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: KF17837 is a novel selective adenosine A2A receptor antagonist .
### Output Text:
<span class="chemical">KF17837</span> is a novel selective <span class="chemical">adenosine</span> A2A receptor antagonist . 