May I comment on the CAS name of Viagra - more properly known by its generic name Sildenafil. All structures in CAS which are tautomers are named by the tautomer which gives the preferred name irrespective of the more probable tautomer (see Index Guide Appendix IV para 122). Hence a hydrogen at N-4 of the heterobicyclic ring system is preferred to one at N-6. At the British Pharmacopoeia Commission we named it: 5-[2-ethoxy-5-(4-methylpiperazin-1-ylsulfonyl)phenyl]-1,6-dihydro-1-methyl- 3-propylpyrazolo[4,3-d]pyrimidin-7-one which is I think the most likely tautomer. Gerry Moss _________________________________________________________________________ Dr GP Moss Telephone: +44 171 775 3262 Department of Chemistry Queen Mary & Westfield College Facsimile: +44 181 981 8745 Mile End Road London E1 4NS, United Kingdom E-mail: g.p.moss@qmw.ac.uk World Wide Web server: http://www.chem.qmw.ac.uk/ IUPAC chemical nomenclature data base: http://www.chem.qmw.ac.uk/iupac/ IUBMB biochemical nomenclature data base: http://www.chem.qmw.ac.uk/iubmb/ chemweb: A list for Chemical Applications of the Internet. To post to list: mailto:chemweb@ic.ac.uk Archived as: http://www.lists.ic.ac.uk/hypermail/chemweb/ To (un)subscribe, mailto:majordomo@ic.ac.uk the following message; (un)subscribe chemweb List coordinator, Henry Rzepa (mailto:rzepa@ic.ac.uk)