There has been some really creative photochemistry coming out of the Kutateladze lab at the University of Denver. I particularly appreciate their approach to highly reactive dienes using photochemical irradiation of aminoxylylene precursors tethered to pyrroles. Irradiation leads to photoinduced tautomerization, which in turn triggers the Diels-Alder cycloaddition shown below. I have drawn a representative example in which an alkaloid-like system is zipped up under UV irradiation. It is interesting to note the effect of concentration on the efficiency of this step. If you read the experimental procedure, the reaction is 0.16M in substrate, which is rather practical for a photochemical process. Another feature of this sequence is that the enamine formed during the Diels-Alder step can be trapped by tosyl azide. I don’t know about you, but I am impressed by the fact that photochemically labile tosyl azide is not leading to any unwanted premature reactions. The rationale for this curious finding is that sulfonyl azides do not absorb above 350 nm, which is why they are not affected by the irradiation at 365nm.