Question: the diels-alder reaction is an example of which class of reactions
[{'answer': 'Cycloaddition', 'provenance': [{'wikipedia_id': '173238', 'title': 'Diels–Alder reaction', 'start_paragraph_id': 1, 'start_character': 379, 'end_paragraph_id': 1, 'end_character': 392, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]