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 series of N - pyrimidinyl - 2 - phenoxyacetamide adenosine A ( 2A ) antagonists is described .
### Output Text:
A series of <span class="chemical">N - pyrimidinyl - 2 - phenoxyacetamide</span> <span class="chemical">adenosine</span> A ( 2A ) antagonists is described . 