Strophasterols A-D (1-4) are rearranged ergostanes with an unprecedented carbon skeleton isolated from the edible mushroom Stropharia rugosoannulata. Strophasterol A (1), originally reported as an ER-stress suppressing as well as anti-MRSA substance, was found to induce fruiting body formation in some mushrooms. The synthesis of 1-4 commenced with an efficient acid-catalyzed double bond migration of an ergosterol derivative from ring-B to ring-D. Oxidative cleavage of the double bond on the D-ring gave a keto carboxylic acid, the acyl radical cyclization of which then afforded the isolated cyclopentane ring unit characteristic to the strophasterols. The stereodivergent synthesis of 1 and 2 was accomplished via diastereoselective hydrogenation of a cyclopentene-type intermediate using two types of catalysts followed by B-ring functionlization. The synthesis of 3 bearing a ketone functionality at the side chain was achieved by using a 1,3-dipolar cycloaddition of a nitrile oxide intermediate as the key step and an improved B-ring functionalization protocol. The synthesis of 4 is now underway.

Synthetic studies on strophasterols A-D

2019 Fall National Meeting

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