A Stereoselective Synthesis of (+)-Gonyautoxin 3 Mulcahy, J. V.; Du Bois, J. J. Am. Chem. Soc. 2008, 130, 12630-12631 Total Synthesis of (+)-Lithospermic Acid by Asymmetric Intramolecular Alkylation via Catalytic C- Bond Activation ʼMalley, S. J.; Tan, K. L.; Watzke, A.; Bergman,. G.; Ellman, J. A. J. Am. Chem. Soc. 2005, 127, 13496-13497 Eric Phillips Short Literature 11/2008
Introduction to C- Insertion Chemistry X, metal catalyst X This process can be thought of as a net oxidation. Initial heteroatom insertion into a C- σ-bond: S Cl Cl Zn cyclohexane S nitrene S Breslow and Sloan, Tetrahedron Lett. 1968, 5349 Difficulties: competing aziridination, poor regioselectivity
Du Boisʼs C- Amination Question: Is there a way to perform the insertion regioselectively? 2 h(ii) catalyst PhI(Ac) 2 Mg Espino and Du Bois, 2001 By employing a carbamate, the nitrogen selectively inserts to form oxazolidinones. Also, full retention of the stereocenter is observed. S 2 TBS h(ii) catalyst PhI(Ac) 2 Mg S TBS Espino, Wehn, Chow, and Du Bois 2001 2 S CCl 3 h(ii) catalyst PhI(Ac) 2 Mg Tces Kim, Mulcahy, Espino, and Du Bois 2006
atural Product elevance 2 2 Saxitoxin Du Bois, 2006 2 Br Br dibromophakellstatin omo, 2003 2 2 2 Cl palau'amine Good Luck!! 3 S 2 2 Gonyautoxin 3 2 2
Du Boisʼs Strategy 2 Tces 3 S 2 insertion 2 Pictet-Spengler 2 Modification of the π-bond was previously undisclosed. Tces 2 Tces Tces aziridination h(ii) PhI(Ac) 2 Friedel-Crafts Tces
Gonyautoxin 3 3 S 2 2 2 + 3 L-serine DCC, Et 3 C 2 Cl 2 65% yield 2 C 1) TBDPSCl 2) DIBAL TBDPS 2 2 Tces TBDPS 1) EtTf S Tces TBDPS 1) Pd(PPh 3 ) 4 BF 3 Et 2 TBDPS 2) 3, 4 Ac 64% yield Tces 2) Tces S Cl 94% yield 54% over 3 steps 20:1 dr CCl 3 C()Cl 2 TBDPS 87% yield C()CCl 3
Gonyautoxin 3 3 S 2 2 2 Tces Tces Tces 2 TBDPS 5 mol % h(ii) TBDPS Et 3 Si TBDPS C()CCl 3 PhI(Ac) 2, Mg 61% yield Ac C()CCl 3 BF 3 Et 2 81% yield C()CCl 3 insertion into the sp 3 C- bond is not observed 1) TBAF 2) Cl 3 C C Bz Tces 2 DMP 79% yield Tces Bz 2 1) s4 2) Tces 2 C()CCl 3 ther types of oxidations resulted sm consumption but no pdt formation. C()CCl 3 54% yield C C()CCl 3 76% yield
Gonyautoxin 3 Tces Bz C()CCl 3 2 2, Pd/C, TFA 2 then 3, 83% yield 2 2 S 3 DMF 3 S 2 2 2 71% yield As with saxitoxin, the ketal is the more stable structure. Epimerization of the sulfate carbon occurs on standing to obtain a 1:3 mixture, β:α.
Ellmanʼs h-catalyzed C- Insertion Bn X h(i) Bn X Unlike the previous chemistry, no oxidant is required and formation of C-C bonds is possible. Bn h Bn h Bn X X X Initial coordination to the imine directs the oxidative addition to the ortho position. Thalji, Ahrendt, Bergman, and Ellman, JACS 2001, 129, 9692-9693. Thalji, Ellman, and Bergman, JACS, 2004, 126, 7192-7193
Ellmanʼs h-catalyzed C- Insertion Enantioselectivity is possible through the use of phosphoramidate ligands. P Bn Bn 94% yield 95% ee Bn Bn 90% yield 70% ee Bn Bn 95% yield 96% ee
Ellmanʼs Synthesis of Lithospermic Acid C 2 C 2 (+)-lithospermic acid C 2 Knöevenagel C 2 h-catalysis 2 C Ar
Ellmanʼs Synthesis of Lithospermic Acid Br C 2 C Br 2 C CBr 4, PPh 3 88% yield n-buli ClC 2 93% yield :py 59% yield 2 Bn Bn C 2 h(i) 2 C Ar ferrocenyl-pcy 2 then Cl 89% yield ee not sufficient
Ellmanʼs Synthesis of Lithospermic Acid Br C 2 C CBr 4, PPh 3 88% yield Br n-buli ClC 2 93% yield :py 59% yield 2 C 2 C 2 56% yield single enantiomer after recryst. [hcl(coe) 2 ] 2 FcPCy 2 toluene, 75 C then Cl 2 C
Ellmanʼs Synthesis of Lithospermic Acid C 2 Ar piperidine pyridine, 100 C C 2 C 2 Ar 85% yield 10:1 anti:syn decarboxylation epimerization EDC, DMAP C 2 C 2 C 2 Ar 80% yield C 2 C 2 C 2 1) 3 Sn C 2 2) TMSI, quinoline 100 C 33% yield over 2 steps