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: A new vitamin D analogue , 22 - oxacalcitriol ( OCT ) , has been shown to have promising characteristics .
### Output Text:
A new <span class="chemical">vitamin D</span> analogue , <span class="chemical">22 - oxacalcitriol</span> ( <span class="chemical">OCT</span> ) , has been shown to have promising characteristics . 