I am looking for international conferences on natural products, unfortunately some conferences are not sufficiently publicized or announced to late!! is there a permanent websites which announce periodically these events?
The American Society for Pharmacognosy is one place to start ( http://www.pharmacognosy.us/ ). Here is a list of their meetings: http://www.pharmacognosy.us/calendar-of-events/future-asp-meetings/
Scroll though this link- it is the international joint meeting in Copenhagan in June. You can see the other natural products organizations in other parts of the world- google them and you can find their meeting dates: http://www.pharmacognosy.us/wordpress/wp-content/uploads/JNPC2016-ASP-presentation.pdf
The American Chemical Society would do such papers under medicinal chemistry or organic chemistry. the web site is www.acs.org.
You may find information on some interesting upcoming conferences at the website of the Market Insider for Medicinal Plants of International Trade Centre (ITC). The calendar is updated periodically and presently includes both conferences and trade shows related to medicinal and aromatic plants from October 2015 through end of 2016: http://www.intracen.org/uploadedFiles/intracenorg/Content/Exporters/Market_Data_and_Information/Market_information/Market_Insider/Medicinal_plants/MI_MAPs_Upcoming_Events_revised_26_Sept_2015.pdf