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: Skin reactions associated with oral coumarin - derived anticoagulants are an uncommon occurrence .
### Output Text:
Skin reactions associated with oral <span class="chemical">coumarin</span> - derived anticoagulants are an uncommon occurrence . 