Total Synthesis of (±)-Pallavicinin and (±)-Neopallavicinin

Bioinspired Total Synthesis of Natural Products

Currently, the frontier challenges in total synthesis pertain to increasing the synthetic efficiency and enabling the divergent synthesis of a number of natural products. Bioinspired synthesis has been well recognized as an effective approach to increasing synthetic efficiency. Especially, when bioinspired synthesis was applied at late-stage skeletal diversification to generate various natural products with distinct carbon skeletons, it held special promise for achieving both goals. In our laboratory, bioinspired synthesis has served as one of two long-standing principles for facilitating the efficient synthesis of natural products. In this Account, we summarize our endeavors and journeys in the bioinspired synthesis of natural products. We categorize our work into three parts based on the imitation of biosynthetic reactions and processes. (1) To mimic the key cyclization steps. Inspired by the biosynthetic process that formed the core skeleton, we developed new synthetic methods to enable the rapid and efficient construction of the core skeletons of the targeted molecules, ultimately leading to their concise total synthesis, for example, seven-step total synthesis of lamellarins D and H featuring three bioinspired oxidative coupling reactions, seven-step total synthesis of clavicipitic acid highlighted by a C-H activation/aminocyclization cascade reaction, eight-step total synthesis of phalarine via a bioinspired oxidative coupling, seven-step total synthesis of α-cyclopiazonic acid, and ten-step total synthesis of speradine C through a bioinspired cascade cyclization reaction initiated by the benzylic carbocation of indole. (2) To mimic the revised biosynthetic pathway proposed by us. In some cases, the proposed biosynthetic processes may be flawed, as they contradict some basic principles of chemistry. Thus, an alternative biosynthetic process must be proposed and investigated. We showcase the total synthesis of euphorikanin A through a bioinspired benzilic acid-type rearrangement and bipolarolides A and B via a bioinspired Prins reaction/ether formation cascade cyclization. (3) To mimic the skeletal diversification process. Nature usually synthesizes a multitude of products from a key common intermediate in a divergent manner. Biogenic skeletal diversification to generate various natural products with distinct carbon skeletons has also drawn our attention. Compared with single-target-oriented synthesis, skeletal-diversity-oriented synthesis of natural products remains underexplored due to its high synthetic challenges. We showcased the divergent total syntheses of ten pallavicinia diterpenoids with three distinct skeletons and six grayanane diterpenoids with three distinct skeletons, which were achieved with unprecedented ease and high efficiency by imitation of the proposed biogenic skeletal diversification process. These two successful projects can serve as inspiration for the application of the bioinspired skeletal diversification strategy to other skeletally diverse natural products.

Synthesis of the bicyclic butenolide core of pallamolide A: a biomimetic approach

Pallamolide A is a 7,8-seco-labdane terpenoid possessing a unique bicyclo[2.2.2]octane core and a spiro-butenolide moiety. A biomimetic synthesis of the bicyclic butenolide core over 10 steps is reported, featuring an unexpected autoxidation ring opening, and a vinylogous Mukaiyama aldol reaction which was spontaneously followed by an unusual intramolecular vinylogous aldol reaction to assemble the spiro-butenolide moiety and bicyclic core of pallamolide A.

Total Synthesis of Pallamolides A-E

We have achieved the first total synthesis of pallamolides A-E. Of these compounds, pallamolides B-E possess intriguing tetracyclic skeletons with novel intramolecular transesterifications. Key transformations include highly diastereoselective sequential Michael addition reactions to construct the bicyclo[2.2.2]octane core with the simultaneous generation of two quaternary carbon centers, a one-pot SmI2-mediated intramolecular ketyl-enoate cyclization/ketone reduction to generate the key oxabicyclo[3.3.1]nonane moiety, and an acid-mediated deprotection/oxa-Michael addition/β-hydroxy elimination cascade sequence to assemble the tetracyclic pallamolide skeleton. Kinetic resolution of ketone 14 through Corey-Bakshi-Shibata reduction enabled the asymmetric synthesis of pallamolides A-E.

Remote B-Ring Oxidation of Sclareol with an Engineered P450 Facilitates Divergent Access to Complex Terpenoids

Though chiral pool synthesis is widely accepted as a powerful strategy in complex molecule synthesis, the effectiveness of the approach is intimately linked to the range of available chiral building blocks and the functional groups they possess. To date, there is still a pressing need for new remote functionalization methods that would allow the installation of useful chemical handles on these building blocks to enable a broader spectrum of synthetic manipulations. Herein, we report the engineering of a P450_BM3 variant for the regioselective C-H oxidation of sclareol at C6. The synthetic utility of the resulting product was demonstrated in a formal synthesis of ansellone B, the first total synthesis of the 2,3-seco-labdane excolide B, and a model study toward (+)-pallavicinin.

seco-Labdanes: A Study of Terpenoid Structural Diversity Resulting from Biosynthetic C-C Bond Cleavage

The cleavage of a C-C bond is a complexity generating process, which complements oxidation and cyclisation events in the biosynthesis of terpenoids. This process leads to increased structural diversity in a cluster of related secondary metabolites by modification of the parent carbocyclic core. In this review, we highlight the diversifying effect of C-C bond cleavage by examining the literature related to seco-labdanes-a class of diterpenoids arising from such C-C bond cleavage events.

Total Synthesis of (±)-Leonuketal

Leonuketal is an 8,9-seco-labdane terpenoid with a unique tetracyclic structure, owing to a diversity-generating biosynthetic C-C bond cleavage event. The first total synthesis of leonuketal is reported, featuring a Ti(III)-mediated reductive cyclization of an epoxy nitrile ether, an unusual ring-opening alkyne formation as part of an auxiliary ring strategy, and the previously undescribed Au(I)-catalyzed cyclization of a β-keto(enol)lactone to assemble the core spiroketal motif.

C-C fragmentation: origins and recent applications

It has been 60 years since Eschenmoser and Frey disclosed the archetypal CC fragmentation reaction. New fragmentations and several variants of the original quickly followed. Many of these variations, which include the Beckmann, Grob, Wharton, Marshall, and Eschenmoser-Tanabe fragmentations, have been reviewed over the intervening years. A close examination of the origins of fragmentation has not been described. Recently, useful new methods have flourished, particularly fragmentations that give alkynes and allenes, and such reactions have been applied to a range of complex motifs and natural products. This Review traces the origins of fragmentation reactions and provides a summary of the methods, applications, and new insights of heterolytic CC fragmentation reactions advanced over the last 20 years.

A concise construction of the chlorahololide heptacyclic core

A concise and efficient strategy for the construction of the heptacyclic core of the chloranthaceae family has been developed. The key strategy comprises an S(N)2-type intramolecular nucleophilic substitution and a biomimetic endo-Diels-Alder cycloaddition.