This is actually a reply to Harijay (I don't Twitter) - you can look at the PubChem synonyms, they usually contain CAS numbers, but aren't marked as CAS numbers as such. You'd have to use a regexp plus do a checksum to ensure that it's a CAS number. ChemSpider might also do this - Rajarshi Guha

Hey thanks Rajarshi I will try and do this - Hari

Try http://rguha.ath.cx/~rguha... (replace the CID at the end of the URL with yours). You will get 0 or more CAS numbers - no checksumming, and not in sync with latest PubChem - Rajarshi Guha

checksumming added, so they should be syntactically valid CAS numbers - Rajarshi Guha

Hey rajashi thanks for the quick program , also i see what you are talking about at the bottom of the pubchem synonyms are the CAS ids only they are not called CAS ids ..maybe someday you can tell me how your app works..thanks again - Hari

we have a local mirror of PubChem - and the app just does an SQL query looking for synonyms for a given CID that match the CAS regex. The Python code then does a checksum for each returned entry, and sends the browser the valid entries - Rajarshi Guha