In an attempt to obtain adequate supplies of the alkaloid flavinine for complete structural characterisation, the plant Croton flavens was reaped from the same location as before. Countercurrent separation again afforded norsinoacutine (I; R=H), but we were surprised not only by the absence of flavinine but also by the presence of sinoacutine (I; R=Me) and a new alkaloid flavinantine (II; R1=R2=Me, R3=H), C₁₉H₂₁NO₄, m.p. 130–132°, [α]ᴅ −14.5° (EtOH). By using two-dimensional TLC to examine the alkaloids of entire and serrate leaf specimens of C. flavens (2), it is now evident that the species is not homogeneous: the serrate-leaf variety contains flavinine and norsinoacutine, while the entire-leaf type has the facility for performing N-methylations and contains flavinantine, sinoacutine, and norsinoacutine. The U.V. (EtOH) [λmax 239 nm (ε 14,910), 286 nm (7078)], i.r. [νmax (CHCl₃) 3448 cm⁻¹ (OH), 1667, 1639, 1626 cm⁻¹ (dienone), 1508 cm⁻¹], n.m.r. [(CD₃)₂SO: δ 2.32 (3H; NMe), 3.72, 3.79 (3H each; 2 OMe), 6.22, 6.72, 7.02 (2H, 1H, 1H respectively; H-1, H-4, H-5, H-8)], and circular dichroism spectra of flavinantine are consistent with a morphinandienone structure. The C-2,C-3 substitution pattern and CD-determined stereochemistry were supported by conversion of flavinantine to N,O-dimethylflavinine methiodide. A comparison with synthetic isosalutaridine (racemate of II; R1=R3=Me, R2=H) (4) established flavinantine as II (R1=R2=Me, R3=H). Flavinantine methiodide was identical to N-methylflavinine methiodide, defining flavinine as II (R1=R3=H, R2=Me). Deuterium exchange of the C-4 proton in flavinantine (n.m.r. control) replaced the i.r. band at 1508 cm⁻¹ with 1488 cm⁻¹ in the deuteriated compound, showing these IR changes can no longer be ascribed solely to C-1 substitution (6). Flavinine and flavinantine are probably biosynthesised from reticuline-type precursors via para-para-intramolecular oxidative coupling, followed by demethylation and re-methylation to give the observed ring A substitution, analogous to the formation of crotonosine from coclaurine (7).