Combining the results from the three independent processes allows us to place a lower limit for AHf°0(c-CH2N2) at 61 kcal/mol. The fact that the heat of formation of diazirine resulting from the measured onsets for processes I, II, and III all lie within 1 kcal/ mol of each other is indicative that the true value of AffACc-CEysy is close to our lower limit. As previous'!.)' discussed, it is doubtful if AHf °0(c-CH2N2) is greater than 66 kcal/mol. A8'f°(c-CH2lSr2) obtained previously (79.3 kcal/mol) by electron impact6 is definitely outside of the above limits. From Table I, it is apparent that the CH2+ (m/e 14) peak obtained by 3507 Paulett and Ettinger is twice as large as the parent ion, w7hereas our data show that the CH2+ peak is only 66% of the parent. The other peak ratios are quite comparable. It is therefore possible that extensive pyrolysis may have taken place in their reaction chamber to produce CH2 + N2 prior to ionization. The appearance potential of 11.0 eV which they measure may be due to "free" CH2, which leads to a much higher value of AHf%c-CH2N2). The results are summarized in Table II.
Abstract:The examination of the aerial parts, roots, and seeds of the endemic plant Rindera umbellata is reported in this paper for the first time. Phytochemical investigation of R. umbellata led to the isolation and characterization of ten pyrrolizidine alkaloids and eleven fatty acids in the form of triglycerides. Pyrrolizidine alkaloids 1-9 were found in the aerial parts, 7 and 8 in the roots, and 6-10, together with eleven fatty acids, in the seeds of this plant species. The structures of compounds 1-10 were established based on spectroscopic studies ( 1 H-and 13 C-NMR, 2D NMR, IR and CI-MS). After trans-esterification, methyl esters of the fatty acids were analyzed using GC-MS. The effect of lindelofine-N-oxide (7) on tubulin polymerization was determined.
The genus Senecio (family Asteraceae) is one of the largest in the world. It comprises about 1100 species which are the rich source of pyrrolizidine alkaloids. Plants containing pyrrolizidine alkaloids are among the most important sources of human and animal exposure to plant toxins and carcinogens. The pyrrolizidine alkaloids of seven Senecio species (S. erucifolius, S. othonnae, S. wagneri, S. subalpinus, S. carpathicus, S. paludosus and S. rupestris) were studied. Fourteen alkaloids were isolated and their structures determined from spectroscopic data ( 1 H-and 13 C-NMR, IR and MS). Five of them were identified in S. erucifolius, four in S. othonnae, two in S. wagneri, four in S. subalpinus, two in S. carpathicus, three in S. paludosus and three in S. rupestris. Seven pyrrolizidine alkaloids were found for the first time in particular species. The results have chemotaxonomic importance. The cytotoxic activity and antimicrobial activity of some alkaloids were also studied.
p-Allylpalladium intermediates are known to participate efficiently in transformations involving nucleophilic species. Exploring these processes, we have developed a method for the preparation of allyl acetates via palladium-catalysed functionalisation of allenes and 1,3-dienes. Reactions of aryl iodides with either of these two classes of compounds and excess sodium acetate in the presence of Pd(OAc) 2 and Ph 3 P as the catalytic system afforded the respective allyl acetates in moderate to good yields. The scope of this process has been investigated. The described method is an addition to the synthetic repertoire for the preparation of allyl acetates, and may be useful, in particular, for the synthesis of structurally complex compounds of this type.Allyl acetates represent a useful class of organic compounds extensively used in the allylation processes catalysed by a range of transition metals. 1 An additional synthetic methodology employing allyl acetates or related esters is the Claisen-Ireland 3,3-sigmatropic rearrangement, which produces g-unsaturated carboxylic acids or their derivatives. 2 If directly prepared, they can also be a source of synthetically very useful allyl alcohols via hydrolytic processes.A frequently used procedure for the preparation of allyl acetates is based on the acetylation of allyl alcohols, 3a which are usually accessible via the vinylation of aldehydes or ketones. 3b-e Although useful, this method often requires the application of sensitive magnesium-or lithium-based organometallic reagents. Direct synthesis of allyl acetates can be achieved by allylic acetoxylation and this methodology has been investigated for a long time leading to the development of a number of oxidative reagents. 4 In recent decades, related palladium-catalysed activation of the allylic C-H bond emerged as an efficient method for the preparation of this class of compounds. 5 An alternative approach is based on the highly regioselective palladium-catalysed functionalisation of allyl acetates employing boronic acids. 6 These reactions were carried out without a ligand and in the presence of the oxidative reagent. A high degree of regioselectivity was attributed to the chelatation of the palladium-intermediate by the acetate substituent. 1,3-Dienes have also shown to be suitable starting materials for the preparation of allyl acetate derivatives via halo-acetoxylation catalysed by palladium. 7Our interest in the palladium-promoted chemistry of allenes prompted an investigation of their reactivity in the presence of an acetate as nucleophilic species (Scheme 1). 8 The initial idea is based on the well-documented reactivity of p-allylpalladium intermediates in the presence of heteroatom nucleophiles (Scheme 1). 9 Intermolecular reactions with carboxylic nucleophiles have not been widely explored, while an intramolecular variant has been used for the preparation of lactones. 10 Although the product of the proposed allene-acetate reaction, an allyl acetate, is a substrate for palladium-catalysed transformations and ...
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