The pros always seem to have a nifty answer to this but it might be as simple as breaking up more concrete - most of the time I'm working in brand-new construction and half the time the plumbers are breaking up part of the floor slab!

Seriously, though, I guess there's not enough room for an angle grinder, but I wonder if you could use a Dremel-type tool? I suspect it'd take ages and you want to use adequate face/eye protection as this can be a dangerous procedure.

You could get a 4" angle grinder in there and cut from the inside. A worn wheel would fit better. You can also use a cut-off .045 wheel on the same grinder. That would be my choice.

HD has a flange assembly that slips into the pipe and is sealed with 3 gaskets on the outside. No idea if it's code, but I used one for an upper floor and it solved the problem. Not sure about using it on the ground floor, though.

I just realized you may not be using it for a toilet. Sorry about that.

How big a line are you trying to connect with?

A donut would slip inside the iron pipe and have a hole for the OD of the mating pipe.

Ended up going at it with a dremel and cut off wheel(s). I scored a line and broke the chunks out with a hammer. Made a somewhat clean cut but it took a while. Thanks for the suggestions everyone on to the next part of this never ending project :-)