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: The dose of ( - ) - propranolol was significantly smaller than that of ( + ) - propranolol in both species but much higher than that required to produce evidence of beta - blockade . 8 .
### Output Text:
The dose of ( - ) - <span class="chemical">propranolol</span> was significantly smaller than that of ( + ) - <span class="chemical">propranolol</span> in both species but much higher than that required to produce evidence of beta - blockade . 8 . 