A New Synthetic Route to the Skeleton of Saxitoxin, a Naturally Occurring Blocker of Voltage ─ Gated Sodium Channels Yusuke Sawayama and Toshio Nishikawa ＊ Department of Applied Biological Sciences, Graduate School of Bioagricultural Sciences, Nagoya University Nagoya 464 ─ 8601, Japan (Received July 2, 2012; E ─ mail: [email protected]) Abstract: Saxitoxin is as potent and specic blocker of voltage ─ gated sodium channels as tetrodotoxin. This unique biological activity has established the importance of these two small natural products in neurophysio- logical experiments. In order to nd new blockers of the voltage ─ gated sodium channels, an efcient synthetic route to the skeleton of saxitoxin was developed. This new synthetic route is based on two key reactions (i) cascade cyclization of a guanidino ─ acetylene initiated by the bromocation (Br ＋) and (ii) transformation of the geminal ─ dibromomethylene moiety to enol acetate, and culminated in the total synthesis of decarbamoyl ─ α ─ saxitoxinol, a naturally ─ occurring analog of saxitoxin. 1． Introduction Saxitoxin (1, STX) and tetrodotoxin (2, TTX) are famous marine natural products isolated as toxic principles of para- lytic shellsh poison (PSP) and puffersh intoxication, respec- tively (Figure 1). 1 These two small natural toxins exert their potent toxicity by specic blockage of sodium ion inux through voltage ─ gated sodium channels (VGSC) without interfering with any other ion channels such as potassium or chloride channels on the neuro cell membrane. 2 Due to this unique biological property and their high afnity for the VGSC, STX and TTX have played signicant roles in the identication and elucidation of the biological function(s) of the channel proteins. 3 Figure 2. (a) The schematic structure of VGSC and (b) the subtypes (isoforms) of human VGSC. Figure 1. Saxitoxin (1, STX) and tetrodotoxin (2, TTX), naturally occurring VGSC blockers. Recently, genetic analyses have revealed that ten subtypes (isoforms) of VGSC (Na v 1.1 ─ 1.9 and Na x) are expressed in different organs of mammals, and that each subtype has differ- ent and unique biological functions (Figure 2). 4 For example, Na v 1.5 is predominantly expressed in cardiac muscle, while Na v 1.7 and 1.8 are found in central neurons and are responsi- ble for the sensation of pain. Interestingly, Na v 1.5, 1.7 and 1.9 are not inhibited by STX and TTX (so ─ called TTX ─ resistant sodium channels). Subtype ─ selective VGSC inhibitors there- fore are highly desired, not only for analyses of channel bio- logical functions, but also for the development of drugs for the treatment of epilepsy, pain and arrhythmia. Several subtype ─ selective inhibitors based on the structures of TTX and STX 5 have been reported (Figure 3); 4,9 ─ anhydro ─ TTX (3), a natu- 6 ral analog of TTX, selectively inhibited Na v 1.6, compound 4, a simplied analog of STX shows inhibitory activity of Na v Figure 3. Subtype ─ selective inhibitors of VGSC. 1 178 （ 70 ） J. Synth. Org. Chem., Jpn. 有機合成化学70-11＿0006論文_Sawayama.indd 70 2012/10/19 14:05:24 7 1.1 and 1.2, and STX ─ related compound 5 inhibits Na v 1.4 ration of diverse STX analogs for developing subtype ─ selective 8 and 1.5. On the other hand, A ─ 803467 (6), a compound struc- blockers of VGSC. turally unrelated to TTX or STX, was also reported to be a Initial Synthetic Strategy potent and selective blocker of Na v 1.8, and has been devel- Our initial idea for the synthesis of STX arose with recog- oped as a therapeutic agent for neuropathic and inammatory nition of the single carbon chain in the STX core skeleton, pain. We have also been interested in developing subtype ─ which inspired us to conceive a new synthetic route commenc- selective inhibitors of VGSC based on STX and TTX. 9 This ing from a simple intermediate B as shown in Scheme 1. We account describes the details of our synthetic efforts toward envisioned that the tricyclic skeleton of STX would be con- STX, focusing on synthesis of the guanidine ─ containing core structed from a bis ─ guanidino ─ acetylene A by double halocy- structure as well as the underlying logic and strategy. 10 clizations (A to C & C to D) for the synthesis of the two cyclic guanidines, and an intramolecular N ─ alkylation for the pyrro- 2． Synthetic Strategy Toward the STX Skeleton lidine synthesis (D to E). We expected these three cyclizations Saxitoxin (1) is a representative densely ─ functionalized would proceed in cascade fashion, or in one ─ pot. The precur- natural product. The molecular formula (C 10H 19N 7O 4), and the sor A for this cascade cyclization could be readily prepared by structure containing two cyclic guanidiniums and a hydrated bis ─ guanylation of an anti ─ diaminoacetylene B, a relatively ketone clearly indicate the special features that characterize a simple compound. 18 heteroatom ─ rich small molecule. Since more than thirty ana- This strategy relies on two reactions; (i) cascade bromocy- logs of STX have been isolated from nature, a synthetic route clizations (halocyclizations) of a guanidino ─ acetylene, and (ii) to STX should enable access to these various analogs, includ- transformation of the geminal ─ dibromomethylene moiety to a ing neosaxitoxin (7, N ─ hydroxylated), gonyautoxin 3 (8, sulfo- ketone (E to STX skeleton). When we initiated this project, a nated) and zetekitoxin AB (9) shown in Figure 4. 11 few examples of the former transformation had been reported. 19 ─ 21 and the latter transformation had been reported only for specic substrates (vide infra). First of all however, we prepared a possible precursor 10, and attempted the cascade reaction by exposing it to sources of the bromocation (Br ＋) such as NBS and pyridinium tribromide (PyHBr 3) (Scheme 2). However, to our disappointment, the desired bicyclic guanidine 12 was not obtained at all. Instead, ve ─ membered guanidine 11 was obtained in a poor yield under a limited range of condi- tions, while six ─ membered cyclic guanidine product was not detected. When the product 11 was isolated and exposed to Br ＋ again, the second cyclization did not proceed. These results Figure 4. Naturally ─ occurring Analogs of STX. led us to suppose that the rate of ve ─ membered cyclization was much faster than that of six, and that the ve ─ membered Due to its unique structure as well as its potent biological ring product might not be a suitable precursor for the second activity, STX has been an attractive synthetic target for natural cyclization. However, we had not yet understood the whole product synthesis since its structure elucidation in 1970s. 12 To picture concerning the bromocyclization of guanidino ─ acety- date ve researchers Kishi, 13 Jacobi, 14 Du Bois, 15 Nagasawa 16 lenes. We therefore decided at this stage to investigate optimi- and Looper 17 have independently reported elegant total syn- zation of the conditions and the regioselectivites for the bro- theses of STX and its analogs. Although their synthetic routes mocyclization of guanidino ─ acetylenes by utilizing model are all unique and efcient, a simpler, more versatile synthetic compounds. route has been desired to make possible the expeditious prepa- In the halocyclization of a guanidino ─ acetylene, there are Scheme 1. Our initial synthetic strategy for construction of the STX skeleton. Vol.70 No.11 2012 （ 71 ） 1179 有機合成化学70-11＿0006論文_Sawayama.indd 71 2012/10/19 14:05:28 Scheme 2. The rst attempt at a cascade bromocyclization of Scheme 3. Two possible modes of bromocyclization for (a) propar- bis ─ guanidino ─ acetylene 10. gyl guanidine F and (b) homopropargyl guanidine I for the synthesis of the STX skeleton. two possible modes of cyclization; the exo ─ and endo ─ modes (Scheme 3). Propargyl guanidine F can undergo 5 ─ exo ─ dig cyclization and/or 6 ─ endo ─ dig cyclization, leading to ve ─ membered ring product G and/or six ─ membered ring product ＋ H, respectively (Scheme 3(a)). On the other hand, homo- the source of bromocation (Br ), no reaction was observed, propargyl guanidine I can undergo 5 ─ endo ─ dig cyclization indicating the poor reactivity of this reagent. We next exam- ＋ and/or 6 ─ exo ─ dig cyclization leading to ve ─ membered ring ined pyridinium tribromide (PyHBr 3) as a more reactive Br product J and/or six ─ membered ring product K (Scheme 3(b)). source (Table 1). When 13a was treated with PyHBr 3 in the Although both 6 ─ endo ─ dig and 5 ─ endo ─ dig cyclizations may presence of K 2CO 3 in CH 2Cl 2, simple bromination of the be possible according to the Baldwin’s rules, 22 we anticipated alkyne moiety took place to give dibromoalkene 15 in a good that the exo ─ mode of cyclization would proceed preferentially yield (entry 1). The same reaction when conducted in a bipha- in both cases to yield the desired cyclic guanidine products G sic solvent system comprising of CH 2Cl 2 and H 2O (1:1) and K, respectively. afforded the spiro ─ hemiaminal 16a as a single diastereomeric product in moderate yield (entry 2). This result indicates that 3． Bromocyclization of Guanidino ─ acetylenes the expected six ─ membered ring product 14 further reacted ＋ 3.1 Bromocyclization of Homopropargyl Guanidines with an excess amount of Br to give the compound 16a by Bromocyclization of homopropargylguanidine to give the trapping of an iminium ion intermediate by an internal six ─ membered cyclic guanidine was examined rst. Com- hydroxy group. The similar spiro ─ product 16b was obtained in pounds 13a ─ c were selected as model substrates having the better yield, when mono ─ protected guanidine 13b was treated same carbon number as the STX skeleton, and exposed to a under the same conditions (entry 3). In order to intercept and variety of conditions for bromination. Our preliminary experi- isolate the six ─ membered ring product 14, substrate 13c with ments revealed that bromocyclization was better than iodocy- the hydroxy group protected with TBS was exposed to the clization because of product stability.