Andransinine: An Unusual Case of Spontaneous Resolution in an Indole Alkaloid Derivative

Racemic andransinine (1), an indole alkaloid derivative
obtained during isolation of alkaloids from Alstonia angustiloba and Kopsia pauciflora, was found to undergo spontaneous
resolution when crystallized in EtOAc, forming racemic conglomerates
(an equimolar mechanical mixture of enantiomerically pure individual
crystals). X-ray analyses of the enantiomers (obtained from crystals
from EtOAc solution and from chiral-phase HPLC) provided the absolute
configuration of each enantiomer as (15R,16S,21R)-(+)-andransinine (1a or I+) and (15S,16R,21S)-(−)-andransinine (1b or I−).