Starting from 2-chloropyridine 1, a series of l,2-diazepin-3-ones 16 was prepared by a five-step synthesis. The key intermediates were the bicyclic oxadiazolium salts 10 which led, through a methoxide ion induced ring opening, to the expected 2-methoxypyridinium ylides 11. The photoinduced ring enlargement of ylides 11 proved to be regiospecific and led to 3-methoxy-l,2-diazepines, the immediate precursors of the final diazepinones 16.
2‐Chlorpyridin (I) liefert mit Mesitylsulfonylhydroxylamin das N‐Amino‐pyridiniumsalz (II), das mit den Säurechloriden (III) über die Acylierungsprodukte (IV) zu den bicyclischen Salzen (V) führt.
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