Transition Metal-Mediated Annulation Approaches for Synthesis of Arylnaphthalene Lignan Lactones

Arylnaphthalene lignan lactones belong to a class of natural lignans, and more than 60 analogs have been isolated. Their pharmacological activities as well as unique structural features have attracted considerable attention from medicinal and synthetic chemists. Since the first synthesis in 1895, many synthetic methodologies with ionic or pericyclic reaction mechanisms have been reported. Transition metal catalysts sometimes provide exceptional synthetic versatility for the syntheses of natural compounds. Recently, transition metal-mediated methodologies were investigated for the construction of basic scaffolds of arylnaphthalene lignan lactones. Five kinds of transition metal catalysts containing gold, manganese, nickel, palladium, and silver have been explored. Most of the metal catalysts successfully created arylnaphthalene lactones by intermolecular or intramolecular annulative cyclization. In this review, all reports of transition metal-mediated annulative construction of arylnaphthalene lignan lactones were compiled, and synthetic approaches, mechanistic aspects, and successful applications were discussed.

Transition metals have proven to be very useful in modern organic synthesis. The transition metal catalyzed reaction is particularly important for the formation of carbon-carbon bonds, and enables formation of bonds that are not possible with conventional methods. It also allows the formation of complex structures in a single reaction. In this review, all the research articles in which transition metals were utilized for the synthesis of arylnaphthalene lignan lactones are comprehensively discussed and the main advantages in each method are elucidated. There are five transition metals involved in the synthesis of arylnaphthalene lignan lactones, and each one is discussed in terms of the reaction mechanism and its applications for synthesis of arylnaphthalene lignan lactones.

Gold-Catalyzed Cyclization
Recently, extensive studies have been devoted to gold-catalyzed reactions in organic synthesis (Hashmi, 2007;Li et al., 2008). Balamurugan's group focused on the intramolecular cyclization of alkynyl ester 1 by an Au catalyst in combination with AgSbF 6 for the synthesis of an arylnaphthalene lignan lactone lignan scaffold (Gudla and Balamurugan, 2011). Catalysis of the cyclization begins with coordination of the distant carbonyl and alkyne group of 1 to a proximal position by the Au catalyst. Vinyl cation 2 was produced by subsequent attachment of the alkyne on the carbonyl group. Arylnaphthalene lignan lactone precursor 4 was synthesized by the removal of water and Au catalyst from 3 that was obtained from the electrophilic benzannulation reaction of 2 (Scheme 1).
Application of this methodology to the synthesis of natural arylnaphthalene lignan lactone is summarized in Scheme 2. The key intermediate 1 was obtained from propargyl alcohol in four steps. Sonogashira coupling of propyn-1-ol 5 with substituted aryl iodide provided 6 and 7 was obtained by the substitution reaction. Boron trifluoride-mediated epoxide opening provided 8 and 1 was obtained by Swern oxidation. The ketone 1 can also be obtained by epoxidation of cinnamyl alcohol 9, propargylation, Cu(OTf) 2 -mediated Meinwald rearrangement and Sonogashira coupling with substituted aryl iodide albeit in low yield. The Au-catalyzed cyclization of 1a-1c was achieved in yields of 84, 68, and 73%, respectively. Regiocontrolled oxidation of 4 by CrO 3 /H 5 IO 6 in acetonitrile (Yamazaki, 1999) afforded 4.5:1 mixture of type II and isomeric type I arylnaphthalene lignan lactone. The advantage of this synthetic approach is the highly mild reaction condition performed at room temperature and easy derivatization with late-stage introduction of the aryl group from intermediate 10. Two years later, Balamurugan's group SCHEME 1 | Mechanism of Au-catalyzed intramolecular cyclization of alkynyl ester.

Manganese-Catalyzed Cyclization
Shia's group studied Mn(OAc) 3 -mediated cyclization to synthesize arylnaphthalene lignan lactone (Wong et al., 2014;Kao et al., 2015). Initially, they investigated the feasibility of the oxidative cyclization of model system 12. The reaction proceeded under mild condition compared to the dehydro-Diels-Alder reaction, which requires high reaction temperatures of 160-300 • C. The mild reaction conditions allow diverse functional groups, including Trimethylsilyl (TMS), ester, ketone, amide, phosphonate, and halogen. The reaction mechanism was proposed as follows. Abstraction of α-H in 12 produced Mn(III) enolate, and electron transfer generated Mn(II) and radical species 14. Then, the radical was added to the alkyne by 5-exo-dig type cyclization to provide 15. Next, the vinyl radical reacted with Mn(II) to form Mn(III)-vinyl complex 16, and naphthalene core 13 can be constructed by vinyl radical addition to benzene by 6-exo/endo type cyclization, and another electron transfer from 17 to Mn(III) ultimately generates an aromatic ring with loss of H + (Scheme 3).
With these successful results in hand, they turned their attention to the natural products synthesis as shown in Scheme 4. Cyclization precursor 12a was synthesized by esterification of substituted phenyl propargyl alcohol 18 with cyanoacetic acid, one-pot Knoevenagel condensation with benzaldehyde, and Hantzsch ester mediated reduction. Mn(OAc) 3 -catalyzed cyclization provided inseparable 3:1 mixtures of 13a and its regioisomer (67:22%). Samarium iodide mediated reductive decyanation and air oxidation afforded retrojusticidin B at 80% yield. Synthesis of justicidin E and helioxanthin was also achieved from 13b following the same procedure affording a 59:20% mixture. Regioselective synthesis of helioxanthin was accomplished from 13c by introducing Br in the 2-position of piperonal to minimize regioisomer formation. The main advantage of this reaction may be functional group tolerance; however, the regioselectivity is not satisfactory unless Br is introduced in the 2-position to inhibit adverse cyclization, where Mn(OAc) 3 is used in stoichiometric amount (2.0 equivalents).

Nickel-Catalyzed Cyclization
Peng's group developed a nickel-catalyzed cyclization to afford dihydronaphthalene and tetrahydronaphthalene in a diastereodivergent manner (Xiao et al., 2018a). Following on, they investigated the construction of an arylnaphthalene core for the synthesis of dehydrodesoxypodophyllotoxin (Xiao et al., 2018c). Key reaction is summarized in Scheme 5 although detailed stereochemical outcomes are not indicated here (Xiao et al., 2018b). Ni(I) is reduced to Ni(0) by Zinc(0) metal and the single electron transfer process produced alkyl radical 21 SCHEME 2 | Synthesis of arylnaphthalene lignan lactone by Balamurugan's group. from 20. The secondary radical of 21 was added to the alkene in 5-exo-trig type producing 22 and then, the primary radical formed the Ni(III) species 2 following the known Ni I -Ni III redox process via 23 and 24. Finally, reductive elimination provided 26 and regenerated the Ni(I) species. The synthesis of arylnaphthalene lignan lactones begins with oxazolidinone imide formation, as shown in Scheme 6. Oxazolidinone imide 28 was obtained by the Horner-Emmons reaction of 6-bromopiperonal 27 with triethyl phosphonoacetate, saponification, and imide formation by reaction of in situ generated pivaloyl anhydride and oxazolidinone. Gilman reagent was added to the β-position of enone diasteroselectively at a ratio of 97:3 affording 29 and subsequent reduction of the imide by NaBH 4 , oxidation with Pyridinium ChloroChromate (PCC) and acetal formation afforded 30. Enol ether 31 was generated in the presence of TMSOTf as Lewis acid. 32 was obtained as a 1.2:1 mixture by bromination with 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCD) (Kato et al., 1976(Kato et al., , 1984 and concomitant mixed acetal formation with allyl alcohol. Nicatalyzed reductive cyclization afforded tetrahydronaphthalene lactol ether 33 with cis and trans mixture of 41 and 35% yield, respectively. Then, acetal 33 was hydrolyzed and oxidized to lactone by PCC to produce cis-34 and trans-34 in 68 and 62% yield, respectively. Dehydrodesoxypodophyllotoxin was obtained by radical halogenation, elimination reaction, and further oxidation from trans-34. Dihydronaphthalene lactone can be obtained from cis-34 by phenylselenylation at −78 • C, m-CPBA oxidation, and selenoxide elimination. This Ni-mediated reductive cyclization is advantageous in terms of diastereodivergent synthesis for tetrahydronaphthalene containing diversity-oriented library synthesis.

Palladium-Mediated Cyclization
Pd-catalyzed copolymerization of diynes and arynes is wellknown for the construction of an aromatic ring depending on the metal species (Peña et al., 1998). In continuation of SCHEME 5 | Mechanism of Ni-catalyzed cyclization reaction.
arylnaphthalene lignan synthesis efforts Sato et al., 1999), Mori et al. developed a new method for construction of the naphthalene ring by a Pd-catalyzed cocyclization of diyne esters and arynes (Sato et al., 2004). A plausible reaction mechanism can be explained by the formation of palladacycle intermediate 36 from diyne ester 35 and further reaction with benzyne to provide arylnaphthalene lignan lactone 37 (Peña et al., 1998; Scheme 7).
As depicted in Scheme 8, synthesis of diyne ester 40 was commenced with Pd-catalyzed coupling of N-methoxy-N-methylcarbamoyl chloride with protected propargyl alcohol 38 (Murakami et al., 1998). After deprotection of 39 in the acidic condition, the resulting alcohol was coupled with 3-[benzo(d)(1,3)dioxol-5-yl]propiolic acid, which was prepared by sequential Corey-Fuchs olefination, carboxylation under n-BuLi, and methyl chloroformate, and hydrolysis of methyl ester to afford diyne ester 40. Aryne precursor 41 was synthesized by silylation of 2-bromophenol, lithiation at low temperature with silyl group migration, and triflation of in situ generated phenol. Arylnaphthalene lignan lactone 42 was obtained with diyne ester 40 and aryne precursor 41 in the presence of CsF and Pd catalyst in 61% yield with only 5 mol% loading of Pd catalyst. However, the functional group interconversion of Weinreb amide of 42 was difficult compared to the ease of core ring formation. Direct reduction of amide with LiAl(OtBu) 3 H (Paris et al., 1998) and L-Selectride with MeOTf (Tsay et al., 1990) were attempted but were unsuccessful. Finally, it was converted to aldehyde with an additional four steps. Lactol 43 was obtained by methanolysis of lactone 42 and concomitant transesterification and partial reduction with DIBAL at low temperature. Reduction of lactol 43 with NaBH 4 afforded diol, which underwent transesterification to provide primary alcohol and aldehyde 44 was obtained upon PCC oxidation. Then, the aldehyde was converted to taiwanin C by the Tsuji-Wilkinson decarbonylation reaction with rhodium catalyst (Tsuji and Ohno, 1965). Taiwanin E was obtained by Baeyer-Villiger oxidation and hydrolysis of the ester.
In 2007, Sato et al. (2007) applied the aforementioned methodology for the synthesis of arylnaphthalene lignan lactone. As shown in Scheme 9, Pd-catalyzed copolymerization of diyne ester 45 and aryne precursor 41 was also successful despite a multistep of reactions for conversion of Weinreb amide 46 to lactol 47 and aldehyde 48. The aldehyde group of 48 was converted to O-triflate 49 by a three-step sequence of Baeyer-Villiger oxidation, hydrolysis, and triflation of the resulting phenolic hydroxyl group. Finally, the triflate was eliminated by Pd-catalyzed hydrogenation to give arylnaphthalene lignan lactone. The desired dehydrodesoxypodophyllotoxin was synthesized from 46 in low yields of 17% through eight steps. In these two syntheses, constructions of the core ring were highly efficient and regioselective. Further manipulation of the functional groups successfully delivered natural arylnaphthalene lactone products.
In 2013, Patel and Argade (2013) further developed Pdcatalyzed [2 + 2 + 2] cocyclization of aryne 50 and unsymmetrical conjugated diene 51 using palladium catalyst with N-heterocyclic carbene ligand. Initial trial of aryne 50 and diene 51 in the absence of catalyst resulted in only [2 + 2] cycloaddition adduct in which the less substituted olefin of 51 reacted with aryne 50. It was assumed that the desired [2 + 2 + 2] cycloadduct was not generated because of two SCHEME 6 | Arylnaphthalene lignan lactone synthesis by Ni-catalyzed cyclization. SCHEME 7 | Mechanism of Pd-catalyzed cocyclization of diyne ester and aryne. electro-positive carbons in diene. Therefore, Argade's group systematically studied metal-mediated [2 + 2 + 2] cocyclization to construct a 6-membered ring. The reaction was ultimately optimized to 62% yield with Pd 2 (dba) 3 and IMes.HCl as the ligands. The mechanistic pathway involves oxidative addition of Pd to aryne 50 and the less hindered alkene of 51 to form a five-membered palladacycle intermediate 52, and insertion of more substituted bonds to form a seven-membered palladacycle 53 via rearrangement. Finally, 55 was obtained by reductive elimination of 53 and spontaneous air oxidation of 54 during workup (Scheme 10).
By applying this methodology, the authors successfully achieved total synthesis of Type I and Type II arylnaphthalene lignan lactones, justicidin B and retrojusticidin B, respectively, as SCHEME 8 | Synthesis of taiwanin C and E by Mori's group. SCHEME 9 | Synthesis of dehydrodesoxypodophyllotoxin by Mori's group.
of 58 and 59. Regioselective hydrolysis of less-hindered ester afforded 61. Justicidin B was obtained by borane reduction of carboxylic acid and lactonization. Retrojusticidin B was obtained in 67% yield by selective ester group reduction in the presence of carboxylate salt with 28% yield of justicidin B as a regioisomer.
Mitsudera's group studied the construction of arylnaphthalene lignan lactone by a regiospecific Pd-catalyzed benzannulation of α, β-bisbenzylidene-γ-lactone for the synthesis of helioxanthin (Mizufune et al., 2001;Scheme 12). Structurally, helioxanthin is a 7,8-substituted naphthalene lignan lactone compound, whereas most natural arylnaphthalene lignan lactones bear 6,7-oxygen substituents. Therefore, regioselectivity is the most significant issue in this synthesis. Benzannulation precursor 65 was synthesized by Stobbe condensation of diethyl succinate with 2-iodopiperonal 62 and Fischer esterification to afford 63. Stobbe reaction of 63 with piperonal afforded 64, and lactone 65 was obtained by Super-Hydride-mediated ester reduction and concomitant lactonization. The geometry of the alkene was confirmed by analysis of the NOESY spectrum. With this substrate in hand, helioxanthin was obtained by benzannulation in 60% yield. The reaction mechanism can be explained as described below. Oxidative addition of Pd to aryl iodide 65 provided intermediate 66, which is stabilized by a coordination of 1,3-diene to Pd even in the presence of steric hindrance generated by the aryl group of the diene. Then syn insertion of the palladium complex to the alkene provided σdihydronaphthalene palladium complex 67 and syn β-hydride elimination afforded helioxanthin. In the other process, the geometry of alkene can be isomerized to (Z) in 68 specified in red color in Scheme 6. In that case, the β-hydride, which is positioned anti to Pd in 69 is eliminated by anti β-hydride elimination process by way of π-allylpalladium complex 70. In this synthesis, construction of the naphthalene ring is proceeded in relatively mild condition with only the catalytic amount of Pd catalyst in a concise, efficient manner with opposite regioselectivity usually obtained as a minor product using the Diels-Alder approach.
by the Ag(II) ion to a highly stable radical species 72, and the radical is added to propiolate 73 to produce vinyl radical 74, which is stabilized by the neighboring aryl group (Yan et al., 2015). Finally, the radical 74 underwent addition to the benzene ring and synthesis of 4-arylnaphthol 77 was achieved by additional oxidation of 75 by Ag(II) ion, and tautomerization of 76 (Scheme 13).
Successful and efficient applications for the synthesis of arylnaphthalene lignan lactone is summarized in Scheme 14. The reaction of substituted ethyl benzoyl acetate 71 and substituted ethyl phenylpropiolate 73 in the presence of silver acetate and Na 2 S 2 O 8 furnished 4-aryl α-naphthol 77a and 77b in 56 and 60% yield, respectively. Surfactant (sodium dodecyl sulfate) was used to enhance water solubility and yield and reduced the use of organic solvent. Lithium Aluminum Hydride (LAH)-mediated reduction of ester located next to phenol and the spontaneous lactonization afforded diphyllin and taiwanin E in 80 and 84% yield, respectively. Justicidin A was also obtained by methylation of diphyllin in quantitative yield. This Ag-catalyzed cyclization proceeded in an atom economic manner, broad substrate scope, excellent regioselectivity, and environmental friendliness albeit with high loading of Ag catalyst.

CONCLUSION
Arylnaphthalene lignan lactones continuously intrigue synthetic organic chemists and medicinal chemists owing to their structural uniqueness and pharmacological activity as well as the possibility as privileged structures. This review focused on transition metal catalyzed approaches for the synthesis of arylnaphthalene lignan lactones, especially on the construction of arylnaphthalene cores by metal catalyst incorporating gold, manganese, nickel, palladium, and silver. Compared with the non-metal catalyzed synthesis of arylnaphthalene lignan lactones, synthetic methodologies using transition metal catalysts provide distinct advantages in terms of mild reaction conditions, chemical yields, and functional group tolerance. In addition, single step construction of a complex core structure through multiple bond formation is scientifically meaningful and provides utility in chemical library synthesis for drug discovery.

AUTHOR CONTRIBUTIONS
DS conceptualized the manuscript. SP researched publications related to the review article. SP and J-HK prepared the schemes and figures. S-HK and DS wrote the draft of the manuscript and prepared the final version of the manuscript with considerable corrections. All authors approved the manuscript in its final form for publication.