Easy activation: A Lewis acid catalyst based on molybdenum relies on activation by molecular oxygen for its high catalytic activity. The novel complex catalyzes the aza-Diels–Alder reaction of acyl hydrazones with Danishefsky dienes (see scheme) and other Lewis acid mediated reactions, such as the Mukaiyama aldol addition, Mannich-type reactions, and the addition of trimethylsilylcyanide to imines. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2006/z600801_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.