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: Procaterol , a new beta - 2 adrenoceptor stimulant , was studied in a double - blind , placebo - controlled , cross - over trial in patients with bronchial asthma .
### Output Text:
<span class="chemical">Procaterol</span> , a new beta - 2 adrenoceptor stimulant , was studied in a double - blind , placebo - controlled , cross - over trial in patients with bronchial asthma . 