Is いけません used as an interjection like いけない？ Realizing you've left your money at home when you're out to buy something, (I think) you can say ; could you, politely, say ? (I suppose, for that matter, my question could also be applied to .)

The short answer is 'no'.

You would sound very strange if you said as an interjection. It would be just too funny. is basically a phrase you say to another person, not to yourself. You say it when you think something is no good or when you want to stop someone from doing something that you do not think is good.

When we say things like {}("Oops! Forgot the money!") on our way to the convenience store, we are indeed talking to ourselves, are we not?

You say , , , etc. as an interjection around Kanto. Western Japan is another story -- around Nagoya and around Osaka, etc.

Finally, the same goes for /; Only the former is used as an interjection.