New 30-norlupane derivatives through chemical-microbial semi-synthesis of betulinic acid and their neuroprotective effect.

•Seven 30-norlupane derivatives (2–8) was obtained through combined chemical and microbial semi-synthesis of betulinic acid.•Metabolites 2–4 and 6–8 are reported for the first time.•Chemical-microbial semi-synthesis was applied as a very useful approach to enrich the structural diversity of natural products.•Metabolite 6 showed a potent neuroprotective activity.