On the structure, biosynthesis, function and phylogeny of isoarborinol and motiol

The solid‐state conformations of the C‐3 acetates of two isomeric hopanoids—1, isoarborinol (D∶C‐friedo‐B1∶A1‐3β,5α,8α,10β,13β,14α,17β,18α,21β) and 2, motion (D∶C‐friedo‐B1∶A1‐neogammacer‐7(8)‐en‐3β‐ol[3β,5α,9α,10β,13α,14β,17α,18β,21α])—have been determined by X‐ray crystallography. The data show that whereas both molecules are planar, 1 orients into a chair‐halfchair‐chair‐chair‐halfchair conformation while 2 orients into a chair‐sofa‐twist‐halfchair‐halfchair conformation. To explain the biogenesis of 1 and 2 from squalene oxide, a step‐wise mechanism is proposed which proceeds through the protosteroid cation (for 1) and dammarenyl cation (for 2). After ring enlargement from the corresponding 13(17)bond followed by concerted 1,2‐migrations and loss of the 11β‐H and 7β‐H as protons, respectively, a 9,11‐double bond (in 1) and a 7,8‐double bond (in 2) is introduced into the nucleus. The mechanism is discussed in relation to the classical view of a non‐stop cyclization process where, for example, squalene oxide folds in a chair‐chair‐chair‐chair‐boat conformation to give a cyclized product (motiol) presumably with the same conformational disposition as the cyclizing material. The three‐dimensional geometry of 1 and 2 was found to be structurally dissimilar from sterols. For instance, 1 and 2 are shorter and volumetrically smaller molecules than cholesterol, and this may explain their diminished importance as membrane inserts compared with sterols in eukaryote evolution.