Synthesis of (±)-Bistellettadine A

Abstract

Esterification of the trienoic acid with o-xylylene dibromide gave the bis ester that underwent a templated Diels-Alder reaction to afford the macrodiolide stereospecifically in a single step. The synthesis of bistellettadine A was completed in four steps by hydrolysis and side chain elaboration.

As part of our continuing interest in the synthesis of guanidine-containing alkaloids, we were intrigued by the biologically active, dimeric, tetra guanidines bistellettadines A (1a) and B (2), which were recently isolated by Fusetani and co-workers from a Stelletta sponge collected from Shikine-jima island, 200 km south of Tokyo (see Figure 1).^1,2 They inhibit Ca²⁺/calmodulin-dependent phosphodiesterase (40% inhibition at 100 μM) and the growth of yeast and E. coli at 10 μg/disk. The structures of the bistellettadines were determined by spectroscopic analysis and the cis relationship of the two unsaturated side chains on the cyclohexene was established by NOE experiments. Bistellettazine A (3), a structurally related dimeric marine natural product with trans unsaturated side chains, was isolated by Capon in 2008.^2f Dimers 1a and 2 are probably biosynthesized by a Diels-Alder reaction of two monomers analogous to stellettadine A (7)^2b (see Scheme 1). The stereochemistry of the Diels-Alder reaction may be controlled in the biosynthesis. However, closely related bis sesquiterpene dimers have been isolated as mixtures of cis and trans isomers.³ This raises the possibility that the Diels-Alder reactions are not stereoselective and that the trans isomer 1b and the cis isomer of 3 were present in the sponges, but were not isolated and characterized.

Figure 1.

Structures of bistellettadines A (1a) and B (2), epi-bistellettadine A (1b), and bistellettazine A (3).

Scheme 1.

Mori's Synthesis of Stellettadine A (7)

Mori and co-workers synthesized stellettadine A (7)^2b by coupling Boc-protected agmatine 4 and acyl chloride 5 to provide the diacylated product 6 in 62% yield (see Scheme 1).⁴ Removal of the Boc group of 6 under acidic conditions followed by treatment of the resulting amine with aminoiminomethanesulfonic acid constructed the second guanidine group. The extra acyl group was removed using KOH to afford stellettadine A (7) in 56% overall yield from 6. The yield of this four-step sequence is 35% from 4, but only 17% based on acyl chloride 5.

Our retrosynthesis analysis of 1a is shown in Scheme 2. A Diels-Alder dimerization of two molecules of 8a or 8b could give 1a (after deprotection if 8a is used). Bis guanidine 8a will be prepared from trienoic acid 10 resulting from the hydrolysis of ester 11 formed by the Wittig reaction of trienal 12⁵ and ylide 13. Alternatively, the Diels-Alder reaction of 10 or 11 could be carried out before introduction of the guanidine side chains to give diacid 9a, which will then be elaborated to 1a. We hoped that the carboxylate salt of 10 might orient in water to give selectively the desired Diels-Alder adduct 9a with cis unsaturated side chains.⁶

Scheme 2.

Retrosynthesis of (±)-Bistellettadine A (1a)

Mori's procedure for the introduction of the guanidine side chain of stellettadine A (7) that proceeds through a bis acyl guanidine could be used for the conversion of acid 10 to monomer 8b, but it cannot be used for the conversion of diacid 9a to 1a. We therefore needed to develop a more efficient procedure for introduction of the guanidine side chain that does not proceed through a diacylated intermediate analogous to 6.

Wittig reaction of readily available dienal 12⁵ and ylide 13 was most effectively carried out in THF at 70 °C for 15 min under microwave irradiation to give 11 in 85% yield (see Scheme 3). The unstable trienoate 11 decomposed partially and gave some Diels-Alder dimer at longer reaction times, higher temperatures, or with traditional heating. Hydrolysis of ester 11 with LiOH in 4:2:1 THF/MeOH/H₂O for 16 h followed by neutralization with 6 M HCl gave crude acid 10. Coupling⁷ of acid 10 and 14⁷ in CH₂Cl₂ using DIPEA, EDC and HOBT gave 15 in 59% overall yield from ester 11, which was treated with bis Boc-protected agmatine 16,⁸ Et₃N, and AgNO₃ in DMF at 0 °C for 3 h and at 25 °C for 4 h to give 8a in 77% yield as a 2:3 mixture of tautomers. Deprotection of the Boc groups of 8a using TFA gave the bis trifluoroacetate salt 8b. Unfortunately, initial attempts to carry out Diels-Alder dimerizations of either 8a or 8b resulted in polymerization of the sensitive trienoyl guanidine.

Scheme 3.

Unsuccessful Approach to Bistellettadine A (1a)

To establish the structures of the two tautomers of 8a, we prepared an inseparable 36:64 mixture of the simpler tautomers 17a and 17b from tiglic acid using the two-step procedure described above for the conversion of 10 to 8a (see Scheme 4). The alkene proton at δ 7.08 in the minor isomer 17a shows a strong NOE only to the geminal methyl group at δ 1.80. However, the alkene proton at δ 6.74 in the major isomer 17b shows strong NOEs not only to the geminal methyl group at δ 1.87, but also to the N-H proton at δ 12.86. The presence of both tautomers indicates that the N-H exchange is slow on the NMR time scale.⁹ Protonation of both 17a and 17b should give the same diacylguanidinium cation. Treatment of the mixture in CDCl₃ with TFA gave 18 as a single compound in which the alkene proton absorbed at δ 6.86.

Scheme 4.

Model Tautomers 17a and 17b

We then investigated the Diels-Alder dimerization of trienoate ester 11 in organic solvents and the lithium salt of trienoic acid 10 in water. Hydrolysis of 11 with LiOH followed by concentration gave the lithium salt, which underwent a clean Diels-Alder dimerization in water under microwave irradiation at 110 °C for 50 min to give an inseparable 5:4 mixture of crude 9a and 9b in 94% yield (see Scheme 5). Heating trienoate ester 11 in toluene at 125 °C for 2 days followed by hydrolysis gave a similar mixture of 9a and 9b in comparable yield. An NOE between H-11 and the C-6 methyl group in 9a suggested that the unsaturated side chains of the major isomer 9a have the desired cis relationship. NOEs between H-11 and H-5 and between H-12 and the C-6 methyl group established that the unsaturated side chains are trans in the minor isomer 9b.

Scheme 5.

Diels-Alder Dimerization of 11 to Give 9a and 9b

Analytical, but not preparative, separation of 9a and 9b was achieved on reverse phase silica gel. We therefore coupled the mixture of 9a and 9b with 14, DIPEA, EDC and HOBT to afford an inseparable 5:4 mixture of 19a and 19b in 67% overall yield from trienoate ester 11 (see Scheme 6). Treatment of this mixture with 16, Et₃N, and AgNO₃ in DMF afforded a 5:4 mixture of 20a and 20b in 69% yield. Stirring this mixture in 9:1 CDCl₃/TFA at 25 °C for 4 days cleaved all six Boc groups to give a 5:4 mixture of 1a and 1b in 68% yield as the tetra trifluoroacetate salts, which were readily separated by reverse phase HPLC. The ¹H and ¹³C NMR spectra of synthetic 1a in DMSO-d₆ correspond very closely to those of natural 1a.^1,10 The spectra of 1b are very similar except that H-11 is shifted downfield by 0.07 ppm to δ 3.14, the C-6 methyl group is shifted upfield by 0.10 ppm to δ 1.00, C-5 is shifted downfield by 2.7 ppm to δ 153.0 and the C-6 methyl group is shifted upfield by 2.4 ppm to δ 21.7. These shifts are consistent with the upfield shift expected for gauche butane interactions between C-12 and C-5 in 9a and between C-12 and the C-6 methyl group in 9b.

Scheme 6.

Completion of Bistellettadine A (1a) Synthesis

Unfortunately, the Diels-Alder dimerization of ester 11 or the salt of acid 10 proceeded cleanly but with poor stereocontrol. Coupling two molecules of acid 10 to a linker by a temporarily tether will form a substrate that can undergo an intramolecular Diels-Alder reaction.¹¹ The tether needs to be easily attached and removed, long enough to accommodate the different lengths of the unsaturated side chains in the Diels-Alder adduct, and yet short enough to facilitate an intramolecular Diels-Alder reaction and favor the formation of the isomer with cis unsaturated side chains. Molecular mechanics calculations suggested that the intramolecular Diels-Alder reaction of 22 should give 23 stereospecifically.¹² We therefore treated acid 10 (2.5-4 equiv) with o-xylylene dibromide (21) and Cs₂CO₃ in CH₃CN at 85 °C (see Scheme 7). After 3 h, 21 was completely converted to bis ester 22 and ∼10% of Diels-Alder product 23 had already formed. Heating for 2 days gave exclusively macrodiolide 23 with the cis configuration of the unsaturated side chains in 42% overall yield from 11 (80% yield from 21). Remarkably, the intramolecular Diels-Alder reaction proceeded stereospecifically under milder conditions than the intermolecular Diels-Alder dimerization of 10 or 11 to form 23 with a 16-membered ring macrodiolide. The stereochemistry of 23 was established by NOEs between H-11 and the C-6 methyl group. Hydrolysis of 23 using LiOH in 5:2 THF/H₂O at 25 °C for 16 h gave 9a. Coupling of 9a with 14 gave 19a in 57% overall yield from 23. Reaction of 19a with 16 gave 20a in 57% yield, which was deprotected using TFA for 4 days to give 1a as a single isomer in 68% yield.

Scheme 7.

Use of Templated Diels-Alder Reaction to Synthesize Bistellettadine (1a)

In conclusion, trienal 12 has been converted to bistellettadine A (1a) in seven steps in 8% overall yield. Remarkably, the templated Diels-Alder reaction of 22 afforded macrodiolide 23 in excellent yield with complete stereocontrol.