In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Angina and ischemic electrocardiographic changes occurred after administration of oral dipyridamole in four patients awaiting urgent myocardial revascularization procedures .
### Output Text:
Angina and ischemic electrocardiographic changes occurred after administration of oral <span class="chemical">dipyridamole</span> in four patients awaiting urgent myocardial revascularization procedures . 