This is a rather hard tesuji. 

Black to capture the five white stones at "a"

These are the intersections MadLab considers while searching:

The answer is Black "1" (see the figure below), which MadLab finds in about 15 seconds on a fast pc. No other moves work, and the variations are quite complex. MadLab can show you how Black "1" works (and why the point to the left of Black "1" does not).

Source: The problem appears several places; for instance in the book The art of connecting stones (Wu Piao, Yutopian). Thanks to Anders Kierulf (www.smartgo.com) for drawing my attention to it.