Scheme 1
![Scheme 1](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs43094-019-0016-7/MediaObjects/43094_2019_16_Sch1_HTML.png)
Reagents and conditions: (a) dry methanol, reflux 9–10 h; (b) methanol, NaOH, THF, RT; (c) thiosemicarbazide or semicarbazide hydrochloride, dry isopropyl alcohol, and glacial acetic acid, reflux 36 h
Reagents and conditions: (a) dry methanol, reflux 9–10 h; (b) methanol, NaOH, THF, RT; (c) thiosemicarbazide or semicarbazide hydrochloride, dry isopropyl alcohol, and glacial acetic acid, reflux 36 h