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: Reduction of pain during induction with target - controlled propofol and remifentanil .
### Output Text:
Reduction of pain during induction with target - controlled <span class="chemical">propofol</span> and <span class="chemical">remifentanil</span> . 