Minor and trace sterols in marine invertebrates 30. Isolation,structure elucidation and partial synthesis of 26-methylstrongylosterol and 28-methylxestosterol - two marine sterols arising by a novel quadruple biomethylation sequence

Two new sterols, 26-methylstrongylosterol ($\text{4}$) and 28-methylxestosterol ($\text{5}$), arising from a hitherto unknown quadruple biomethylation sequence, together with the biogenetically important “missing link”, durissimasterol ($\text{6}$), were isolated from the Indo-Pacific sponge $\text{Strongylophora}$$\text{durissima}$.