclopropanes

Wessig

Mühling

2005

Angew Chem Int Ed

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In the last few years organic photochemistry has made remarkable progress in the area of asymmetric synthesis.[1] A range of new strategies has been developed that drastically enhance the diastereo-and enantioselectivity of certain photochemical processes. Here we report on an entirely novel concept concerning the synthesis of enantiomerically enriched cyclopropanes based on a photochemically induced intramolecular 1,2-chirality transfer.During chirality transfer a prochiral moiety incorporates the chiral information of a chiral moiety, which simultaneously loses this information. A distinguishing feature in the intramolecular transfer of central chirality is the distance between the source and the target of the chiral information. While relatively many examples of 1,3-chirality transfer exist, [2] which are mainly based on the transfer of chirality along an allylic system, there are only a few examples of 1,2-chirality transfer. [3] Some time ago we developed the concept of spin-center shift, [4, 5] which provides a completely new access to cyclopropanes. In contrast to the classical Norrish-Yang reaction, [6] phenyl ketones 1 with a suitable leaving group X in a position are used. At the stage of 1,4-diradicals 2 this results in a shift of the radical center from the carbonyl C atom to the adjacent C atom as a result of HX elimination. The resulting 1,3-diradicals 3 cyclize with often complete diastereoselectivity and good to very good yields to give cyclopropanes 4 (Scheme 1). [4] By extensive quantum chemical calculations we could elucidate details of the mechanism and found that the leaving group X already has a significant impact on the first reaction step, the photochemical H-shift (1!2, Scheme 1). [4] Owing to hyperconjugation between the s* orbital of the CÀX bond and the p system of the excited carbonyl group, X prefers a pseudoaxial arrangement with respect to the approximately chairlike six-membered transition state.[4b] If we consider enantiomerically pure ketones of type 5 with two identical substituents in b position, we can formulate two different transition states for the g-H abstraction (TS-5 a and TS-5 b), which differ in the arrangement of the leaving group X (axial or equatorial, Scheme 2). Based on the quantum chemical calculations, [4b] TS-5 a with X in the axial position should be favored over TS-5 b. In TS-5 a the g-methylene group is attacked, whereas in TS-5 b the attack takes place at the g'-methylene group, which means that the carbonyl oxygen atom must distinguish between the two diastereotopic methylene groups. During the photochemically induced H-transfer and the following elimination of HX, the enantiomeric diradicals 6 a and 6 b are formed, and the chiral information of the a-C atom has been transferred to the adjacent prochiral b-C atom (1,2-chirality transfer). Consequently, after the cyclization the enantiomeric cyclopropyl ketones (R)-7 and (S)-7 are obtained, whereby the formation of (R)-7 should be favored over (S)-7. Thus, during such a process desymmetrization relat...

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Eine neue asymmetrische Formylierung von Aldehyden

Abstract: Die Synthese und Handhabung der nachstehend aufgefiihrten Substanzen erfolgte unter strengem FeuchtigkeitsausschluD.

Cited by 9 publications

References 26 publications

Asymmetric Nucleophilic Acylation of Aldehydes via 1,1-Heterodisubstituted Alkenes

Asymmetric Nucleophilic Acylation of Aldehydes via 1,1-Heterodisubstituted Alkenes

1,2‐Chiralitätstransfer bei der Synthese von Cyclopropanen

1,2‐Chirality Transfer in the Synthesis of Cyclopropanes

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