New pyrrole alkaloids with bulky N-alkyl side chains containing stereogenic centers from Lycium chinense

Four new pyrrole alkaloids, methyl 2-[2-formyl-5-(methoxymethyl)-1H-pyrrol- 1-yl]propanoate (1), methyl 2-[2-formyl-5-(methoxymethyl)-1H-pyrrol-1-yl]-3-(4- hydroxyphenyl)propanoate (2), dimethyl 2-[2-formyl-5-(methoxymethyl)-1H-pyrrol- 1-yl]butanedioate (3), and dimethyl 2-[2-formyl-5-(methoxymethyl)-1H-pyrrol-1- yl]pentanedioate (4), were isolated from the AcOEt extract of the fruits of Lycium chinense Miller (Solanaceae). The stereogenic center C(2) in the bulky N-alkyl side chain in each of 1-4 seems to hold the H-atoms of nearby CH ₂ groups, CH₂(7′) and CH₂(3) (if R≠H), leading to two different chemical shifts in the ¹H-NMR spectrum due to their diastereotopic characteristics. In the ¹H-NMR data of each of 2-4, the enhancement of H-C(2) signal was inhibited by the R group, probably due to steric hindrance, and its chemical shift was influenced by the anisotropy effect. The structures of 1-4 were elucidated by analysis of various spectroscopic data, including 1D- and 2D-NMR.