Identification and isolation of new neolignan and sesquineolignan species: Their acid-catalyzed ring closure and specific accumulation in the fruit wall of Cirsium eriophorum (L.) Scop.

• Cirsium eriophorum fruit is a new source of neolignans and sesquineolignans.
• Fruit wall separated by germination contains high level of new neolignan/sesquineolignan.
• Specific transformation of compounds by acid-catalyzed cyclization was characterized.
• Acid-treated wall is the best source of balanophonin/picrasmalignan in the plant kingdom.

In Cirsium eriophorum fruit, the main neolignan and sesquineolignan compounds, denominated prebalanophonin (preBA) and prepicrasmalignan (prePI), were determined for the first time in the plant kingdom using a combination of optimized acid treatments and complementary spectroscopic (HPLC–MS, GC–MS, CD and NMR) methods. Analysis of fruit parts separated via germination, demonstrated the specific accumulation of these compounds, since preBA and prePI were exclusively found in the fruit wall. Based on quantitative approaches obtained by HPLC-UV measurements, the fruit wall was found to contain extraordinarily high amounts of preBA (4.57%) and prePI (2.88%) allowing their high-yield isolation by preparative HPLC. Optimized acidic treatment (2 N trifluoroacetic acid, 50 °C, 15 min) of the wall extract resulted in the quantitative transformation of preBA and prePI into balanophonin (BA) and picrasmalignan (PI), as a result of acid-catalyzed cyclization by the SN2 reaction. Consequently, acid-treated wall extract was found to be the richest raw material containing BA and PI (5.3% and 3.10%), reported to date.

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