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A versatile synthetic approach to the tulearin class of macrolactones has been developed and deployed to make a stereoisomer of tulearin A. The knowledge gained about structure and synthesis will expedite the assignment of the stereostructure of this new anti-cancer agent.
In 2008, Kashman et al. reported the isolation of tulearin A (1) (Figure 1) from the marine sponge Madagascar Fascaplysinopsis sp. collected in the Salary Bay north of Tulear.1 The structure of 1 was elucidated through interpretation of MS, IR, and various one-dimensional and two-dimensional NMR experiments. However, the relative and absolute stereochemistry of 1 remain unknown. Tulearin A (1) exhibits potent anti-proliferative activity against human leukemic cell lines K562 and UT7. Activity against K562 cells was notably better; a 0.5 µg/mL solution of 1 inhibited ~60% of proliferation vs. 35% inhibition against the UT7 cells.1
Due to its promising biological activity and lack of a three-dimensional structure, we designed a synthetic strategy that was capable of producing any stereoisomer of tulearin A (1). These stereoisomers could be used to help assign the stereochemistry and to provide information about the stereostructure/activity relationship. Here we describe the single stereoisomer of tulearin 2.
Our retrosynthetic plan (Figure 2) was to construct the complete structure through the union of three fragments: stannane 25 (the C20–C26 fragment), an acid of type I (the C1–C12 fragment), and vinyl iodide 20 (the C13–C19 fragment) through an esterification, ring-closing metathesis, and Stille coupling.
Both acid I and iodide 20 originate from readily available (S)-citronellal (3), which already contains the methyl stereochemistry at C5 and C15. The stereocenters at C2 and C3 come from an asymmetric aldol reaction, while the substituents at C8 and C9 are installed through a Sharpless asymmetric dihydroxylation.
The alcohol at C17 is formed through a Noyori reduction. The carbamate is challenging because the alcohols at C8 and C9 have to be orthogonally protected to allow its installation before the final deprotection. We envisioned that all of the fragments could be adjusted by choice of reagents to give any stereoisomer.
The synthesis of the C1–C12 fragment I began with known aldehyde 42 derived from (S)-citronellal (Scheme 1). Aldehyde 4 and phenyltetrazolylsulfonyl ester 5 were joined in a Julia-Kociensky reaction3 (KHMDS, THF, −78 °C to −45 °C, 69%) to provide alkene 6 with excellent E selectivity. Sharpless asymmetric dihydroxylation4 of 6 with AD-mix-α in the presence of methanesulfonamide (t-BuOH/H2O 1:1, 0 °C, 96 h) gave rise to a diol that spontaneously cyclized to form the 5-membered lactone 7 exclusively in 85% yield. Thus, the hydroxy groups at C8–C9 (tulearin A numbering) were differentiated, and C8 group was protected as a p-methoxybenzyl ether [PMBOC(=NH)CCl3, La(OTf)3, toluene, 12 h, 85%] to give lactone 8. The p-methoxybenzyl (PMB) group was chosen because it can later be removed in the presence of silyl protecting groups installed on the hydroxy groups present at C3 and C9.
To cleave the lactone and introduce the double bond for the eventual ring-closing metathesis (RCM), lactone 8 was reduced (DIBAL-H, CH2Cl2, −78 °C, 96% yield) and the resulting hemi-acetal was treated with the methylphosphonium ylide (MePPh3, n-BuLi, THF, −78 °C to 60 °C, 2 h, 80%) to give alkene 9. The liberated hydroxy group at C9 was protected with TBSOTf (2,6-lutidine, CH2Cl2, 1 h, 96%), and the primary alcohol was deprotected selectively with Oxone5 (MeOH, H2O, 2 h) to yield alcohol 10 (88%). The free alcohol was then oxidized to aldehyde 11 with Dess-Martin periodinane (CH2Cl2, 2 h, 85%). To construct the final two stereogenic centers with the carboxylic acid already in place, the versatile Crimmins chiral auxiliary A6 was chosen and conditions were used to provide the syn product. Reagent A was treated with TiCl4, (−)-sparteine, and N-methyl pyrrolidinone (CH2Cl2, 0 °C) to give an enolate intermediate that was reacted with aldehyde 11 to afford the aldol product 12 in good yield (84%) and with good diastereoselectivity (dr = 94/6). After protection (TBSOTf, 2,6-lutidine, CH2Cl2) and saponification7 (LiOH, H2O2, THF), compound 13 was transformed into acid 14 in 99% yield (over the two steps). Thus, the C1–C12 fragment 14 was obtained in 21.7% yield from (S)-citronellal (3) over 14 steps.
Alcohol 20, representing the C13–C19 fragment of target 2, was also prepared from (S)-citronellal (3) (Scheme 2). Treatment of 3 with the propynyl Grignard reagent (THF, −78 °C), followed by a Swern oxidation [(COCl)2, DMSO, Et3N, CH2Cl2, −78 °C], afforded ketone 15 in 71% yield over the two steps.
Noyori reduction8 with (S,S)-Noyori catalyst B (10 mol % in i-PrOH) gave alcohol 16 in moderate yield (48%).9 Alcohol 16 was transformed into the alkyne 18, precursor of 20, through a facile five-step sequence (Scheme 3). To begin, the hydroxy group in 16 was protected (TIPSCl, imidazole, DMF), then the double bond was oxidatively cleaved with ozone followed by in situ reduction with NaBH4 (CH2Cl2/MeOH (3:2), −78 °C to rt) to give alcohol 17 in 85% yield (two steps). A three-step sequence10 involving mesylation (MsCl, Et3N, CH2Cl2), iodination (NaI, THF), and elimination (t-BuOK, THF) produced alkyne 18 in good yield (91%). Finally, the alkyne was converted to the desired vinyl iodide 19 through the (E)-selective hydrostannylation (Bu3SnH, Pd(PPh3)2Cl2, THF), followed by tin-iodine exchange with iodine (I2, CH2Cl2, 45% yield).11 Deprotection of the C17 hydroxy group proceeded smoothly to furnish iodide 20 (TBAF, THF) in 72% yield. Thus, the C13–C19 fragment 20 was obtained in 10.6% yield from (S)-citronellal (3) over 11 steps.
With both major fragments 14 and 20 in hand, the fragment couplings were undertaken. Several avenues were explored, and we present in Scheme 3 the optimum strategy for both yield and RCM (E) to (Z) selectivity. Coupling of acid 14 with alcohol 20 (DCC, DMAP, CSA, CH2Cl2)12 produced ester 21 in 74% yield. The RCM with Grubbs 1st generation metathesis catalyst gave macrocyclic lactone 22, as a 1.9:1 (E/Z) mixture.13 Without separation, 22 was deprotected with DDQ14 (CH2Cl2/H2O: 20/1, rt). A single flash chromatography purification on silica gel gave (E)-macrocycle 23 in 43% yield (over two steps). The free C8 hydroxy group was converted to the carbamate using a milder base that had been described15 (Cl3CONCO, CH2Cl2, then NaHCO3, MeOH) to afford macrocycle 24 in 86% yield. The Stille coupling with the easily prepared stannane 25, based on conditions reported by Marshall et al.16 [Pd2(dba)3•CHCl3, AsPh3, LiCl, NMP], proceeded in moderate yield (31%) to provide compound 26. Finally, the latter was deprotected with TBAF at 0 °C, to minimize basic saponification of the carbamate moiety,17 and compound 218 was recovered after HPLC purification.19
Analysis of the αD, 1H and 13C NMR spectra of 2 showed that it was a stereoisomer of tulearin A. The J3 coupling constant between H2 and H3 was significantly smaller in the spectrum of the natural product (2.9 Hz) than in isomer 2 (6.9 Hz), so the natural product probably has the 1,2-anti configuration. Other resonances in the 1H NMR spectrum of compound 2 tended to be similar to those of the natural product in both chemical shifts and coupling constants, but because the stereocenters of tulearin are somewhat isolated from each other, further conclusions about stereochemistry are premature.
In summary, a stereoisomer of tulearin A, compound 2, has been prepared in 0.34% overall yield from (S)-citronellal (20 steps for the longest linear sequence) in a total of 32 steps. The synthesis takes into account stereochemical flexibility, so the work paves the way to make other isomers and analogues of tulearin in an expeditious fashion.
We are grateful to Dr. Sablé and Dr. Herman from Sanofi-Aventis (Vitry sur Seine) for purifying compound 2. DPC thanks l'état et la région Ile-de-France for a Chaire Blaise Pascal and NIH-NIGMS for funding.
Supporting Information Available Experimental procedures and characterization of all new compounds.