N-Acylnitroso derivatives 6 which were prepared by in-situ oxidation of the corresponding hydroxamic acids 5 reacted instantaneously and in high yields with dihydropyridine 4. The Diels-Alder adducts 8 were formed rcgiospecifically with the acylnitroso dienophiles 6a<, whereas the dienophiles 6d f gave mixtures of both regioisomers 7 and 8. These and some other results [2] were best explained by the FMO theory. The Diels-Alder adducts 7 and 8 gave the corresponding 'unti'-&-glycols when reacted with Os04/N-methylmorpholine N-oxide. Hydrogcnolysis of the N-O bond followed by peracetylation led to the expected aminolyxose derivatives 14 and 16. A similar sequence, using 4 and the hydroxamic-acid derivative 18 of (+)-D-mandelic acid led, with a poor asymmetric induction, to a mixturc of the expected optically active aminolyxose compounds 19A/19B.
Reaction of pyran-2-thiones 4 with nitroso derivatives led surprisingly to type-8 (19) adducts which proved to be isomeric with the initially expected primary Diels-Alder cycloadducts 5. Methyl 2-thioxo-2H-pyran-5-carboxylate (40, when reacted with nitrosobenzene at -lo", led quantitatively to the thieto-oxazine intermediate 13, which turned out to be the cornerstone of the complex cycloaddition-rearrangement 5 4 reaction pathway (Scheme 3 ) . Differential scanning calorimetry, as performed for the 18a-19a conversion, permitted to demonstrate that this multistep rearrangement is overall a highly exothermal process, the final product 19 representing an energy-sink along this reaction pathway.
Die Hydroxamsäuren (I) werden durch Tetrapropylammoniumperiodat (II) in die Acylnitrosoverbindungen (III) übergefuhrt und diese an Dihydropyridin‐N‐carbonsäuremethylester (IV) unter Bildung der isomeren Addukte (V) und (VI) angelagert.
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