May be this information is helpful if you are planning experiments on antisyphilitic activity of plant extracts:

Species of Datisca, Euphorbia, Lobelia (Toxic Plants of North America by GE Burrows and RJ Tyrl) and Acacia leucephloea (Indian Medicinal Plants: An Illustrated Dictionary by C.P. Khare) were reported to have the activity (folklore use).

http://www.ncbi.nlm.nih.gov/pubmed/7934084

http://www.scielo.br/pdf/bjmbr/v38n12/v38n12a16.pdf

http://holisticonline.com/herbal-med/hol_herb_glossary.htm

I am suggesting to go through the article www.ijppr.com 4(3) 2012 pp 92-96. Mentioned Agava americana

Guaiacum officinale was imported just in the year of 1508 to Spain. The powder of the wood, named Lignum sanctum, Lignum vitae oder Lignum benedictum

was used against lues venerae (syphilis).At the time in Germany the drug was used, too. The book "De Guajaci medicina et morbo gallico liber unus" was writen from Ulrich von Huttens in the year of 1519. This book was spread over the royal community for healing this illness. The special application of the drug against syphilis was developed from the spanish physician Monardes.

This not antisyphilis but a herb used for the pain of late stage syphilis is the NZ herb Pukatea.

Please take a look at these useful links.

https://en.wikipedia.org/wiki/Euphorbia_antisyphilitica

https://en.wikipedia.org/wiki/Polemonium_caeruleum

https://www.science20.com/humboldt_fellow_and_science/blog/euphorbia_hirta_l_has_been_used_treatment_syphilis_traditional_medicines

https://www.researchgate.net/deref/http%3A%2F%2Fwww.scielo.br%2Fpdf%2Fbjmbr%2Fv38n12%2Fv38n12a16.pdf

I suggest you https://en.m.wikipedia.org/wiki/Polemonium_caeruleum

I am suggesting going through the attached article. In this article, A Ayurveda classical literature Rasa tarangini therapeutic indications have been described. In this article, on page no 10 (344 to 348 no) there was a reference of Javitri ( Myristica fragrans), Lavanga ( Syzygium aromaticum ), Guduchi ( Tinospora cordifolia), Ananta moola ( Hemidesmus indicus ) have been indicated with some mercurial preparations.

Kindly find the attached article.