I don't know of any centuries in June, but there is a hillclimb race up Whiteface in mid-June http://whitefacerace.com/bikeraceinfo.html (it's not quite in New England but it's close). I did the run last year- it was great!

For challenging rides http://www.northeastcycling.com/NE_Climbs_main.html is a great webside. It compares all the major NE hillclimb races with elevation profiles etc (I did the Equinox race last summer and am planning on going back). The Gap ride section of the website is also really helpful. I can recommend doing part or all of the Vermont 6 gap ride. So far I've only done the App Gap- Middlebury Gap loop but I really enjoyed it and would love to do the 4 gap or 6 gap ride sometime.

Have fun!