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: Phenylephrine but not ephedrine reduces frontal lobe oxygenation following anesthesia - induced hypotension .
### Output Text:
<span class="chemical">Phenylephrine</span> but not <span class="chemical">ephedrine</span> reduces frontal lobe oxygenation following anesthesia - induced hypotension . 