guaguanco - 27-3-2004 at 22:48
I'm probably missing something obvious, but I haven't run across a heterocylic chemical with no duplicated elements in the ring.
Something like an -O-C-N- aziridine-type structure, or a -N-O-C-S-P- ring...
I'm sure an example exists. And the ring has to have 3 or more members, so carbon monoxide doesn't count!