Total Synthesis of Saxitoxin
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1 Total ynthesis of axitoxin John Baird April 11,
2 First isolated in 1957 by chantz et al Isolation and Properties X-Ray tructure obtained in 1975 Freely soluble in water and methanol ptical rotation tereo view of X-ray structure Amorphous solid with formula C Reported to be one of the most toxic non-protein compounds known mw g/mol 2 Jacobi, P. A. et. al Croat. Chem. Acta 1986, 267
3 Biological Properties of axitoxin Potent neurotoxin isolated from toxic Alaska butter clams (axidomus giganteus) and soft shell clams (Mytilus californias) Established in 1920 s that shellfish became toxic during the time of red tides during which Gonyaulax catenella (algal species) blooms. Under optimal temperature and light conditions blooms of red algae proliferate to generate red tides Responsible for paralytic shellfish poisoning. o known cure, treatment available ymptoms include tingling, numbness, weakness, and limp paralysis Toxicity of 8µg/Kg in mice; mg would prove fatal to humans;100x more poisonous than strychnine, 1000x (sarin gas), 2000x (ac) Biological action: blocks sodium ion channels in neuron membranes (medicinal interest) 3 Jacobi, P. A. et. al Croat. Chem. Acta 1986, 267
4 Key ynthetic Challenges 2 2 Molecule possesses: All carbon atoms except one carry heteroatoms 3 contiguous stereogenic centers Tricyclic skeleton posessing two guanidines 2 Polarity/solubility of the molecule makes manipulation/purification cumbersome 4
5 Total yntheses First by Kishi, made racemic saxitoxin in 1977, and later reported the synthesis of unnatural (-)-decarbamoylsaxitoxin in econd synthesis of racemic saxitoxin by Jacobi in 1984 Most recently, synthesis of natural (+)-axitoxin by Du Bois (2006) Kishi Jacobi Du Bois
6 Kishi Retrosynthesis Me Y P X C 6
7 Preparation of Vinylogous Carbamate 2 2 Me 1) p-ta, C 6 5 C 3 reflux 2) 2 2 2, C 3, reflux P 2 5 benzene 2 74% ac 3, C 2 Cl 2, reflux 1) C 3 CCBrC 2 C 3 2) K, C 3, 50 0 C 50% (from amide) Please propose a mechanism for this reaction Me 7 Kishi, Y. et. al. J. Am. Chem. oc. 1977, 98, Kishi, Y. et. al. etercycles. 1980,14, 1477.
8 Preparation of Diguanidine Core Me Me i(=c=) 4 Bn + rt to C 75% ) 2 2 2, C ) Cl, C 2 Cl 2, C Bn C 3 C, BF 3 Et 2 Bn 3) 90 0 C, C 6 6 4) 3, C % 75% Mechanism? 2 Bn 1) Et 3 + BF - 4 ac 3 C 3 C 2, CF 3 C 2 (9:1) Bn C 2 Cl 2, rt 50 0 C, 18 h 2) EtC 2 4, C 50% 33% 2 Kishi, Y. et. al. J. Am. Chem. oc. 1977, 98, Kishi, Y. et. al. etercycles. 1980,14, 1477.
9 Completion of the ynthesis Bn BCl 3 C 2 Cl 2, 0 0 C 2 1) Ac 2, pyr, rt 2) B, C 3 C aq, 15 0 C 3) C 3, C 2 75% 2 30% 2 2 Cl 2 =C=, C 2 2 then % 2 Kishi, Y. et. al. J. Am. Chem. oc. 1977, 98, Kishi, Y. et. al. etercycles. 1980,14, 1477.
10 Kishi ynthesis- ummary First total synthesis (racemic) 1977 Key teps: Eschenmoser sulfide contraction Curtius Rearrangement Enamine condensation 17steps, 0.2% overall yield
11 2 Jacobi Retrosynthesis R X Et 2 R R R Takes advantage of 3+2 cycloaddition Jacobi, P. A. et. al J. Am. Chem. oc. 1984, Jacobi, P. A. et. al Croat. Chem. Acta. 1986, 2674
12 Preparation of Cycloaddition Precursor ClCC 2 C 2 Et, ncl 4 C % Et (C 2 ) 3, BF 3 Et 2 74% Et K, 2 80% TFAA, C % PhC 2 2, TF 74% Ph Me Me BF 3 Et 2 C 2 Me Ph 12 Jacobi et al J. Am. Chem. oc. 1981,
13 Preparation of Pyrazolidine C 2 Me Ph 65-75% Me Ph 1) ame, Me 2) ab 4, Me 72% Ph a, C 43% Ph Further attempts at reduction gave decomposition 13 Jacobi, P. A.; Martinelli, M. J. J. Am. Chem. oc. 1984, 106, Jacobi, P. A. et. al Croat. Chem. Acta 1986, 267
14 Completion of the Formal ynthesis 2 2 Ph B 3 DM 98% Ph 1) Pd, C 2 2) Cl Ph 2 Ph a 3, C 75% 1) Ac 2, Pyr 2) Et 3 + BF4 - KC 3 99% Et Ac Et 14 EtC C 40-50% 2 2 Intersects with Kishi s synthesis Jacobi, P. A.; Martinelli, M. J. J. Am. Chem. oc. 1984, 106, Jacobi, P. A. et. al Croat. Chem. Acta 1986, 267
15 Jacobi ynthesis- ummary 2 2 Intersected Kishi intermediate 2 Utilized [3+2] cycloaddition to access core calable and efficient route has been used to prepare 1.0g of saxitoxin precursor econd synthesis 1984 (racemic) 15 steps (3.3% yield)
16 Du Bois Retrosynthesis R 3 Me 2 R ZnX + Ts Et Et Ts Will take advantage of C- amination reaction 16 Fleming, J. J.; Du Bois, J. J. Am. Chem. oc. 2006, 3926
17 ovel Iminium Ion Equivalents 17 Du Bois, J. et. al, J. Am. Chem. oc. 2003, 125, 2028
18 ovel Iminium Ion Equivalents Cont d Proposed T model xathiazinane heterocycles are easily prepared chiral building blocks 18 Du Bois, J. et. al, J. Am. Chem. oc. 2003, 125, 2028
19 Et Et Preparation of xathiazinane Cl Rh 2 (tpa) 4 3mol% DMA/C 3 C PhI(Ac) 2, Mg C 2 Cl 2, rt, 4h Et Et Et 75% 92% Et Ts ZnCl 1) 2, Pd/CaC 3 /Pb, TF BF 3 Et 2 2) a 3, nbu 4 I, DMF 90% 70% Ts PMB p-mec 6 4 C 2 Cl 1) Me 3 P, TF/ 2 nbu 4 I, K 2 C 3, C 3 C 2) Me(Cl)C=Mbs ipr 2 Et, C 3 C 85% PMB 3 Mbs= p-mec Me 3 72% Mbs 19 Du Bois, J. et. al, J. Am. Chem. oc. 2003, 125, 2028 Fleming, J. J.; Du Bois, J. J. Am. Chem. oc. 2006, 3926
20 Preparation of Elaborated Azide PMB PMB 1) Tf 2, C 5 5, DMAP, C 2 Cl 2 ( 4 ) 2 Ce( 3 ) 6 Me 2) a 3, DMF, C 3 Me tbu/c 2 Cl 2 Mbs 70% Mbs 74% Mbs 2 Mbs 2 3 Me Mbs KtBu, Cl 2 C=Mbs then (Me 3 i) 2 70% (+20% M) 3 Me aq. C 3 C, 70 0 C 3 95% Me R 20 Fleming, J. J.; Du Bois, J. J. Am. Chem. oc. 2006, 3926
21 Mbs Preparation of 9-membered Ring Mbs Me R 1) Me 3 P, TF/ 2 2) Ag 3, Et 3, C 3 C 65% 2 2 Mbs 2 Mbs Cl 3 CC()C, TF/C 3 C, 2 Mbs C; then K 2 C 3, Me Mbs 82% Mbs 21 Fleming, J. J.; Du Bois, J. J. Am. Chem. oc. 2006, 3926
22 Completion of the ynthesis Mbs 2 2 Mbs % scl 3, xone, a 2 C 3 EtAc/C 3 C/ 2 Mbs 57% Mbs Mbs 2 Mbs 2 2 B( 2 CCF 3 ) 3, CF 3 C 2 82% 2 2 DCC, C CCF 3 DM 2 70% Fleming, J. J.; Du Bois, J. J. Am. Chem. oc. 2006, 3926
23 Du Bois ynthesis- ummary 2 2 First total synthesis of (+) saxitoxin 2 Utilized C- amination and oxathiazinane iminium ion equivalent developed within the Du Bois labs Key strategies relied upon stereocontrolled formation of the 9-membered ring and condensation to prepare the bicyclic guanidine core 19 steps from commercially available (R) glycerol acetamide 1.6% overall yield
24 2 2 Conclusion Three syntheses within the past 29 years; although first non-racemic synthesis published last month. Three very different approaches to the core: Kishi- build each cyclic guanidine upon pyrrolidine 2 Jacobi- [3+2] cycloaddition Du Bois- condensation of 9-membered ring 2 Kishi Jacobi Du Bois
25 3+2 cycloaddition selectivity disfavored 25
26 Keto hydroxylation Reaction 26 Plietker, JC, 2004, 8287
27 27 Keto hydroxylation cont d
28 28 Proposed Biosynthesis
29 29 Biosynthesis
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