The structure of intermediate products of ‘fragmentation’ of 10-bromodihydrocinchonine

10-Bromodihydrocinchonine 1d, similarly to analogical derivatives of other main cinchona alkaloids, transforms into nicinquine and isonicinquine 2d formally loosing its C2 carbon atom in a form of formaldehyde. This reaction was found to proceed via the so-far unstudied intermediate compounds (5a) 4-S-(Z-propenyl)- and (5 4-S-(E-propenyl)-6-R-7-S-(quinolyl-4)-8-oxa-1-R-azabicyclo[4.3.0]nonane which at the same time are products of a novel rearrangement of the parent cinchonine. The stereostructure of these compounds was determined using, mainly, NMR techniques. The energy minima of conformers 5 and 5a were supported by molecular mechanics calculations. The mechanisms for the 1d → 5 → 2d sequence have been discussed. The alkaloid 5 is sterically preferred to its Z-isomer. The accompanying nucleophilic substitution (1d → 6) and elimination (1d → 7) are also stereospecific.