Ozonolyse von Enolethern. 10. Mitteilung

Schank, Kurt; Beck, H. P.; Pistorius, Susanne

doi:10.1002/hlca.200490182

Cited by 13 publications

(2 citation statements)

References 63 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Given Caspie and Rodig's observations concerning the cleavage of steroidal systems, our initial choice of spirodienone substrate was guided by the recognition that in order to promote efficient ozonolysis, it would be necessary to address the inherent electron-deficiency of these cross-conjugated ketones. Since it is well-known that electron-releasing substituents increase the reactivity of alkenes toward ozone, , we opted to examine the cleavage of dienone substrates activated by the presence of a β-methoxy substituent. Our investigation therefore commenced with the preparation of a series of dienones 12 , which where accessed through the oxidative spirocyclization of alkyl hydroxamates 10 , under conditions previously developed in this laboratory (Table ) .…”

Section: Resultsmentioning

confidence: 99%

Nitrenium Ion Azaspirocyclization−Spirodienone Cleavage: A New Synthetic Strategy for the Stereocontrolled Preparation of Highly Substituted Lactams and N-Hydroxy Lactams

Wardrop

Burge

2005

J. Org. Chem.

View full text Add to dashboard Cite

[reaction: see text] Although 1,4-cyclohexadienes 2, obtained through the Birch reduction of arenes 1, have found widespread use as masked beta-oxo carbonyl synthons 3, the possibility that 2,5-cyclohexadienones 5 might also be employed to the same end has been overlooked despite their ready availability. As part of our ongoing investigation of the synthetic chemistry of nitrenium ions, we have developed a novel and efficient strategy for the stereoselective preparation of di- and trisubstituted azetidinone, pyrrolidinone, and piperidinone derivatives, which features the ozonolytic cleavage of azaspirocyclic 2,5-cyclohexadienones 12. For example, ozonolysis of spirodienone 12c in CH2Cl2 and reductive workup with dimethyl sulfide generated unstable beta-formyl ester 21, whereas cleavage in MeOH followed by reduction with thiourea led to hemiacetal 22. While both 21 and 22 partially decompose upon exposure to silica gel, they can be trapped in situ, with a variety of weakly basic nucleophiles, to usefully substituted products. The requisite spirodienone substrates are readily accessible through the nitrenium ion cyclization of alkyl omega-arylhydroxamates 10, which proceeds with moderate to high diastereoselectivity.

show abstract

Section: Resultsmentioning

confidence: 99%

Nitrenium Ion Azaspirocyclization−Spirodienone Cleavage: A New Synthetic Strategy for the Stereocontrolled Preparation of Highly Substituted Lactams and N-Hydroxy Lactams

Wardrop

Burge

2005

J. Org. Chem.

View full text Add to dashboard Cite

show abstract

“…Analogously, in the ozonolysis of enol esters, beside a carbonyl compound, a mixed carboxylic acid anhydride is formed [7] [15]. The C=C bonds of enamines can be more resistant towards ozone than those of structurally related enol ethers [15].…”

mentioning

confidence: 96%

Functionalized Imides by Regioselective Ozonation

Poschenrieder¹,

Stachel²,

Eckl³

et al. 2006

Helvetica Chimica Acta

View full text Add to dashboard Cite

Ozonations of alkoxy-and (acyloxy)-substituted alkylidene-lactams 1 and 5 or of the alkylidene-sultams 9 and 10 proceeded by regioselective cleavage of the exocyclic C=C bonds (Schemes 1 and 2). These bonds are part of an enamide system and, therefore, possess considerable polarity as shown by 13 C-NMR spectra. As a result, the partly known maleimides 3 and 6 or the sulfonimides 11 were obtained. Compounds 3 and 11 reacted with diazomethane to give the highly reactive bicyclic derivatives 8 and 12, respectively. The cinnamylidene-lactames 16a,b were converted by selective ozonolysis mainly into the formylmethylene lactames 17a,b (Scheme 3). The amino-substituted aldehyde 20 bears a structural relationship to the lactone antibiotic basidalin 21a. The tendency of some donor-substituted maleimides to undergo [2 + 2] cycloadditions was assessed (Scheme 4). The configuration of the photodimers 22a,b and 24a,b was established by X-ray crystallography.Introduction. -The cleavage of alkene C=C bonds by ozone to give aldehydes, ketones, and carboxylic acids was introduced into organic chemistry at the beginning of the 20 th century by Harries [1]. Ozonolyses take place under extremely mild reaction conditions and often with excellent yields. The individual course of an ozonization, however, is strongly dependent on the constitution of the starting material, the solvent, and the choice of reductive or oxidative workup conditions (for a comprehensive review, see [2]).A basic three-step mechanism was formulated by Criegee and is now bearing his name [3]. In a sequence of concerted 1,3-dipolar cycloaddition and cycloreversion, carbonyl oxides [4] are formed which are captured by internally generated carbonyl compounds in another 1,3-dipolar cycloaddition to yield ozonides. As a result, for unsymmetrical alkenes, substituent rules for the regioselective OÀO splitting in the cycloreversion step have been formulated [5] [6]. There are, however, exceptions from these rules, and the ozonide-forming step may be bypassed due to low dipolarophilicity of the carbonyl intermediate. Both is examplified in the ozonolysis of ketene acetals [7]. Modifications of the original Criegee mechanism have been introduced later [8]. An alternative mechanistic concept of ozonations was recently brought up for discussion [9]. According to the donor-acceptor concept, ozonations are regarded as oxidation reactions with radical-ion intermediates.It is well known that the C=C bond of enol ethers is readily cleaved by ozone forming a carbonyl compound and a carboxylic acid ester [10 -14]. Analogously, in the ozonolysis of enol esters, beside a carbonyl compound, a mixed carboxylic acid anhydride is

show abstract

Der Mechanismus der Alken‐Ozonolyse – eine kritische Betrachtung

Schank

2004

Helvetica Chimica Acta

View full text Add to dashboard Cite

The Mechanism of Alkene Ozonolysis ± a Critical Examination Alkene ozonolyses have been carried out in experimental organic chemistry for more than a century, and a lucid description of the underlying reaction mechanism has been developped and integrated in most chemistry textbooks. Nevertheless, there are a great number of exceptions hard to rationalize by the accepted mechanism. Attempts in the past to give more-convincing answers to this problem failed. Therefore, the search for an alternative mechanistic model without contradictions is compelling. The present paper offers a redox model based upon the known properties of ozone both as a strong oxidant and an O-atom-transferring agent, especially when reacted with alkenes of sufficiently high electron density to act as reducing partners. Harries

show abstract

Ozonolyse von Enolethern. 10. Mitteilung

Cited by 13 publications

References 63 publications

Nitrenium Ion Azaspirocyclization−Spirodienone Cleavage: A New Synthetic Strategy for the Stereocontrolled Preparation of Highly Substituted Lactams and N-Hydroxy Lactams

Nitrenium Ion Azaspirocyclization−Spirodienone Cleavage: A New Synthetic Strategy for the Stereocontrolled Preparation of Highly Substituted Lactams and N-Hydroxy Lactams

Functionalized Imides by Regioselective Ozonation

Der Mechanismus der Alken‐Ozonolyse – eine kritische Betrachtung

Contact Info

Product

Resources

About