Fractionation and chromatography of an ethanol extract of the stembark of Guatteria melosma Diels (Annonaceae) afforded melosmine (2) and melosmidine (3), two members of a new class of aporphine alkaloids (7,7-dimethyltetradehydroaporphines). A consideration of the spectral data of melosmine (2) and its O,O-dimethyl-(5), tetrahydro-(6), O,O-dimethyltetrahydro-(7), N-methyltetrahydro-(8), N-methyl-O,O-dimethyltetrahydro-(9) and O,O-diacetyl-(10) derivatives suggested the structure to be 2 (2,3-dimethoxy-7,7-dimethyl-7H-dibenzo [de,g] quinolin-1,9-diol). This was confirmed by x-ray crystallographic analysis. A consideration of the spectral data of melosmidine (3) and its O-methyl-(5), tetrahydro-(11), N-methyltetrahydro-(12) and O-acetyl-(13) derivatives indicated that melosmidine was 3 (1,2,3-trimethoxy-7,7-dimethyl-7H-dibenzo [de,g] quinolin-9-ol). O-Methylmelosmidine (5) is identical to O,O-dimethylmelosmine (5). In addition, isoboldine (4) and two uncharacterized alkaloids, GMA 1 and GMA 2, were isolated. © 1982, American Chemical Society. All rights reserved.