Methylation of dihydroprotoberberine-type enamines, dihydropalmatine (Ia) and dihydroberberine (Ib), with formaldehyde gave 13-methylprotoberberines, dehydrocorydaline (IIa) and 13-methylberberine (IIb) and dimeric protoberberines, 13, 13'-ethylenebis(palmatine) (IVa) and 13, 13'-ethylene-bis(berberine) (IVb), respectively. The structure of these dimeric products was deduced from chemical and spectral data, and established by an alternative synthesis.
