As you have found out, the Denons are a closed headphone design, but not a sealed headphone design.
As people have suggested here, the AKG K550 is a good choice for an around-ear sealed headphone. They sound good for the price, and are also comfortable to wear with glasses. While they can provide a good seal (some people find them too big though) the materials used should allow you to put on or remove your glasses without taking the headphones off.
Another suggestion would be Sony's Z1000 which you may be able to find discounted now that it is discontinued, or the MDR-7520 which many people say is essentially the same headphone. The MDR-1R is another option. (though I haven't heard them) They are similarly isolating, but are quite a bit smaller than the K550 and have better padding. The clamping force is a little higher, and due to the materials used you will not be able to put on your glasses without taking the headphones off first - at least not until the pads soften up. They're more comfortable, sound better in my opinion, and they have a removable cable (that uses a standard 3.5mm jack) which I find important.
If you are wanting on-ear headphones that isolate well, B&W's P5 are very good in that regard. They have a high clamping force though, and I find on-ear headphones to be uncomfortable to use with glasses, as they press the top of my ear against the arm. Around-ear headphones don't have that problem for me. Sennheiser's HD25-II or Beyerdynamic's DT1350 should also be good choices for sealed on-ear headphones.