We report the first total synthesis of bolivanine in a 14-step pathway involving the synthesis of onoseriolide. Our synthesis features a palladium-catalyzed intramolecular cyclopropanation involving an allylic metal carbene and a Diels-Alder/intramolecular hetero-Diels-Alder cascade, allowing the single-step assembly of a tricyclic system with proper configuration. The synthetic efforts validate our modified biogenetic hypothesis and allow us to confirm the absolute configuration of bolivianine.

In this article, we describe our efforts on the total synthesis of bolivianine (1) and isobolivianine (2), involving the synthesis of onoseriolide (3). The first generation synthesis of bolivianine was completed in 21 steps by following a chiral resolution strategy. Based on the potential biogenetic relationship between bolivianine (1), onoseriolide (3), and β-(E)-ocimene (8), the second generation synthesis of bolivianine was biomimetically achieved from commercially available (+)-verbenone in 14 steps. The improved total synthesis features an unprecedented palladium-catalyzed intramolecular cyclopropanation through an allylic metal carbene, for the construction of the ABC tricyclic system, and a Diels-Alder/intramolecular hetero-Diels-Alder (DA/IMHDA) cascade for installation of the EFG tricyclic skeleton with the correct stereochemistry. Transformation from bolivianine to isobolivianine was facilitated in the presence of acid. The biosynthetic mechanism and the excellent regio- and endo selectivities in the cascade are well supported by theoretical chemistry based on the DFT calculations.

The syntheses of members of a family of lindenane sesquiterpenoid [4+2] dimers led to the total syntheses of sarcandrolide J and shizukaol D. Inspired by a modified biosynthetic pathway, a cascade featuring furan formation/alkene isomerization/Diels-Alder cycloaddition was devised to construct the congested polycyclic architecture of the target molecules with the correct stereochemistry. This study presents a pioneering synthetic entry to this family of natural products and paves the way for fully exploring their biological functions.

The dimeric lindenane sesquiterpenoids are mainly isolated from the plants of Chloranthaceae family. Structurally, they have a crowded molecular scaffold decorated with more than 11 stereogenic centers. Here we report divergent syntheses of eight dimeric lindenane sesquiterpenoids, shizukaols A, C, D, I, chlorajaponilide C, multistalide B, sarcandrolide J and sarglabolide I. In particular, we present a unified dimerization strategy utilizing a base-mediated thermal [4 + 2] cycloaddition between a common furyl diene, generated in situ, and various types of dienophiles. Accordingly, all the three types of lindenane [4 + 2] dimers with versatile biological activities are accessible, which would stimulate future probing of their pharmaceutical potential.

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