The isolation and structures of four new alkaloids, mesembrenol (5, R = Me; R′ = H), O-acetylmesembrenol (5, R = Me; R′ = Ac), 4′-0-demethylmesembrenol (S, R = R′ = H), and 4′-0-demethylmesembranol (4, R = H) are reported. The position of the phenolic hydroxyl in 4′-0-demethylmesembranol is determined by the application of a radioisotope dilution method. A discussion of the circular dichroism and nuclear magnetic resonance spectra of (+)-mesembrenone (2) and the nmr spectra of related alcohols mesembrenol and 6-epi-mesembrenol (7) is presented in providing information on the conformational preference of ring C in which it is shown that the equivalent forms of the half-chair, as represented in structures 2a, Sa, and 7a, is preferred. © 1970, American Chemical Society. All rights reserved.