Zur Thermolyse von Verbindungen mit geometrisch fixierter Vinyloxiran‐Einheit: Stereospezifische Synthese von 4,5‐anellierten
cis
‐ und
trans
‐2,3‐Dihydrofuranen
Abstract:Das Furangerust ist wegen seines haufigen Vorkommens in Naturstoffen ein wichtiges Strukturelement, fur dessen Darstellung zahlreiche Methoden zur VeFfiigung stehen'). Eine erst in jungster Zeit genauer studierte Synthesembglichkeit fur eine der reduzierten Formen des Heterocyclus, des 2,3-Dihydrofurans, besteht inder intramolekularen Ringerweiterung von Vinyloxiranen2). Die Vorteile dieses Verfahrens, das formal der Vinylcyclopropan-Cyclopenten-Isomerisierung entspricht, liegen in dem relativ leichten Zugang … Show more
“…This method afforded the cis:trans dihydrofuryl ester 16 as a mixture of isomers (8:1). This ratio was independent of the stereochemistry of the starting epoxide and represents preferential disrotary ring closure of the less hindered ylid intermediate to afford the desired cis isomer, Scheme . , Following this protocol, we isolated pure cis dihydrofuryl ester 16c in 66% yield after flash column chromatography. 3 Alkenyl Epoxide−Dihydrofuran Rearrangement…”
[reaction: see text] Epiasarinin, an endo-endo furofuran, has been synthesized from piperonal via a five-step route with good stereocontrol. The sequence involves Darzens condensation, alkenyl epoxide-dihydrofuran rearrangement, and a Lewis acid mediated cyclization.
“…This method afforded the cis:trans dihydrofuryl ester 16 as a mixture of isomers (8:1). This ratio was independent of the stereochemistry of the starting epoxide and represents preferential disrotary ring closure of the less hindered ylid intermediate to afford the desired cis isomer, Scheme . , Following this protocol, we isolated pure cis dihydrofuryl ester 16c in 66% yield after flash column chromatography. 3 Alkenyl Epoxide−Dihydrofuran Rearrangement…”
[reaction: see text] Epiasarinin, an endo-endo furofuran, has been synthesized from piperonal via a five-step route with good stereocontrol. The sequence involves Darzens condensation, alkenyl epoxide-dihydrofuran rearrangement, and a Lewis acid mediated cyclization.
The first documented report of a possible Cope rearrangement was probably that of Baeyer, who prepared eucarvone by hydrobromination of carvone in 1894. Although the transformation was briefly studied at that time, it was not until the 1950s that this and other Cope‐type rearrangements received detailed attention. The thermal isomerization of
cis
‐divinylcyclopropane to cycloheptan‐1,4‐diene was reported by Vogel in 1960 during his studies of the Cope rearrangement of 1,5‐hexadienes annulated by a homologous series of carbocyclic rings. Scores of mechanistic studies followed this discovery upon the realization that the rearrangement could be related to the conceptually similar vinylcyclopropane–cyclopentene isomerization discovered a year earlier. It was also recognized that this rearrangement might be operating in the formation of cycloheptatriene from norcaradiene during photolysis of diazomethane in benzene. The topic received considerable attention in the 1960s, an era of mechanistic investigations of various concerted transformations. During the 1970s it enjoyed exploitation in many synthetic strategies, and the following decade the elements of this rearrangement were incorporated into tandem or multistep procedures in a preconceived manner. Many aspects of the various permutations of the Cope rearrangement have been previously reviewed.
The purpose of this review is to summarize the mechanistic, stereochemical, and practical results in this area in the context of the evolution of synthetic achievements during the last 40 years. Also described in this chapter are the transformations of several of the simple heteroatom permutations of this rearrangement in order to render appropriate comparisons of various systems. A discussion addresses those rearrangements of cyclopropanes, oxiranes, thiiranes, and aziridine rings substituted with vicinal vinyl groups. Excluded from this review are rearrangements of those divinylsubstituted three‐membered rings that contain more than a single heteroatom within the reacting manifold. Brief mention of these systems along with a guide to the literature is found in the last section of this chapter.
The literature is covered through December 1990. Many of the principal researchers in this field have been contacted and many unpublished transformations have been included in the tables.
Synthesis, Thermolysis, and Photolysis of Substituted 3,4-Epoxy-cycloalkenes1)The substituted epoxy-cycloalkenes 5-7 are transformed thermally at 200-250'C into the benzene derivatives 10, 15, and 18 after initial C/O-bond cleavage of the oxirane ring. At temperatures higher than 300°C (short time pyrolysis) 6 and 7 give, in addition to the aromatic compounds 15/18, the furans 16 and 19, respectively. On photochemical excitation 5 -7 react with C/C-ring opening to the annelated 2,3-dihydrofurans 11, 17, and 20, respectively. The epoxycycloheptene 8 undergoes analogous ring expansion leading to 21 on electronic as well as thermal activation (ca. 80% yield). Whereas 21 does not form any defined product up to 380°C. and the monosubstituted bicyclus 11 suffers fragmentation affording methyl 3-furancarboxylate (12) and ethylene, both dihydrofurans 17 and 20 are quantitatively converted back into the epoxy-cycloalkenes 6 and 7, respectively, at temperatures below 200°C; the cycloreversion reaction affording 16/19 competes only above 300°C. The cyclic dipoles of type IV, assumed to occur as intermediates during !he 2,3-dihydrofuran-vinyloxirane isomerisations 17 .6/20 .7, can be trapped with dimethyl acetylenedicarboxylate or N-phenylmaleimide at significantly lower temperature (15O/12O0C) to give the 1 : 1-cycloadducts 38-40. However, because no such products could be obtained in case of the photoreactions of 5 -8, a concerted [ I ,3]-C-migration has to be considered as a mechanistic alternative of the light-induced formation of 11, 17, 20, and 21.
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