Stereochemie aliphatischer Carbokationen, 13. Protonierte Cyclopropane als Zwischenstufen von 1,2‐Alkylverschiebungen

Kirmse, Wolfgang; Loosen, K.; Prolingheuer, E.-C.

doi:10.1002/cber.19801130114

Cited by 7 publications

(1 citation statement)

References 17 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We attribute this to an unsymmetrically protonated three-membered ring [22] [24], which may be due to steric hindrance and which is especially notable in the case yielding the two isomeric compounds 6 and 35. We summarize in Scheme 4 all these mechanistic considerations, which may explain the reported cyclopropane-alkene isomerization.…”

Section: C(23)c24) +mentioning

confidence: 99%

Stereochemical Aspects of Acid‐Catalyzed Cyclopropane Ring‐Opening Reactions. A Stereospecific Pathway to Crinosterol and Brassicasterol

Lang¹,

Djerassi²

1982

Helvetica Chimica Acta

View full text Add to dashboard Cite

SummaryIt is shown that the acid-catalyzed ring-opening of the two diastereoisomeric 23,24-methylenecholesterols 3 and 5 on treatment with gaseous HCl in acetic acid leads stereospecifically to the naturally occurring crinosterol (4) and brassicasterol (6), respectively (Scheme I ) . This isomerization can be viewed as a biomimetic model of an in vivo methylation process of the type already known in plant sterol metabolism (cJ: cycloeucalenol+ obtusifoliol, 1 4 2). The synthetic application of this method provides a convenient labelling of sterol side chains for tracer experiments. The mechanistic features of the reaction with respect to its particular stereospecificity are discussed.Introduction. -Are sterols containing a cyclopropane ring moiety metabolic dead ends or do they represent biosynthetic intermediates? This question has become relevant with the recent isolation of many such sterols [ 11. Lederer originally postulated [2] that cyclopropanes may serve as intermediates for methyl groups through a biochemical reductive opening, but he rejected this consideration on the basis of negative experimental results. However in vivo C-alkylation by isomerization of a cyclopropane intermediate to a methylated product (with an additional double bond) has been shown experimentally to be operative in plant sterol metabolism [3]. Thus cycloeucalenol (1) seems to play a key role in the biosynthesis of phytosterols since enzymatic opening of the three-membered ring leads to the A'-isomer obtusi-

show abstract

Section: C(23)c24) +mentioning

confidence: 99%

Stereochemical Aspects of Acid‐Catalyzed Cyclopropane Ring‐Opening Reactions. A Stereospecific Pathway to Crinosterol and Brassicasterol

Lang¹,

Djerassi²

1982

Helvetica Chimica Acta

View full text Add to dashboard Cite

show abstract

The Oxidation of Alcohols by Modified Oxochromium( VI )–Amine Reagents

Luzzio

1998

Organic Reactions

View full text Add to dashboard Cite

The development of oxochromium(VI)‐amine reagents as oxidants in organic synthesis has evolved since the early studies of Sisler which described the use of adducts of heterocyclic nitrogen bases and chromium(VI) oxide. The later work of Sarrett and coworkers in steroid synthesis soon established the utility of chromium(VI) oxide‐pyridine adducts as the first selective oxochromium(VI)‐amine reagents. Several modifications of the Sarrett oxidation followed. The present concern with the toxicity and environmental implications of oxochromium(VI) has provided encouragement for the study and use of catalytic oxochromium reagents in conjunction with stoichiometric co‐oxidants such as peroxides. Such technology is welcome, particularly when applied to the large‐scale preparations found in industry where the disposal of byproducts is a constant problem. Modified oxochromium(VI)‐amine reagents on polymer supports have been prepared and examined with the objective of providing a regenerable reagent system which is serviceable on a practical scale. A significant improvement in routine oxidations utilizing PCC and PDC entails the addition of adsorbents such as Celite®, alumina, silica gel, molecular sieves, or zeolites. The distinct advantages associated with the use of adsorbents in oxochromium(VI)‐amine‐mediated oxidations are discussed. The importance of buffers and desiccants in oxochromium(VI) oxidations is discussed. Virtually any number of oxochromium(VI)‐amine reagents may be prepared by varying the amine ligand and/or acid species associated with oxochromium(VI), and many of these compounds have been tested using simple substrate alcohols for oxidation to carbonyl compounds. The search for a milder and more efficient oxidant than the Sarrett‐type oxochromium(VI) reagent systems as well as the nonchromium oxidants based on dimethyl sulfoxide or dimethyl sulfide led to an examination of the utility of pyridinium chlorochromate (PCC) in selective oxidations of alcohols. PCC was superior in terms of ease of preparation, shelf stability and economy. Since the introduction of PCC in 1975 the reagent has become a commercially available “textbook” compound and is usually the reagent of choice for routine large and small scale oxidations of alcohols to carbonyl compounds. Unlike the oxochromium(VI)‐pyridine adducts, PCC is an acid salt, and its properties have proven advantageous in a number of tandem oxidative reactions such as oxidative transpositions and oxidative cationic cyclizations. Several reviews address the versatility of PCC and its extension to many different types of oxidative conversions in single and multistep organic syntheses. While PCC was gaining widespread use, the utility of pyridinium dichromate (PDC) in solvents such as dimethylformamide (DMF) and dichloromethane was reported. PDC/DMF was useful for oxidizing allylic alcohols to the corresponding, α,β‐unsaturated carbonyl compounds and non‐conjugated aldehydes and primary alcohols to the corresponding carboxylic acids. The utilization of beyond the simple oxidation of alcohols, and it is now employed for various transformations. Many other types of substituted pyridines, nitrogen heterocycles and amines form chromates, dichromates, and chlorochromates which allow many different types of oxidative processes for a wide range of substrate molecules. This chapter surveys the many applications of oxochromium(VI) complexes in oxidative conversions of alcohols to carbonyl compounds. Alternative reagent systems based on modifications involving ligands, buffering agents, or adsorbents, as well as nonchromium(VI) oxidants are discussed. A variety of substrates have been included in the Tables so as to guide the synthetic chemist in selecting reagents and conditions that will be optimal for a given transformation. Oxidative conversions of alcohols through transpositions and cationic cyclizations giving carbonyl compounds as endproducts are included.

show abstract

Stereochemie aliphatischer Carbokationen, 12. Alkylverschiebungen zwischen sekundären C‐Atomen

Kirmse

Prolingheuer

1980

Chem. Ber.

Self Cite

View full text Add to dashboard Cite

Verschiedene optisch aktive, P-verzweigte Amine wurden aus Aminosauren dargestellt. Die entsprechenden [2-D]Amine waren aus Isobutyraldehyd bzw. 2-Methylbutanal (37) zuganglich. Die Stereochemie am Endpunkt von 1,2-Methylverschiebungen wurde bei der Desaminierung von 4 und 21 mit salpetriger Saure untersucht. Uberwiegende, jedoch unvollstandige Inversion am Endpunkt der Umlagerungen ist mit konformativer Kontrolle besser vereinbar als mit Nachbargruppen-Beteiligung. Bei der Desaminierung von 31 tritt eine degenerierte Ethylverschiebung und eine nicht degenerierte Methylverschiebung auf; fur 44 gilt das Umgekehrte. Vollstandige Inversion am Ausgangspunkt der Methylwanderungen und das Fehlen anschlieBender, ruckschreitender H-Verschiebungen machen methylverbruckte Carbokationen als Zwischenstufen sehr wahrscheinlich. Partielle Racemisierung am Ausgangspunkt von Ethylwanderungen wurde auf Protonenverschiebungen in ethyl-verbruckten Zwischenstufen zuruckgefuhrt. Umgelagerte, offene Kationen tragen signifikant zur Gesamtreaktion bei, wenn durch Selbstaggregation der Alkylammonium-Ionen Micellen gebildet werden. Stereochemistry of Aliphatic Carbocations, 12" AIkyl Shifts between Secondary Carbon AtomsSeveral optically active, P-branched alkylamines have been synthesized from amino acids. The corresponding [?-Dlamines were obtained from isobutyraldehyde and 2-methylbutanal (37), respectively. The stereochemistry at the terminus of 1,2-methyl shifts has been elucidated in the nitrous acid deaminations of4 and 21. Predominant, although incomplete inversion at the migration terminus is consistent with conformational control rather than neighboring group participation. The deamination of 31 involves a degenerate 1,'-ethyl shift and a nondegenerate 1,Zmethyl shift; the reverse holds for 44. Complete inversion at the origin of the methyl migrations and the absence of subsequent retrogressive H shifts strongly support the intermediacy of methyl-bridged carbocations. Partial racemization at the origin of the ethyl migrations has been traced to proton shifts within ethyl-bridged intermediates. Rearranged open cations contribute significantly to the overall reaction if micelles are produced by self-aggregation of the alkylammonium ions.Die Kontroverse uber offene (,,klassische") und verbruckte (,,nicht-klassische") Carbokationen galt bisher vorwiegend bicyclischen Systemen, insbesondere dem Norbornyl-Kation. Die stereochemischen Kriterien fur das Auftreten verbruckter Ionen sind auf Bicyclen nur bedingt anwendbar, da sie von ,,angeborenen" stereochemischen Praferenzen uberdeckt werden (em-Selektivitat bei allen Reaktionen des Norbornan-Gerusts). Stereochemie aliphatischer Carbokationen, 12 105Wir untersuchten daher die Stereochemie von 1,2-Alkylverschiebungen an acyclischen, konformativ beweglichen Verbindungen. Hier wird die Stereochemie am Endpunkt der Umlagerungen durch konformative Kontrolle') und/oder Nachbargruppen-Beteiligung 3, bestimmt, wahrend die Stereochemie am Ausgangspunkt der Umlagerungen von der Natur der Zwischen...

show abstract

Stereochemie aliphatischer Carbokationen, 13. Protonierte Cyclopropane als Zwischenstufen von 1,2‐Alkylverschiebungen

Cited by 7 publications

References 17 publications

Stereochemical Aspects of Acid‐Catalyzed Cyclopropane Ring‐Opening Reactions. A Stereospecific Pathway to Crinosterol and Brassicasterol

Stereochemical Aspects of Acid‐Catalyzed Cyclopropane Ring‐Opening Reactions. A Stereospecific Pathway to Crinosterol and Brassicasterol

The Oxidation of Alcohols by Modified Oxochromium( VI )–Amine Reagents

Stereochemie aliphatischer Carbokationen, 12. Alkylverschiebungen zwischen sekundären C‐Atomen

Contact Info

Product

Resources

About