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Synthesis and Diethylamine Induced Ring-Opening of exo and endo Hexahydro-7-oxa-2,5,6a-triaza-cyclopenta[a]pentalene-1,3-diones. -The title reaction results in stereoselective formation of endo-fused ring systems of type (III). The degree of selectivity slightly depends on the nature of the maleimide substituents. In the presence of secondary amines, the endo and exo products undergo an interesting and unprecedented ring-opening giving imidazolines of type (V). The expected double cis-elimination to pyrrolidin-3-ols does not occur. - (COSKUN*, N.; MERT, H.; ARIKAN, N.; Tetrahedron 62 (2006) 7, 1351-1359; Dep. Chem., Uludag Univ., TR-16059 Bursa, Turk.; Eng.) -Jannicke 24-128

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The EI mass spectra of 3,4-disubstituted-1,2,4-oxadiazin-5-ones 1−6 and -thiones 7,8 and 3,5-disubstituted 1,2,4-oxadiazin-6-ones 9,10 were recorded and their fragmentation pathways solved and compared with each other. The fragmentation routes of 5-ones and 5-thiones do not differ very much from each other but compounds 9 and 10 behave differently as could be expected based on their lactone type structures. Only compounds 4−6 exhibit a loss of CO and compound 7 a loss of NO. The loss of a benzyl group dominates the behaviour of compounds 1 and 2 which showed only few additional fragmentations. Some earlier data on some 3,4-disubstituted-1,2,4-diazin-5-ones 11−13 and -thiones 14,15 have been reanalyzed and discussed in further detail.