Maybe by journal type. If a product is extracted from X bacteria, then it will be reported as not a synthesis. Refine by polyketide?...hope this helps. There are still a plethora of natural products that haven't been synthesized yet. Heres an example that probably wont be synthesized lol:
Fenical, W.; Jensen, P. R.; Nature Chemical
Biology. 2006, 12, 2, 666-673.