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: Activation of presynaptic alpha2 - adrenoceptors is the most widely accepted mechanism of action of the antisympathetic drug clonidine ; however , other target proteins have been postulated to contribute to the in vivo actions of clonidine .
### Output Text:
Activation of presynaptic alpha2 - adrenoceptors is the most widely accepted mechanism of action of the antisympathetic drug <span class="chemical">clonidine</span> ; however , other target proteins have been postulated to contribute to the in vivo actions of <span class="chemical">clonidine</span> . 