Methoxy‐ und Chlor‐substituierte Oxallyl‐Zwischenstufen

Fölisch, Baldur; Krimmer, D.; Gehrlach, E.; Käshammer, Dietmar

doi:10.1002/cber.19881210909

Cited by 60 publications

(38 citation statements)

References 21 publications

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“…(2), R = Z]. [6] Whereas reactions of vinyloxocarbenium species are believed to proceed by a concerted (4+2) cycloaddition followed by a Wagner-Meerwein-type ring expansion, [5] allylic cations with a Z group at the 2-position have been suggested to undergo "true" (4+3) cycloaddition reactions, either through a concerted or a stepwise process.[7] The synthetic utility of these cycloadditions is limited however by issues concerning the generation of the allylic cation reaction partner. The precursors required for allylic cations can be synthetically challenging, and the carbocationic species themselves are generally quite unstable, thus giving rise to various by-products.…”

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confidence: 99%

Scope and Mechanism of the (4+3) Cycloaddition Reaction of Furfuryl Cations

et al. 2011

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The (4+3) cycloaddition reaction between a conjugated diene and an allylic cation constitutes a convenient and conceptually straightforward method to prepare seven-membered rings. [1,2] In recent years, investigations into this type of cycloaddition, including a number of elegant applications in total synthesis, [3] have demonstrated its potential for development as a synthetic method that could approach the (4+2) cycloaddition (Diels-Alder) reactions in terms of selectivity and efficiency. Of particular interest are allylic cations bearing a cation-stabilizing substituent (Z group) at the 2-position [Eq. (1)] [4] and stabilized allylic cations such as vinyloxocarbenium ions [Eq. (2)] [5] or, typically, cationic reaction partners incorporating both of these features [Eq. (2), R = Z]. [6] Whereas reactions of vinyloxocarbenium species are believed to proceed by a concerted (4+2) cycloaddition followed by a Wagner-Meerwein-type ring expansion, [5] allylic cations with a Z group at the 2-position have been suggested to undergo "true" (4+3) cycloaddition reactions, either through a concerted or a stepwise process.[7] The synthetic utility of these cycloadditions is limited however by issues concerning the generation of the allylic cation reaction partner. The precursors required for allylic cations can be synthetically challenging, and the carbocationic species themselves are generally quite unstable, thus giving rise to various by-products.In the course of synthetic studies towards the natural product rameswaralide, [8] Pattenden and Winne uncovered an unexpected smooth intramolecular dehydrative reaction between a diene and a furfuryl alcohol that gave a novel fused polycycle containing a seven-membered ring [Eq. (3), TFA = trifluoroacetic acid].We surmised that the mechanism of this intriguing cycloaddition reaction involves the initial formation of a furfuryl cation intermediate, [9] which then undergoes an intramolecular (4+3)-type cycloaddition with the appended diene (cf. Scheme 1). The initially formed bicyclic oxocarbenium ion can then be converted into the observed product through straightforward loss of a proton and regeneration of the furan ring. The cationic reaction partner in this sequence can thus be regarded as an allylic cation with a stabilizing oxygen substituent at the 2-position [cf. Eq. (1)], and moreover, if the oxonium ion resonance structure is considered (Scheme 1, bottom), it can be viewed as a vinylogous vinyloxocarbenium ion [cf. Eq. (2)]. If it is concerted, the observed cycloaddition can then be regarded as a rare example of a "higher order" pericyclic reaction, that is, a [6p + 4p] cycloaddition with an unprecedented 2-oxafulvene-type 6p reactant. [10] As the prospect of an efficient (4+3) cycloaddition method using simple furfuryl alcohol starting materials is highly attractive from a synthetic point of view, we decided to investigate the scope and mechanism of this unprecedented Scheme 1. Proposed mechanism of cycloaddition via a furfuryl cation intermediate.

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Scope and Mechanism of the (4+3) Cycloaddition Reaction of Furfuryl Cations

et al. 2011

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“…[12,13] Treatment of (acyclic) dihalogeno ketones with sodium trifluoroethoxide in trifluoroethanol (NaTFE/TFE) generates oxyallyl intermediates which undergo electrocyclization to form labile (mono)halogenocyclopropanones. These intermediates are cleaved by trifluoroethoxide to form trifluoroethyl esters.…”

Section: Discussionmentioning

confidence: 99%

“…[36] The solvolysis of the primary cycloadduct(s) cannot occur when the oxyallyl intermediate(s) are generated in lithium perchlorate/diethyl ether. [11,12] However, in this solvent the reaction between 30 and furan, in the presence of triethylamine, gave only a 23 % yield of the expected dichlorooxabicycles. Three of them could be isolated by MPLC (ratio of isomers ca.…”

Section: A 27a)mentioning

confidence: 97%

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Base‐Induced Trifluoroethanolysis of Acyclic Di‐ and Trihalogeno Ketones: Favorskii Rearrangement and [4+3] Cycloaddition

Föhlisch¹,

Franz²,

Kreiselmeier³

2005

Eur J Org Chem

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Reactions of five acyclic dihalogeno‐ and trihalogeno ketones, representing the α,α‐, α,α′‐, α,α,α‐ and α,α,α′‐di‐ and trihalogeno substitution pattern, with sodium 2,2,2‐trifluoroethoxide in 2,2,2‐trifluoroethanol (NaTFE/TFE) in the presence of furan were investigated with the aim of obtaining [4+3] cycloadducts of the corresponding oxyallyl intermediates. Preference of Favorskii rearrangement over cycloaddition was observed with dichloromethyl isobutyl ketone (1a) and 1,3‐dibromobutan‐2‐one (12) that formed mainly the trifluoroethyl esters of 3‐chloro‐2‐isopropylpropanoic acid (9a), and (Z)‐but‐2‐enoic acid (isocrotonic acid) (13), respectively. 9a was dehydrohalogenated to form trifluoroethyl 3‐isopropylacrylate (11a). [4+3] Cycloaddition was favored with the 1,1,3‐trihalogenobutan‐2‐ones 24 and 25, leading to 2,4‐dihalogenated 8‐oxabicyclo[3.2.1]oct‐6‐en‐3‐ones (26, 27) as a mixture of endo‐exo‐stereoisomers. With trichloromethyl isobutyl ketone (30) the isomeric 2,4‐dichloro‐substituted oxabicycles (34) were formed in lithium perchlorate/diethyl ether/furan/triethylamine, while in NaTFE/TFE solvolytic displacement of one chloro substituent occurred, providing 2‐chloro‐4‐trifluoroethoxy‐4‐isopropyl‐8‐oxabicyclo[3.2.1]oct‐6‐en‐3‐one (36). By analogy, 27 reacted with sodium methoxide/methanol to form the methoxy‐substituted oxabicycles 39a and 39b. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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“…Bicyclo[3.2.1]oct-6-en-3-one (4a): TBA (3.80 g, 10.6 mmol) was converted with freshly monomerized cyclopentadiene in an ultrasound bath according to the above protocol and conditions outlined in Table 1 to give 4a [29] as beige crystals (purity > 95%/GC); yield: 0.80 g (62% over 2 steps); mp 98 -100 8C.…”

Section: General Procedures For the Zn-mediated [4 þ 3]-cycloadditionmentioning

confidence: 99%

Facile Synthesis and Ring‐Opening Cross Metathesis of Carbo‐ and Heterocyclic Bicyclo[3.2.1]oct‐6‐en‐3‐ones Using Gaseous Olefinic Reaction Partners

et al. 2006

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Abstract:The title compounds were prepared by a facile [4 þ 3]-cycloaddition strategy involving sonochemistry. The oxyallyl species required for the reaction with the corresponding diene was generated from a suitable perbromo ketone with activated zinc under sonification conditions. The resulting dibromo bicyclo compounds were reduced to the target products using a Cu/Zn couple. Ring-opening cross metathesis of the resulting bicyclic species was established as an efficient method for the diastereoselective preparation of cis-3,5-disubstituted cycloketones. Especially gaseous olefins gave symmetrical and unsymmetrical products as versatile platforms for subsequent transformations. This is the first example of such a ringopening reaction of a non-strained ring system.

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Methoxy‐ und Chlor‐substituierte Oxallyl‐Zwischenstufen

Cited by 60 publications

References 21 publications

Scope and Mechanism of the (4+3) Cycloaddition Reaction of Furfuryl Cations

Scope and Mechanism of the (4+3) Cycloaddition Reaction of Furfuryl Cations

Base‐Induced Trifluoroethanolysis of Acyclic Di‐ and Trihalogeno Ketones: Favorskii Rearrangement and [4+3] Cycloaddition

Facile Synthesis and Ring‐Opening Cross Metathesis of Carbo‐ and Heterocyclic Bicyclo[3.2.1]oct‐6‐en‐3‐ones Using Gaseous Olefinic Reaction Partners

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