Abstract
AbstractPathways of formation are discussed for the thirteen toxisterols, the isolation and structure determination of which have been dealt with in the preceding article. It is suggested that toxisterols A (C‐8 spiro compounds) and C (bicyclo[3.1.0]hexenes) originate from previtamin D through relaxation of the excited “tZc” form of its hexatriene system. The symmetry‐allowed conrotatory closure to a 5–6–10 cyclopropane ring constitutes a plausible first step for both reactions. The description of the formation of the bicyclohexene moiety of toxisterol C1 as a two‐step process is consistent with its stereochemistry; the configuration corresponds to an overall [π4a + π2a] route and not to the [π4s + π2a] pathway expected for a concerted photochemical process. For the formation of toxisterols A a series of steps following the initial cyclopropane ring closure ‐ comprising a hydrogen shift from C‐4 to C‐9 and bond formation between C‐4 and C‐8 ‐ to provide the C‐8 spirostructure is proposed. Toxisterol D1 can be formed from previtamin D (or tachysterol) by a photoinduced antarafacial [1,5] hydrogen shift. The cyclobuteno compound toxisterol E1 originates from excited tachysterol through a disrotatorye ring closure of the 6‐7‐8‐9 butadiene moiety.A rationalization is offered for the novel photochemical 1,6‐addition of alcohol to the conjugated triene system of previtamin D/tachysterol yielding the toxisterols B, as well as for the photoreduction to form toxisterol R1. The article concludes with comments on the general picture of photoconversions as it emerges from previous and present investigations.
Published Version
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