There are many reactions in Biological system which follow Diel Alders Reaction and this is specially found during formation of various pharmacologically active ployketides involving polyketide synthase enzymes. Here are two papers for you reference
When talk about Diels-Alderases, I guess you would certainly like to know Ben Liu and coworkers’ work on spinosyn biosynthesis. See their Nature paper and a review on Diels-Alderases:)