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: To date , 23 men with a median age of 50 years and good performance status have entered the trial .
### Output Text:
To date , 23 men with a median age of 50 years and good performance status have entered the trial . 