3,4,5-trimethoxycinnamic acid, aka sinapinic acid methyl ether, is a naturally occurring derivative of cinnamic acid. It is very closely related in structure to sinapinic acid, ferulic acid, and caffeic acid. It is also a major metabolite of elemicin. The acute toxicity of this compound is extremely low, with comparable LD50/toxicity values to ferulic acid and sinapinic acid, both of which are non-toxic nutrients and fundamental building blocks for lignins (the fundamental structural component of wood and plant fibers). Along with other cinnamic acid derivatives, it is found ubiquitously in all organisms in the plant kingdom.
3,4,5-trimethoxycinnamic acid has been isolated from Polygala tenuifolia, a Chinese traditional medicine used sometimes as a nutritional supplement or nootropic. It has also been found to occur naturally in humans, black pepper (Piper nigrum), and has been isolated in trace amounts in cow’s milk  . It is likely to be ubiquitous to all living organisms, since cinnamic acids in general are so widespread in nature. In mice studies it has GABAergic effects, as a positive allosteric modulator . This indicates possible anxiolytic, antidepressant, and neuroprotective activity .
One study conducted on animal models of gastritis found that 3,4,5-trimethoxycinnamic acid was highly effective at reducing acidic gastric lesions in mice. The study found this compound to be the most effective out of ten cinnamic acid derivatives studied, and almost twice as potent by weight as cimetidine, a commonly prescribed gastritis medication and H+ proton pump inhibitor . From the paper: “The effects of pretreatment with cinnamic acid derivatives on HCl/ EtOH-induced gastritis were investigated (Fig. 1). In the non-treated control group, the index of gastric lesions was 74.2 ± 15.7 mm. In the treatment groups; in cinnamic acid (200 mg/kg) 26.0 ± 18.7 mm (inhibition, 65.0%), p-methoxycinnamic acid (200 mg/kg) 33.5 ± 18.7 mm (inhibition, 54.9%), and 3,4,5-trimethoxycinnamic acid (200 mg/kg) 10.2 ± 5.0 mm (inhibition, 86.3%). Cimetidine (200 mg/ kg) the positive control had a index of 35.0 ± 8.5 mm (inhibition, 52.8%).”
One study has also found 3,4,5-trimethoxycinnamic acid to reduce or relieve opiate withdrawal, induced by naloxone, in morphine-dependent rats and mice .
Cinnamic acid and its derivatives (such as phenyl ring substituted analogs) can undergo electrolytic amination to form the L-phenylalanine derivatives . One study found that caffeic acid (3,4-dihydroxycinnamic acid) can be aminated in an electrode cell to form L-DOPA (3,4-dihydroxyphenylalanine). This can be further decarboxylated (simply by boiling the precursor in anhydrous solvent) to form dopamine (3,4-dihydroxyphenethylamine).
This means that substituted analogs of cinnamic acid (such as ferulic acid and sinapinic acid) can be used as precursors to the corresponding ring-substituted phenylalanines (e.g. 3-methoxy-4-hydroxy-L-phenylalanine, 3,5-dimethoxy-4-hydroxy-L-phenylalanine, etc).
Available as the free acid, and trans isomer only.
 – http://www.hmdb.ca/metabolites/HMDB0002511
 – http://www.mcdb.ca/metabolites/BMDB02511
 – https://www.sciencedirect.com/science/article/pii/S1347861315001413
 – https://www.ncbi.nlm.nih.gov/pubmed/15305046
 – https://synapse.koreamed.org/Synapse/Data/PDFData/0228NPS/nps-23-299.pdf
 – https://www.ncbi.nlm.nih.gov/pubmed/30632088
 – http://jes.ecsdl.org/content/136/6/1845.abstract