HETEROCYCLES, Vol. 83, No. 3, 2011 491 HETEROCYCLES, Vol. 83, No. 3, 2011, pp. 491 - 529. © The Japan Institute of Heterocyclic Chemistry Received, 27th October, 2010, Accepted, 24th December, 2010, Published online, 7th January, 2011 DOI: 10.3987/REV-10-686 SYNTHESIS AND BIOLOGICAL ACTIVITY OF LAMELLARIN ALKALOIDS: AN OVERVIEW Tsutomu Fukuda,a Fumito Ishibashi,a and Masatomo Iwaob * aGraduate School of Science and Technology, and aDepartment of Applied Chemistry, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan, e-mail: [email protected] Abstract – Lamellarins are natural products isolated from marine invertebrates having a unique heterocyclic ring system. Many of these natural products exhibit potentially useful biological activities such as antitumor and anti-HIV activities. In this review, we summarized the synthesis and biological activity of naturally occurring lamellarins and their analogues. 1. INTRODUCTION Marine invertebrates are a rich source of biologically active compounds with unprecedented molecular structures.1 In 1985, Faulkner and co-workers reported the isolation of novel marine alkaloids named lamellarins A–D from the prosobranch mollusk Lamellaria sp. They demonstrated that these lamellarins possess a unique 14-phenyl-6H-[1]benzopyrano[4’,3’:4,5]pyrrolo[2,1-a]isoquinolin-6-one system by X-ray crystallographic analysis.2 Up to now, over 40 lamellarins (A–Z and –, including their acetates and sulfates) have been isolated from a variety of ascidians and sponges.2-14 These lamellarins can be divided into three structural types. Most lamellarins possess a fused pentacyclic framework with a saturated (type I) (Table 1) or unsaturated (type II) C5–C6 bond15 (Table 2). Isolated by Capton and co-workers,5,6 type III lamellarins (lamellarins O–R) have simple non-fused pyrrolic structures (Figure 1). Pentacyclic lamellarins exhibit important biological activity. For example, Quesada and co-workers have demonstrated that lamellarin D triacetate and lamellarin K triacetate display potent cytotoxicity against multidrug-resistant (MDR) cancer cell lines as well as their respective parental cell lines.16 More interestingly, the less cytotoxic lamellarin I has effectively increased the cytotoxicity of approved anticancer agents, such as doxorubicin, vinblastine, and daunorubicin, towards MDR cell lines.16 Faulkner and co-workers have also reported that lamellarin  20-sulfate inhibits HIV-1 integrase and is active against HIV-1 virus at non-toxic concentrations.11 The purpose of this review is to summarize the 4 92 HETEROCYCLES, Vol. 83, No. 3, 2011 synthesis and biological activity of pentacyclic lamellarins (Type I and II). Because of their lower biological activity, non-fused lamellarins (Type III) and structurally related 3,4-diarylpyrrole marine alkaloids17 are not reviewed in the present study. Table 1. Lamellarin alkaloids (Type I) R4 R3 R2 R1 R5 R6 O N R7 O 5 R9 R8 6 Type I (saturated between C5 and C6) lamellarin R1 R2 R3 R4 R5 R6 R7 R8 R9 ref. A OH OMe H OH OMe OMe OMe OMe OH 2, 4, 10 C OH OMe H OH OMe OMe OMe OMe H 2, 4, 9,10 C diacetate OAc OMe H OAc OMe OMe OMe OMe H 13 C 20-sulfate OSO – OMe H OH OMe OMe OMe OMe H 10 3 E OH OMe H OMe OH OMe OMe OH H 3, 10 F OH OMe H OMe OMe OMe OMe OH H 3, 14 G OMe OH H OMe OH OMe OH H H 3, 10, 12 G 8-sulfate OMe OH H OMe OH OMe OSO – H H 10 3 I OH OMe H OMe OMe OMe OMe OMe H 4, 13, 14 J OH OMe H OMe OMe OMe OH H H 4, 14 K OH OMe H OH OMe OMe OMe OH H 4, 7, 13, 14 K diacetate OAc OMe H OAc OMe OMe OMe OH H 13 K triacetate OAc OMe H OAc OMe OMe OMe OAc H 13, 14 L OH OMe H OMe OH OMe OH H H 4, 10, 12 L triacetate OAc OMe H OMe OAc OMe OAc H H 14 L 20-sulfate OSO – OMe H OMe OH OMe OH H H 10 3 S OH OH H OH OH OMe OH H H 7 T OH OMe H OMe OH OMe OMe OMe H 8 T diacetate OAc OMe H OMe OAc OMe OMe OMe H 14 T 20-sulfate OSO Na OMe H OMe OH OMe OMe OMe H 8 3 U OH OMe H OMe OH OMe OMe H H 8, 9, 13 U 20-sulfate OSO Na OMe H OMe OH OMe OMe H H 8 3 V OH OMe H OMe OH OMe OMe OMe OH 8 V 20-sulfate OSO Na OMe H OMe OH OMe OMe OMe OH 8 3 Y 20-sulfate OSO Na OMe H OMe OH OH OMe H H 8 3 Z OMe OH H OH OH OMe OH H H 10  OH OH H OMe OH OH OH H H 12  OH OMe OMe H OMe OMe OMe OH H 13  OAc OMe H OAc OMe OMe OAc H H 14 HETEROCYCLES, Vol. 83, No. 3, 2011 493 Table 2. Lamellarin alkaloids (Type II) R4 R2 R1 R5 R6 O N R7 O 5 R8 6 Type II (unsaturated between C5 and C6) lamellarin R1 R2 R4 R5 R6 R7 R8 ref. B OH OMe OH OMe OMe OMe OMe 2, 4, 10 B 20-sulfate OSO – OMe OH OMe OMe OMe OMe 10 3 D OH OMe OH OMe OMe OH H 2 D triacetate OAc OMe OAc OMe OMe OAc H 4, 10 H OH OH OH OH OH OH H 3 M OH OMe OH OMe OMe OMe OH 4, 13 N OH OMe OMe OH OMe OH H 8 N triacetate OAc OMe OMe OAc OMe OAc H 4, 10 W OH OMe OMe OH OMe OMe OMe 8 X OH OMe OMe OH OMe OMe OH 8 X triacetate OAc OMe OMe OAc OMe OMe OAc 13  OH OMe OMe OH OMe OMe H 13  20-sulfate OSO Na OMe OMe OH OMe OMe H 11 3  OH OMe OMe OMe OMe OMe OH 13  OH OMe OMe OMe OMe OMe OMe 14  OH OMe OMe OMe OMe OMe H 14  OAc OMe OAc OMe OAc OMe OMe 14 HO OH HO OH MeO N CO2Me N CO2Me R R O lamellarin O (R=H) lamellarin Q (R=H) lamellarin P (R=OH) lamellarin R (R=C6H4-p-OH) Type III Figure 1. Lamellarin alkaloids (Type III) 2. SYNTHESIS OF LAMELLARINS In spite of their unique structure, the synthesis of lamellarins was neglected for over ten years after their initial isolation by Faulkner in 1985. The prominent report by Quesada in 1996 on the potent cytotoxic activity of lamellarins against MDR cancer cell lines prompted many organic chemists to synthesize these molecules. In 1997, three research groups (Steglich, Ishibashi–Iwao, and Banwell–Flynn) reported the 4 94 HETEROCYCLES, Vol. 83, No. 3, 2011 first synthesis of lamellarins via different approaches. Since these initial studies, a wide range of synthetic methods have been developed to generate the pentacyclic structures. These methods can be divided into two categories. The first category relies on a ring construction from isoquinoline derivatives while the second category utilizes pyrroles as starting materials. 2-1. SYNTHESIS VIA ISOQUINOLINES The pentacyclic lamellarin framework can be regarded as a functionalized pyrrolo[2,1-a]isoquinoline system. This explains why most lamellarin syntheses described in this section have adapted methods developed for the preparation of pyrrolo[2,1-a]isoquinolines starting from isoquinoline derivatives.18 2-1-1. SYNTHESIS BY ISHIBASHI–IWAO Ishibashi, Miyazaki, and Iwao reported the first total syntheses of lamellarins D and H in 1997.19 They utilized the N-ylide-mediated cyclization that was devised by Iwao and Kuraishi in 1980 for the synthesis of pyrrolo[2,1-a]isoquinolines20 as the key ring construction step (Scheme 1). OBn OBn MeO MeO BnO BnO BnO OMOM OMOM MeO 1. LDA, THF, -78 °C MeO O MeO O MeO N BnO OMOM MeO N MeO NH BnO 2. 1 MeO 2 CO2Me BnO 3a BnO 3b (63%) OBn R1O R2O OR1 MeO R2O BnO BrCH2CO2Et OMOM 1. HCl, MeOH R2O O MeO O 2. Et 3N N MeO (34%: 3 5) O N CO2Et R1O BnO Br 5 (R1=Bn, R2=Me) H2, Pd(OH)2 (82%) 4 BBr3 lamellarin D (6) (R1=H, R2=Me) (68%) lamellarin H (7) (R1=R2=H) Scheme 1. Synthesis of lamellarins D (6) and H (7) via N-ylide-mediated cyclization The appropriately substituted benzylisoquinoline (1), which was prepared from isovanillin in 6 steps, was deprotonated with 1.1 equiv of lithium diisopropylamide (LDA) at −78 °C and the resulting anion was reacted with the benzoate (2) at room temperature for 3.5 h to give the C-acylated compound as a tautomeric mixture (3a and 3b) in 63% yield. Interestingly, the use of larger amounts of LDA or prolonged reaction times decreased the yield of 3. The lamellarin skeleton was subsequently constructed in three steps without isolation of intermediate compounds. Thus, compound (3) was treated with 20 HETEROCYCLES, Vol. 83, No. 3, 2011 495 equiv of ethyl bromoacetate at 70 °C for 22 h to generate a quaternary ammonium salt (4). Removal of the O-methoxymethyl (MOM) protecting group using methanolic hydrochloric acid followed by treatment with triethylamine gave lamellarin (5) in 34% yield. Selective removal of the benzyl group by hydrogenolysis over Pearlman’s catalyst gave lamellarin D (6) in 82% yield. Exhaustive dealkylation of 3 with 6 molar equiv of boron tribromide gave lamellarin H (7). This strategy was successfully applied to produce a small library of non-natural lamellarins for structure–activity relationship (SAR) investigations relative to cytotoxicity (see section 3-1).21 2-1-2. SYNTHESIS BY BANWELL–FLYNN Banwell, Flynn, and Hockless reported a highly convergent synthesis of lamellarin K using an intramolecular 1,3-dipolar cyclization reaction in 1997 (Scheme 2).22 Derived from dibromostyrene (8) in situ, zinc acetylide (9) was coupled with iodide (10) in the presence of palladium catalyst to give the unsymmetrically substituted acetylene (11) in 84% yield. Baeyer–Villiger oxidation of 11 followed by methanolysis and esterification with iodoacetic acid gave iodide (12). N-alkylation of dihydroisoquinoline (13) with 12 and treatment with diisopropylethylamine (Hünig’s base) gave lamellarin 15 in 81% yield via intramolecular 1,3-dipolar cycloaddition between azomethine ylide and acetylene moieties. Selective removal of the O-isopropyl group with aluminum chloride produced lamellarin K (16). I OMe MeO Br 1. BuLi (2.2 equiv) MeO MeO OMe Br 2. ZnCl2 OHC 10 Oi-Pr i-PrO i-PrO ZnCl2 i-PrO Oi-Pr Pd(PPh3)4 8 9 (84% from10) 11 OHC 1. MCPBA MeO OMe 2. NH3, MeOH MeO OMe MeO N (92%) i-PrO Oi-Pr i-PrO Oi-Pr MeO 3. ICH2CO2H, DCC, DMAP Oi-Pr 13 O (97%) O O MeO 12 O N I I - MeO Oi-Pr i-PrO MeO HO MeO 14 Oi-Pr OH MeO MeO DIEA MeO O AlCl3 MeO O (81% from 12) N (96%) N MeO O MeO O i-PrO HO 15 lamellarin K (16) Scheme 2. Synthesis of lamellarin K (16) via intramolecular 1,3-dipolar cyclization Albericio and Álvarez extended the Banwell–Flynn strategy to a solid phase synthesis of lamellarins (Scheme 3).23 Iodophenol (17) was anchored onto the Merrifield resin by Mitsunobu reaction. 4 96 HETEROCYCLES, Vol. 83, No. 3, 2011 Palladium-catalyzed Sonogashira coupling of 18 with the arylacetylene (19) gave 20. Baeyer–Villiger oxidation of 20 followed by methanolysis gave phenol 21, which was subsequently esterified with iodoacetic acid to give 22. N-alkylation of 3,4-dihydro-6,7-dimethoxylisoquinoline (23) with 22 followed by treatment with Hünig’s base produced 1,3-dipolar cycloaddition product 24. Cleavage of 24 with aluminum chloride gave a crude mixture that consisted of lamellarin U (25) and lamellarin L (26) as the major compounds. Pure 25 and 26 were isolated by semipreparative HPLC in 10% and 4% overall yields, respectively. A number of cleavage/deprotection conditions were also tested at the final stage to produce various lamellarins.24 OMe Oi-Pr HO O O OMe Merrifield resin OHC 19 MeO I MeO I MeO Oi-Pr DEAD, PPh3, DIEA CuI, Pd[(PPh3)2Cl2] 17 18 THF-Et3N (3:1), 20 h 20 OHC O OMe ICHCOH, DMPA, DIP O OMe 2 2 1. mCPBA, DMF DMF, RT, 12 h MeO Oi-Pr MeO Oi-Pr 2. KOH, MeOH-THF 21 HO O 22 O I MeO MeO MeO MeO MeO MeO MeO Oi-Pr OH OH N 1. O HO HO MeO 23 MeO O AlCl3 MeO O MeO O 2. DIEA, 83 °C, 48 h N N N MeO O MeO O HO O 24 lamellarin U (25) lamellarin L (26) Scheme 3. Solid-phase synthesis of lamellarins U (25) and L (26) 2-1-3. SYNTHESIS BY RUCHIRAWAT In 2001, Ruchirawat and Mutarapat reported an efficient synthesis of lamellarin G trimethyl ether (32) starting from 3,4-dihydro-1-benzylisoquinoline (27) (Scheme 4).25 Reaction of 27 with phenacyl bromide (28) in the presence of potassium carbonate in acetonitrile gave 5,6-dihydropyrrolo[2,1-a]isoquinoline (29) via N-alkylation of 27 with 28 followed by intramolecular condensation of the resulting enamine with carbonyl moiety (Tschitschibabin reaction26). Vilsmeier reaction of 29 followed by alkaline hydrolysis of the mesyl group gave hydroxy-aldehyde (31a) in good yield. Oxidation of 31a with manganese dioxide via the putative cyclic hemiacetal (31b) gave lamellarin G trimethyl ether (32) in low yield (20%). The concomitant oxidation of the phenolic moiety generated a quinone as by-product. This undesirable side reaction was prevented using palladium-catalyzed Tamaru oxidation27 HETEROCYCLES, Vol. 83, No. 3, 2011 497 (bromobenzene-palladium acetate-triphenylphosphine-potassium carbonate) which gave 32 in good yield. O MeO Br OMe MeO MeO OMe MeO MeO OMe MeO MsO OMe MeO MeO 28 DMF, POCl3 MeO MeO N+HCl- K2CO3, (M63e%CN), reflux MeO N OMs (82%) MeO N CHOOMs MeO MeO 27 29 30 MeO MeO OMe MeO MeO OMe MeO MeO OMe KOH, EtOH MeO MeO Pd(OAc)2, PPh3 MeO reflux PhBr, K2CO3 (81%) MeO OH MeO O (80%) MeO O N CHO N N MeO MeO OH MeO O 31a 31b lamellarin G trimethyl ether (32) Scheme 4. Synthesis of lamellarin G trimethyl ether (32) via Tschitschibabin reaction Ruchirawat improved the final lactone synthesis using a strategy involving lithium-bromine exchange, carbonate migration, and cyclization (Scheme 5).28 Tricyclic intermediate (33) was synthesized from 27 as described above and brominated by N-bromosuccinimide to give 34. Compound 34 was treated with tert-butyllithium at −78 °C, and then warmed up to room temperature to give lamellarin G trimethyl ether (32) in 67% yield. Acid-catalyzed Friedel–Crafts transacylation followed by lactonization could also directly convert 33 to 32 in excellent yield.29 MeO MeO OMe MeO MeO OMe MeO MeO OMe MeO MeO tert-BuLi, THF MeO OEt NBS OEt -78 °C ~ RT MeO O (95%) MeO O (67%) MeO O N O N Br O N MeO MeO MeO O 33 34 lamellarin G trimethyl ether (32) p-TsOH or Amberlyst-15, toluene, heat (91%: p-TsOH) (94%: Amberlyst-15) Scheme 5. Improved procedures for the lactone formation Ruchirawat developed another highly efficient synthesis of lamellarins starting from 3,4-dihydro-1-benzylisoquinolines.30 For example, the synthesis of lamellarin K is depicted in Scheme 6. Benzylisoquinoline (35) was reacted with -nitrocinnamate (36), which was prepared in four steps from isovanillin in 56% overall yield, in the presence of sodium bicarbonate in acetonitrile to give 4 98 HETEROCYCLES, Vol. 83, No. 3, 2011 5,6-dihydropyrrolo[2,1-a]isoquinoline (37) in 70% yield. This key reaction may proceed via Michael addition of the enaminic tautomer from 35 to 36 followed by ring closure (Grob cyclization31). Debenzylation followed by base-mediated lactonization gave lamellarin K (16) in excellent yield. This procedure was successfully applied to the synthesis of several natural and non-natural lamellarins32 for extensive SAR studies (see Section 3-1).33 OMe OBn BnO BnO MeO OBn HO MeO OH MeO OBn 36 MeO MeO EtO2C NO2 1. H2, Pd/C MeO MeO N NaHCO3( 7M0e%C)N, reflux MeO N COO2BEnt 2. N(a9H3,% T)HF MeO N O MeO MeO O OBn BnO HO 35 37 lamellarin K (16) Scheme 6. Synthesis of lamellarin K (16) via Grob cyclization Recently, Ruchirawat extended the same strategy to the synthesis of azalamellarins (lactam congeners). For example, the synthesis of azalamellarin D is shown in Scheme 7.34 Grob cyclization of 36 and 38 produced compound (39), which reacted with allylamine in the presence of trimethylaluminum to give amide (40). The cyclization of amide through copper (I)-mediated C–N bond formation yielded amide OMe OBn BnO BnO MeO OBn BnO MeO OBn MeO EtO2C NOB2r 36 MeO NH2 MeO MeO BnO N NaHCO3(,3 M3%eC)N, reflux MeO N COB2rEt A2lM00e 3°,C T, H4F5 ,m MinW MeO N BNHr BnO BnO O 38 39 40 BnO MeO BnO MeO OBn OBn MeO MeO CuTC, DMF 1. ClRh(PPh3), MW, 250 °C, 60 min MeO N MeO NH MW, 150 °C 2. OsO (cat.), NaIO, THF/HO, 4 4 2 30 min N rt, overnight N (71%) BnO O BnO O (42%) 42 41 AcO MeO HO MeO OAc OH 1. H2/Pd, AcOEt 2. Ac2O, DMAP, Et3N, CH2Cl2 MeO MeO 3. DDQ, DCE 5% KOH/EtOH MeO NH MeO NH (60%) (71%) N N AcO O HO O 43 azalamellarin D (44) Scheme 7. Synthesis of azalamellarin D (44) HETEROCYCLES, Vol. 83, No. 3, 2011 499 lactam (41).35 Rhodium-catalyzed double-bond isomerization followed by oxidation with osmium tetraoxide produced N-deallylated compound (42).36 Sequential debenzylation, acetylation, dehydrogenation, and alkaline hydrolysis gave azalamellarin D (44). 2-1-4. SYNTHESIS BY GUITIAN Eguchi reported that the 1,3-dipolar cycloaddition of 1-substituted 3,4-dihydroisoquinoline N-oxides with alkynes at room temperature gave stable 4-isoxazolines, which rearranged to 5,6-dihydropyrrolo[2,1-a]isoquinolines upon heating in toluene.37 Guitian utilized this reaction in the synthesis of lamellarins I and K (Scheme 8).38 The N-oxides (nitrones) (46a, b) were prepared in moderate yields by reduction of 3,4-dihydro-1-benzylisoquinolines (45a, b) with sodium borohydride followed by sodium tungstate-catalyzed oxidation with hydrogen peroxide.39 Reaction of 46a, b with alkyne (47) in toluene at 120 °C in a sealed tube produced 49a and 49b in 35% and 61% yields, respectively, via 1,3-dipolar cycloaddition-thermal rearrangement. Selective removal of isopropyl groups in 49a, b, concomitant with lactonization, gave lamellarins I (50) and K (16), respectively. RO OMe MeO MeO Oi-Pr RO RO EtO2C Oi-Pr MMeeOO N 1. N(a9B6H-947, %M)eOH MMeeOO N O i-PrO 47 MeO EtO2C O Oi-Pr N MeO 2. H2O2, Na2WO3 MeO toluene, 120 °C, 18 h MeO MeOH OR OR (45-49%) RO 45a (R=Me) 46a (R=Me) 45b (R=i-Pr) 46b (R=i-Pr) 48a, b RO MeO Oi-Pr RO MeO OR MeO MeO MeO Oi-Pr AlCl3 MeO O N CO2Et N MeO MeO O RO RO 49a (R=Me), 35% lamellarin I (50)(R=Me), 43% 49b (R=i-Pr), 61% lamellarin K (16) (R=H), 45% Scheme 8. Synthesis of lamellarins I and K via 1,3-dipolar cyclization of nitrones 2-1-5. SYNTHESIS BY NYERGES Nyerges and co-workers developed a new route to prepare 1,2-diaryl-5,6-dihydropyrrolo[2,1-a]isoquinolines via 1,5-electrocyclization of azomethine ylides derived from 3,4-dihydroisoquinoline derivatives (Scheme 9).40,41 Perkin condensation of arylacetic acids (51) and benzaldehydes (52) gave stilbenic acids (53). These acids were converted to 3,4-dihydroisoquinolines (56) via amides (55) using standard Bischler–Napieralski reaction. Quaternization of 56 followed by 5 00 HETEROCYCLES, Vol. 83, No. 3, 2011 treatment with triethylamine in ethanol at room temperature gave 1,2-diaryl-4,5-dihydropyrrolo- [2,1-a]isoquinolines (58). Deallylation of 58d using palladium-catalyst gave lamellarin (59). R1 R1 R4 R1 R3 OHC Et3N, Ac2O, reflux R4 1. SOCl2 R2 R2 (48-66%) R2 MeO MeO O CO2H R3 CO2H R3 2. NH2 NH R4 51 52 53 MeO MeO 54 55 (90-95%) R1 R3 R1 R1 R3 R3 R2 POCl3 R2 BrCH2R5 R2 R4 Et3N, EtOH MeO R4 (90-96%) MeO N R4 (95-98%) MeO N R5 (52-68%) N R5 MeO MeO MeO Br 56 57 a R1=OMe, R2=R3=R4=H 58 b R1=OMe, R2=R4=H, R3=NO2 c R1=OMe, R2=R4=H, R3=OMe d R1=R2=OMe, R3=H, R4=OCH2CH=CH2 R5=COEt, Ph, CH=CH 2 2 MeO MeO MeO 10% Pd-C, TsOH MeO EtOH, H2O, reflux MeO O MeO O (68%) N CO2Et N MeO MeO O 58d 59 Scheme 9. Synthesis of 5,6-dihydropyrrolo[2,1-a]isoquinolines (58) via 1,5-electrocyclization of azomethine ylides and its application to the synthesis of lamellarins 2-1-6. SYNTHESIS BY YADAV Recently, Yadav and co-workers reported a unique synthesis of lamellarin G trimethyl ether (32) (Scheme MeO OH MeO O O O O O O MeO MeO OH MeO 62 MeO MeO AlCl3, CH2Cl2 MeO O DCC, DMAP, DMF MeO O (65%) (89%) 60 61 MeO 63 MeO MeO MeO O O MeO OMe NHHCl MeO NBS, Sm(OTf)3 MeO Br MeO 65 MeCN, 20 °C MeO O K2CO3, MeCN, air, reflux MeO O (93%) (63%) N MeO MeO O 64 lamellarin G trimethyl ether (32) Scheme 10. Synthesis of lamellarin G trimethyl ether (32) via reaction between 64 and 65