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Lancilactone C is a tricyclic triterpenoid that inhibits human immunodeficiency virus (HIV) replication in H9 lymphocytes with no cytotoxicity. Its tricyclic skeleton comprises trans-dimethylbicyclo[4.3.0]nonane and 7-isopropylenecyclohepta-1, 3, 5-triene. The latter unique structure, in which all carbon atoms are sp2 hybridized, is not found in other triterpenoids and needs to be verified synthetically. Herein, we have accomplished the first total synthesis of lancilactone C (proposed structure) by developing a new domino [4 + 3] cycloaddition reaction involving oxidation, Diels–Alder reaction, elimination, and electrocyclization. We have also revised the structure based on the total synthesis of lancilactone C according to its plausible biosynthetic pathway.

Schinortriterpenoids, such as pre-schisanartanin A and arisanlactone C, are complex and highly oxygenated polycyclic terpenes. In this study, the left-hand fragment of this class terpenes, which possesses oxygen-functionality at C19, was synthesized through [3+2] cycloaddition with excellent diastereoselectivity. This stereoselectivity was investigated by computational studies. Further selective transformation provided a tricyclic skeleton with the desired stereochemistry.

This report describes the total synthesis of the complex, oxygenated tetracyclic alkaloid, lyconesidine B. The key synthetic challenge involves diastereoselective generation of a decahydroquinoline ring with a quaternary carbon at the angular position via domino cyclopropanation, ring-opening, and reduction. Another crucial step is the domino ene-yne metathesis involving a quaternary ammonium ion, leading to the construction of a decahydroazaazulen framework.

We report the first total synthesis of shagenes A and B, which are tricyclic terpenoids containing a cis-substituted cyclopropane, via ring-closing metathesis of an enamide and Ir-catalyzed double-bond isomerization of an alkylidenecyclopropane. Chemo- and diastereoselectivity in the distorted cis-substituted structures were controlled by the alkylidenecyclopropane reactivity and using the ketone functionality as a remote directing group for the Ir catalyst, respectively. The total synthesis suggested the absolute configuration of shagenes.

A concise synthesis of the ACDE tetracyclic ring system of calyciphylline A-type alkaloids was investigated. The intramolecular Diels–Alder reaction of a tetrasubstituted olefin with furan enabled the construction of the ACD ring system bearing two contiguous quaternary carbons in one step, and subsequent intramolecular [3+2] cycloaddition successfully gave the E ring.

A concise route for construction of the ACDE ring skeleton in calyciphylline A type alkaloids was developed using an intramolecular [5+2] cycloaddition reaction of an oxidopyrylium species bearing a tetrasubstituted olefin. Key to the success of this reaction was the combination of acid and base, which accelerated the construction of this skeleton containing a spiro ring and vicinal quaternary carbon centers. The resultant tricyclic ADE ring compound was converted to an ACDE ring model through C−H oxidation and an aza‐Wittig reaction.

A palladium-catalyzed intramolecular carboborylation of 1, 3-diene has been developed for the synthesis of iminoindolines with a quaternary carbon centre. This method was applied to a substrate bearing several functional groups to afford a complex iminoindoline, which was subsequently converted into an ABCD ring model compound of communesins via an intramolecular Friedel–Crafts-type reaction.

Communesins, isolated from the mycelium of a strain of Penicillium sp., are cytotoxic heptacyclic indole alkaloids bearing a bis-aminal structure and two contiguous quaternary carbon centers. Toward a total synthesis of communesin F, we synthesized a pentacyclic ABCDG ring skeleton via carboborylation of 1, 3-diene and a Friedel–Crafts-type cyclization, resulting in the formation of an azepine ring through a Bi(OTf)₃-catalyzed SN2’ reaction.

C-H functionalization reactions involve the activation of otherwise unreactive C-H bonds, and represent atom economical methods for the direct transformation of simple substrates to complex molecules. While transition metal-catalyzed C(sp²)-H functionalization reactions are regularly used in synthesis, C(sp³)-H functionalization is rarely applied to the synthesis of complex natural products because of the difficulties associated with controlling selectivity. With this in mind, we focused on the development of new palladium (Pd)(0)-catalyzed C(sp³)-H functionalization reactions for the synthesis of complex molecules, resulting in several new methods capable of solving these problems. We initially developed a concise synthetic method for the facile construction of oxindoles and spirooxindoles via a Pd-catalyzed benzylic C(sp³)-H functionalization reaction. This method was subsequently extended to the synthesis of various heterocycles, including 2-arylindoles, benzocarbazole, indolocarbazole, indoloquinazolinone, and indoloquinazolinedione, as well as the total synthesis of several pyrrolophenanthridine alkaloids without the need for any protecting groups. This method was also successfully applied to the synthesis of the right-hand fragment of benzohopane from tetrahydro-2H-fluorene, which was constructed by a Pd-catalyzed benzylic C(sp³)-H functionalization. In this review, we provide a detailed discussion of our most recent investigations pertaining to Pd(0)-catalyzed benzylic C(sp³)-H functionalization.

Valuable synthetic routes to the Lycopodium alkaloid lycodine (1) and its unsymmetric dimers, complanadines A (4) and B (5), have been developed. Regioselective construction of the bicyclo[3.3.1]nonane core structure of lycodine was achieved by a remote functionality‐controlled Diels–Alder reaction and subsequent intramolecular Mizoroki–Heck reaction. A key coupling reaction of the lycodine units, pyridine N‐oxide (66) and aryl bromide (65), through C−H arylation at the C1 position of 66 provided the unsymmetric dimer structure at a late stage of the synthesis. This strategy greatly simplified the construction of the dimeric architecture and functionalization. Complanadines A (4) and B (5) were synthesized by adjusting the oxidation level of the bipyridine mono‐N‐oxide (67). The diverse utility of this common intermediate (67) suggests a possible biosynthetic pathway of complanadines in Nature. Both enantiomers of lycodine (1) and complanadines A (4) and B (5) were prepared in sufficient quantities for biological evaluation. The effect on neuron differentiation of PC‐12 cells upon treatment with culture medium, in which human astrocytoma cells had been cultured in the presence of 1, 4, or 5 was evaluated.

A transition-metal-catalyzed cyclopropanation followed by ring opening was investigated for the synthesis of octahydroquinolines 4 and decahydroquinolines 5 having a quaternary carbon center at the angular position, which are core structures of the fawcettimine-type alkaloids. A tandem reaction was also established for the synthesis of decahydroquinolines 5 and the tricyclic compound 6 through an iminium ion intermediate, readily produced by acidic treatment of cyclopropane 2.

Tetrahydrobiphenylene consists of cyclobutene fused with benzene and cyclohexene rings. In this paper, a direct method for synthesizing tetrahydrobiphenylenes based on a palladium (Pd)(0)-catalyzed C(sp²)-H functionalization was investigated. The developed method was applied to the synthesis of several tetrahydrobiphenylenes having an oxygen functionality at the ring juncture. The derivatization of a tetrahydrobiphenylene is also reported.

Avenaol, isolated from the allelopathic plant black oat, was the first C20 germination stimulant related to strigolactones. Structurally, it consisted of a bicyclo[4.1.0]heptanone skeleton containing a cyclopropane ring bearing three main chains projecting in the same direction (i.e. all-cis-substituted cyclopropane). Herein, we report the total synthesis of avenaol using a robust strategy involving the formation of an all-cis-substituted cyclopropane via an alkylidenecyclopropane. The key factors in the success of this total synthesis include the Rh-catalysed intramolecular cyclopropanation of an allene, an Ir-catalysed stereoselective double-bond isomerisation, and the differentiation of two hydroxymethyl groups via the regioselective formation and oxidation of a tetrahydropyran based on the reactivity of a cyclopropyl group. This strategy effectively avoids the undesired ring opening of the cyclopropane ring and the formation of a caged structure. Furthermore, this study confirms the proposed structure of avenaol, including its unique all-cis-substituted cyclopropane moiety.

A new method has been developed for the synthesis of tetrahydro-2H-fluorenes based on a Pd(0)-catalyzed benzylic C(sp³)-H functionalization. Importantly, the success of the cyclization step was dependent on there being substituents at the two positions ortho to the benzylic group to avoid an undesired C(sp²)-H functionalization. This method was subsequently used to prepare the right-hand fragment of the hexacyclic triterpenoid benzohopanes, and therefore represents a powerful tool for the construction of the related compounds. A new method has been developed for the synthesis of tetrahydro-2H-fluorenes based on a Pd(0)-catalyzed benzylic C(sp³)-H alkenylation of an enol triflate bearing substituents at its two ortho positions (see scheme). This method was used to prepare the right-hand fragment of benzohopanes.

The synthesis of benzocyclobutenes from simple arenes bearing a directing group was investigated via the palladium-catalyzed cyclization of norbornene derivatives. This approach allowed for the facile construction of benzocyclobutenes along with the double functionalization of the C - H bonds at the positions ortho and meta to the directing group. This result shows that the key palladacyclopentene intermediate in the Catellani reaction can be prepared by the directed double ortho C - H activation of the substrate. The results of this study also revealed that the combination of an N-protected amino acid with benzoquinone (BQ) was effective for this transformation.

Communesins are a class of heptacyclic indole alkaloids that contain two aminal moieties and two contiguous quaternary carbon centers. We have investigated the construction of the pentacyclic skeleton of the communesins by employing the oxidative rearrangement of aurantioclavine derivatives, which are believed to be biosynthetic intermediates of the polycyclic communesin alkaloids. The quaternary C-7 carbon center was constructed in a stereoselective manner, whereas the installation of the C-11 stereocenter requires an epimerization process. The isolation of a 2-ethoxyindolenine prior to the reduction of the nitro group and cyclization steps was critical to the success of this strategy. The pentacyclic skeleton of the communesins has been prepared by the oxidative rearrangement of aurantioclavine derivatives, which are believed to be biosynthetic intermediates of the communesin alkaloids. The quaternary C-7 carbon center was constructed in a stereoselective manner, and the isolation of a 2-ethoxyindolenine intermediate was critical to the success of the strategy.

CPZEN-45 was developed as an antibiotic against Mycobacterium tuberculosis by the chemical modification of caprazamycins. CPZEN-45 has been synthesized in this study by the Cu-catalyzed intramolecular amidation of a complex vinyl iodide precursor bearing uridine and sugar moieties with a secondary amide, allowing for the construction of its 1, 4-diazepin-2-one core.

A new method has been developed for the stereoselective construction of the AB rings of the quinolizidine skeleton of ­matrine-type alkaloids with a cis-cis stereochemistry. The key features of this method involve: (i) construction of the quinolizidine by reduction of an acylpyridinium cation; and (ii) late-stage introduction of methoxypyridine by sequential Stille coupling and diastereo­selective hydrogenation reactions.

The indoloquinazolinone ring system has attracted considerable attention as a pharmacophore, because it shows various biological activities. The reported synthetic methods for the compound are simple and direct, but are not effective for the direct synthesis of indoloquinazolinone with a methylene group at the C-6 position. A palladium(0)-catalyzed cyclization of chloroquinazolinone via C H functionalization was developed for a concise synthesis of indoloquinazolinone derivatives. The presence of a substituent at the C-6 position is important for obtaining the product in good yield. The conformation of the reaction intermediate, in particular the N C Pd bond angle, is important for the regioselectivity of the reaction.

Synthesis of 3, 3-disubstituted 2-aminoindolenines was achieved by palladium-catalyzed allylic amidination with an isocyanide. It was found that isocyanides are effective building blocks in palladium-catalyzed allylic functionalizations, analogous to carbon monoxide. This approach enables the direct construction of the indolenine ring along with the formation of a quaternary carbon and the introduction of an amino substituent in one step under mild conditions.

Assoanine, pratosine, hippadine, and dehydroanhydrolycorine belong to the pyrrolophenanthridine family of alkaloids, which are isolated from plants of the Amaryllidaceae species. Structurally, these alkaloids are characterized by a tetracyclic skeleton that contains a biaryl moiety and an indole core, and compounds belonging to this class have received considerable interest from researchers in a number of fields because of their biological properties and the challenges associated with their synthesis. Herein, a strategy for the total synthesis of these alkaloids by using C-H activation chemistry is described. The tetracyclic skeleton was constructed in a stepwise manner by C(sp(3))[BOND]H functionalization followed by a Catellani reaction, including C(sp(2))[BOND]H functionalization. A one-pot reaction involving both C(sp(3))[BOND]H and C(sp(2))[BOND]H functionalization was also attempted. This newly developed strategy is suitable for the facile preparation of various analogues because it uses simple starting materials and does not require protecting groups.

Spirooxindole and iminoindoline structures can be found in a wide range of complex natural products, and compounds containing these groups invariably display interesting biological activities. Considering their interesting biological properties and complex architecture, significant research efforts have been directed towards developing new synthetic methods for constructing these structures. This review summarizes our studies towards developing novel synthetic strategies capable of accessing these structures, as well as the applications of these methods to synthesizing natural products, including elacomine and dehaloperophoramidine.