Abstract
Shizukaols possess a common heptacyclic framework containing more than ten contiguous stereocenters and potential biological activities. Here we report that the total syntheses of shizukaols A (1) and E (2), two lindenane-type dimers from the Chloranthaceae family, are achieved via a modified biomimetic Diels–Alder reaction. The common crucial biomimetic diene 23 and ethylene species (6, 17) are obtained through either a highly Z-selective olefination of α-siloxy ketone with ynolate anions or an intramolecular Horner–Wadsworth–Emmons olefination from commercially available Wieland–Miescher ketone (7). This synthetic approach here opens up practical avenues for the total syntheses of the intriguing Chloranthaceae family members, as well as the understanding of their relevant biological action in nature.
Highlights
Shizukaols possess a common heptacyclic framework containing more than ten contiguous stereocenters and potential biological activities
In consideration of the likely instability of 5, as well as the long synthetic steps in the conversion of our preliminary [4+2] cycloaddition products to shizukaol A (1), we reason that the synthesis of a crucial heptacyclic core with suitable functional groups for subsequent conversions to naturally occurring shizukaols A (1) and E (2) should be a more practical approach
The epoxide moiety in triene 9 is expected to prevent a further [4+2] cycloaddition of relevant alkenyl unit. On basis of these considerations and the aforementioned biogenetic hypothesis, we think that the critical step for the construction of skeleton relevant to that of shizukaols A (1) and E (2) would be the expedient and pragmatic conversion of readily accessible precursor and biomimetic epoxide 9 into the endo-Diels–Alder reaction product via the relevant transition state
Summary
Shizukaols possess a common heptacyclic framework containing more than ten contiguous stereocenters and potential biological activities. The common crucial biomimetic diene 23 and ethylene species (6, 17) are obtained through either a highly Z-selective olefination of αsiloxy ketone with ynolate anions or an intramolecular Horner–Wadsworth–Emmons olefination from commercially available Wieland–Miescher ketone (7) This synthetic approach here opens up practical avenues for the total syntheses of the intriguing Chloranthaceae family members, as well as the understanding of their relevant biological action in nature. The pyrolysis of shizukaol A (1) in a sealed tube resulted in a retro Diels–Alder fragmentation of the linking six-membered ring to furnish two products: the relatively unstable diene 5 and the previously known chloranthalactone A (6)[1,2] This led to a biogenetic hypothesis for the lindenane dimeric family in which enzymatic endo-Diels–Alder reaction of two lindenane-type components (such as 5 and 6) forms a common linking sixmembered ring. Encouraged by our preliminary results of endo-Diels–Alder reaction[28,29], we accomplished the total syntheses of two members of this dimeric family, shizukaols A (1) and E (2), through mimicking a biosynthetic approach
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