High temperature bromination. Part 22: Bromination of 1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalene

The high-temperature bromination of 1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalene and its carboethoxy derivative was studied. Reaction of the title compound with 1 mol of bromine in refluxing carbon tetrachloride resulted in the formation of ring-opening products. In the case of the carboethoxy derivative, bromination took place both regio- and stereospecifically at the benzylic positions, the cyclopropane ring did not undergo bond cleavage. A mechanism for the formation of the products and their dehydrobromination reactions is discussed.     

Syntheses of the terpenoid precursors cyclopent-2-enone and cyclohex-2-enone diesters

Summary Two reaction pathways were elaborated for the practical and convenient synthesis of the title compounds. The first route applies a bromination-dehydrobromination sequence to introduce the double bond into 1-alkoxycarbonyl-2-oxocycloalkylacetic and propionic esters (4a–c,7a,b). The application of 2,6-lutidine for dehydrobromination of -bromocycloalkanones diesters (5a–c,8a,b) provides sufficient selectivity to carry out this step without affecting the sensitive ester group. Alternative pathways, involvingMichael reaction of diethyl 2-acetylsuccinate or -glutarate with acrolein and subsequent intramolecular aldol condensation, are presented in the case of cyclohex-2-enone derivatives2a,b.

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Zusammenfassung Für praktische und bequeme Synthesen der Titelverbindungen werden zwei Reaktionswege präsentiert. Der erste Weg basiert auf einer Bromierungs-Dehydrobromierungs-Sequenz zur Einführung der Doppelbindung in die 1-Alkoxykarbonyl-2-oxocycloalkylessig- und-propionsäureester (4a–c,7a,b). Die Anwendung von 2,6-Lutidin zur Dehydrobromierung der -Bromocycloalkanone-diester (5a–c,8a,b) sichert die ausreichende Selektivität, ohne die empfindlichen Estergruppen anzugreifen. Im Falle der Cyclohex-2-enon-Derivate (2a,b) zeigen wir einen alternativen Weg, der auf derMichael-Reaktion von Diethyl-2-acetyl-succinat bzw. -glutarat mit Acrolein basiert; die anschließende intramolekulare Aldol-Kondensation liefert die Zielprodukte.

     

Bromination and Azidation Reactions of 2-Styrylchromones. New Syntheses of 4(5)-Aryl-5(4)-(2-chromonyl)-1,2,3-triazoles

Summary. The bromination of 2-styrylchromones, bearing electron neutral substituents, with two molar equivalents of piridinium tribromide gave 2-(2-aryl-1,2-dibromoethyl)chromones and 3-bromo-2-(2-aryl-1,2-dibromoethyl)chromones. The presence of electron-donating substituents on their B ring led to a mixture of compounds due to the higher reactivity of their C(2)=C(3) and C=C double bonds, whereas the strongly electron-withdrawing group hindered the bromination. The dehydrobromination of 2-(2-aryl-1,2-dibromoethyl)chromones with triethylamine gave a diastereomeric mixture of (E)- and (Z)-2-(-bromostyryl)chromones. Some novel 4(5)-aryl-5(4)-(2-chromonyl)-1,2,3-triazoles have been obtained from the reactions of 2-(2-aryl-1,2-dibromoethyl)chromones, 2-(-bromostyryl)chromones, and 2-styrylchromones with sodium azide. The reactions of 2-styrylchromones with sodium azide are more efficient, general, and constitute a one-pot synthetic method of 4(5)-aryl-5(4)-(2-chromonyl)-1,2,3-triazoles allowing the preparation of 1,2,3-triazoles bearing either electron-donating or electron-withdrawing substituents in their aryl ring. The structure of all new compounds was established by extensive NMR spectroscopic studies.     

Vitamin B12-catalyzed conversion of some γ- and δ-bromoalkanols to polyhydroxy alkanols

Vitamin B₁₂-catalyzed allylic dimerization of some γ- and δ-bromoalkanols with activated Zn-dust in mixture ethanol/water (1:1) has been studied. Investigated bromoalkanols were prepared from corresponding tertiary Δ⁴- and Δ⁵-alkenols by means of benzeneselenyl bromide and subjected to chemical reduction with vitamin B₁₂. All investigated bromoalkanols, after dehydrobromination, underwent to oxidative allylic coupling and hydratation in the presence of protic solvents to give predominantly polyhydroxy alkanols.     

The Chemistry of Binor-S and its Cyclopropyl Ring Transformations

Selective ring opening of the cyclopropyl moiety of binor-S was accomplished by several methods including acidic hydrolysis, bromination, and bydrobromination. The crystal structure of dibromide adduct 3 was solved by X-ray diffraction analysis. Debrominations of 3 yielded either 1 or 4a , whereas dehydrobromination yielded a 3-substituted monobromide 6a . The mechanism of conversion of 3 to 6a is depicted as involving intermediate 7 ; the existence of 7 is supported by the isolation of olefin 8 . Oxidation of alcohol 4c produced ketone 11a which was either oxidized to lactone 12 or transformed to a methylene derivative 11b . Hydroboration of 11b followed by quenching with hydrogen peroxide produced a hydroxymethyl derivative 14 .     

Competitive,substrate-dependent reductive debromination/dehydrobromination of 1,2-dibromides with triethylamine

The interaction of various 1,2-dibromides with NEt₃ under various conditions (THF and DMF, respectively) at different temperatures was investigated. Our results from these reactions show that substrate dependent dehydrobrominations compete with reductive debrominations. A comprehensive discussion of these competitive pathways is offered.     

A modified synthesis of (±)-occidentalol

Summary An alternative synthesis of methyl 4,8a-dimethyl-3-oxo-trans-perhydronaphthalene-6-carboxylate is described.

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Eine modifizierte Synthese von (±)-Occidentalol

Zusammenfassung Es wird eine alternative Synthese von Methyl-4,8a-dimethyl-3-oxo-trans-perhydronaphthalin-6-carboxylat beschrieben.

     

Synthesis of 3-[2-(1,3-butadienyl)]-1H-indoles en route to murrapanine analogue

A three-step procedure has been developed for the synthesis of 3-[2-(1,3-butadienyl)]-1H-indoles. TBAF was proved to be an effective reagent for dehydrobromination and carbomethoxy deprotection in one step to give 3-[2-(1,3-butadienyl)]-1H-indoles from the corresponding bromo-derivatives. Suitably substituted indolyl-1,3-butadiene has been successfully applied to prepare murrapanine analogue via Diels-Alder reaction.