Dedicated to Prof. Albert Eschenmoser on the occasion of his 75th birthday Seven triterpenes were selected for study of their photoaging behavior by graphite-assisted laserdesorption/ionization mass spectrometry as the primary analytical tool. The triterpenes serve as simplified reference systems for complex triterpenoid resins, which are used as varnishes on paintings. Some of the triterpenes are main components of such natural resins; others were chosen for their particular functional groups or carbon skeleton to evaluate specific influences on the aging process. Progressive aging resulted in incorporation of oxygen and simultaneous loss of hydrogen, recognizable in the mass spectra by the appearance of signals with mass increments of 14 and 16 Da. Degradation of aging products also led to compounds with lower masses than the initial triterpenes. The general aging behavior of the triterpenes studied was found to be rather similar, although differences in reactivity, and, therefore, in the extent of oxidation were found, depending on the functional groups. Compounds with aliphatic ketone groups were much more reactive under the test conditions than others. A mixture of five triterpenes showed the highest reactivity. Yellowing, an important aspect of aging with regard to varnishes on paintings, was found to be related to a general breakdown of the initial triterpenes and was not caused by a single compound.

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Organic chemistryOrganic chemistry Z 0200 Putrescine, Spermidine, Spermine, and Related Polyamine Alkaloids -[528 refs.]. -(BIENZ, S.; DETTERBECK, R.; ENSCH, C.; GUGGISBERG, A.; HAUSERMANN, U.; MEISTERHANS, C.; WENDT, B.; WERNER, C.; HESSE, M.; Alkaloids (Academic Press) 58 (2002) 83-338; Org.-Chem. Inst., Univ. Zuerich, CH-8057 Zuerich, Switz.; Eng.) -Lindner 30-284

A straightforward synthesis of the enantiomerically pure nitro derivatives 31 and epi-32, which are particularly useful intermediates for the synthesis of highly functionalized g-lactams, is presented. ()-(R)-3-Hydroxy-3-phenylpropanoic acid (20) and its ethyl ester 25 were prepared from ()-l-mandelic acid (21). Condensation of 20 with pivalaldehyde furnished the novel enantiomerically pure 1,3-dioxan-4-one 17, the absolute configuration of which was established by X-ray crystal-structure analysis. Treating the lithium enolate of 17 with the nitro alkene 18 led, in a Michael-type addition, to a 1 : 1 mixture of two diastereoisomeric products. The stereocontrol of the addition was limited to the novel stereogenic center next to the lactone function. When the lithium enolate of 25 was treated with 18, the same selectivity was observed but with a lower chemical yield. Very facile separation of the isomers was achieved later in the synthetic sequence, when one isomer cyclized selectively to the nitro lactone 31, while the other one was isolated as hydroxy ester epi-32. The relative configuration of racemic epi-32 could be established by X-ray crystal-structure analysis.Introduction. ± In the course of our investigations on the total synthesis of caesalpinine A (1; Scheme 1) [1], a cyclic spermidine alkaloid [2], we were interested in finding an easy access to the lactam 2. The nitro ester 3 was assumed to be a useful precursor to 2, as it should undergo lactamization as soon as the NO 2 group is reduced. In 1979, Fra¬ter had found that b-hydroxybutyrate 4 can be alkylated with high diastereoselectivity, when its corresponding enolate is treated with alkyl bromides (Scheme 2) [3]. Seebach and co-workers applied this method for nitro alkenes 5 as alkylating agents [4]. Subsequent hydrogenation of the NO 2 groups afforded the corresponding g-lactams through spontaneous cyclization of the in situ formed amino esters. The configuration of the novel stereogenic center in the a-position to the ester was in agreement with the findings of Fra¬ter. The stereoselectivity can be explained by