Total Syntheses of Minfiensine Douany, A. B.; umphreys, P. G.; verman, L. E.*; Wrobelski, A. D., J. Am. Chem. Soc. 2008, ASAP. D: 10.1021/ja800163v Shen, L.; Zhang, M.; Wu, Y.; Qin, Y.*, Angew. Chem. nt. Ed. 2008, Early View. D: 10.1002/anie.200800566 verman Adam oye Current Literature April 26th, 2008 Wipf Group Qin Adam oye @ Wipf Group Page 1 of 19 6/15/2008
Strychnos alkaloids normacusine B C 2 vincorine sungucine C 18-desoxy-Weiland-Gumlich aldehyde minfiensine 3 C strychnine strychnophylline Biological activities: folk medicine, cytotoxic, antimalarial, anticancer Synthetic intrigue- compact complexity n strychnine: For its molecular size it is the most complex substance known. -Sir Robert Robinson, 1952 Robinson, R. Prog. rg. Chem. 1952, 1, 2. Adam oye @ Wipf Group Page 2 of 19 6/15/2008
Strychnos alkaloids in current literature Padwa methodology: R 1 R1! steps Ac Ac R 2 DMB (±)-Valparcine (±)-Tubifolidine Ac (±)-Strychnopivotine (±)-Strychnine Boonsombat, J.; Zhang,.; Chughtai, M. J.; artung, J.; Padwa, A. J. rg. Chem., 2008, ASAP Flavisiamine A-D C 2 C 2 C 2 C 2 Sekiguchi et al. eterocycles, 2008, Prepress Adam oye @ Wipf Group Page 3 of 19 6/15/2008
verman disconnections n this full article publication: -Summary of 2005 synthesis with full account of what didn t work -2 nd generation synthesis featuring a more concise end game strategy Catalytic asymmetric eck eck cyclization/ Carbonylation minium ion addition Adam oye @ Wipf Group Page 4 of 19 6/15/2008
2005 synthesis 2 TPS p-ts, Ph 50 C (95%) TPS R Comins' Reagent, amds TF, -78 C (82%) TPS Tf C 2 R = R = C 2 CC 2, LMDS, TF, - 78 C (89%) i. 9-BB,, 0 C to rt Comins' reagent, CsF, Cs 2 C 3 ii. a, rt iii. PdCl 2 (dppf), TF, rt (71%) TPS C 2 DMF, rt (85-95%) Tf C 2 Douany, A. B.; verman, L. E.; Wrobleski, A. D. J. Am. Chem. Soc. 2005, 127, 10186-10187 Adam oye @ Wipf Group Page 5 of 19 6/15/2008
Catalytic asymmetric eck reaction 20 mol % Pd(Ac) 2, ligand, PMP, toluene, 100 C, 70 h or Tf C 2 10 mol % Pd(Ac) 2, ligand, PMP, toluene, µw, 170 C, 30-45 min (75-87%, 99% ee) C 2 oxazoline ligands [Pd]- BAP Ph 2 P R = i-pr R = t-bu R C 2 (~60%) C 2 TFA (75%, 2 steps) C 2 C 2 Cl 2 C 2 2 C Douany, A. B.; verman, L. E.; Wrobleski, A. D. J. Am. Chem. Soc. 2005, 127, 10186-10187 Key intermediate Adam oye @ Wipf Group Page 6 of 19 6/15/2008
2005 synthesis 2 C R [Pd] 2 C R 2 C or Tandem Pd-mediated eck-carbonylation 2 C Epoxide route: m-cpba 1. PhSea, Et/TF, 60 C a, BnBr, TBA Bn 2 C C 2 Cl 2, rt 2. 2 2, 60 C TF (87%) (50 %, 2 steps) (88%) 2 C 2 C 2 C 2 C C 2 Bn CCF 3 CF 3 C 2 Bn 2 Bn 2 C C 2 C C 2 TFA, C 2 Cl 2, 0 C (92%) C troublesome - deprotection Adam oye @ Wipf Group Page 7 of 19 6/15/2008
Protecting group switch 1. TFA, C 2 Cl 2, rt 1. i. PhSea, Et, TF, 70 C ii. 2 2, 70 C TES Pd(PPh 3 ) 4 TES 2 C 2. AllocCl, K 2 C 3, 18-C-6, TF (90 %, 2 steps) 2 C Alloc 2. TESCl, imidazole, C 2 Cl 2 (75%, 2 steps) 2 C Alloc pyrrolidine, TF, rt (88%) 2 C Ts K 2 C 3, C, 70 C (96%) TES TES TES TES 2 C 2 C 2 C 15 mol % Pd(Ac) 2 dppb, C 80 C 2 C (16%) (48%) (not observed) TBAF, TF (89%) Structure of alcohol confirmed by X-ray Allylic C- insertion??? o tandem Pd reactivity numerous Pd catalysts, ligands, C pressures, solvents TES C 2 C Adam oye @ Wipf Group Page 8 of 19 6/15/2008
Pd-mediated C- insertion vs. Rearrangement/eck nstead a more conservative (rational) proposal olefin migration TES TES TES + TES TES C + R 2 C R 2 C R 2 C TES 5-exo-eck cyclization 2 C but is r catalyst > r rearrangment? Adam oye @ Wipf Group Page 9 of 19 6/15/2008
Phosphine-free reductive eck conditions; completion of the molecule r catalyst > r rearrangment? igh catalyst rates under phosphine-free eck conditions TES TES 2 C 1 mol % Pd(Ac) 2, K 2 C 3, Bu 4 Cl a 2 C, DMF, 80 C (80%) 2 C 1. CsF, DMF (100%) 2. DMP (99%) 2 C i. LMDS, TF, -78 C C 2 ab 4, C 2 1. BzTf, pyridine, C 2 Cl 2, 60 C ii. CC 2 (71%) 2 C TF, 0 C (60%) 2 C 2. KMDS, TF, -78 C (83%, 2 steps) C 2 1. LiAl 4, TF, -78 C 2 C 2. a,, 2, 100 C (85%, 2 steps) (+)-minfiensine 22 steps, 4.1 % overall yield (a) Jeffery, T. Tetrahedron Lett. 1985, 26, 2667 2670. (b) Jeffery, T. Tetrahedron 1996, 52, 10113 10130. Douany, A. B.; verman, L. E.; Wrobleski, A. D. J. Am. Chem. Soc. 2005, 127, 10186-10187 Adam oye @ Wipf Group Page 10 of 19 6/15/2008
ydroboration/oxidation regioselectivity 2nd generation synthesis (2008): 1. B 3 S 2 ; 2 2, a 2. TPAP, M, C 2 Cl 2 2 C 1. 9-BB, TF, 100 C; 2 2, a 2. TPAP, M, C 2 Cl 2 2 C 2 C 2 C 2 C (63%) (21%) (25%) (63%) 1. BX, toluene DMS, 70 C 2. TFA, C 2 Cl 2 (40%, 2 steps) Ts 2 C 1. TFA, C 2 Cl 2 2, Ts K 2 C 3, C (65%, 2 steps) 2 C!- "- leaving group K 2 C 3, C 65! C 2 C Adam oye @ Wipf Group Page 11 of 19 6/15/2008
2 nd -generation end game 10 mol % PdCl 2 (dppf) K 2 C 3 (4 eq.) amds, Comins' Reagent Tf 2 C, 70 C (74%) 2 C TF, -78 C (86%) 2 C Pd(Ac) 2, Ph 3 P, Et 3 C 2 1. LiAl 4, TF, -78 C C,, DMF (89%) 2 C 2. a,, 2, 100 C (85%, 2 steps) (+)-minfiensine formally 15 steps 1st end game- 16 steps from catalytic asymmetric eck, 11.8% yield 2nd end game- 9 steps from cat. asym. eck, 19% yield Adam oye @ Wipf Group Page 12 of 19 6/15/2008
Summary of verman routes 1 st generation: cat. asym. eck minium ion cyclization 2 C 2 C Alloc 2 C TES reductive eck (Phosphine-free) TES 2 C (+)-minfiensine 2 nd generation: cat. asym. eck!" vinylation minium ion cyclization 2 C 2 C 2 C (+)-minfiensine Adam oye @ Wipf Group Page 13 of 19 6/15/2008
Qin indole alkaloids thodology-driven synthesis: Three-step, one-pot cascade cyclization 2 C 2 Et Cu(Tf) 2 C 2 Et Et 2 C C 2 Et (83%) Qin et al. rg. Lett., 2006, 8, 6011-6014 Br Br 2 3 Qin et al. J. Am. Chem. Soc., 2007, 129, 13794-13795 (±)-Communesin F 23 steps, 3% overall yield Adam oye @ Wipf Group Page 14 of 19 6/15/2008
Qin disconnections Concise total synthesis of (±)-minfiensine: Copper carbene insertion cascade Pd-mediated!"vinylation minium ion addition Adam oye @ Wipf Group Page 15 of 19 6/15/2008
Starting material synthesis Bn methyl propiolate TFA, CCl 3 (94%) Bn C 2 Ts C 2 Et LMDS TF, -40 C Ts C 2 Et 10 mol % Pd/C, 2 (1 atm), TF (83%, 2 steps) Ts C 2 Et 1. Li,, TF, 2 2. DCC, DMAP, Et 3, ldrum's acid, C 2 Cl 2 then, reflux (72%, 2 steps) Ts C S 2 3 Ts 2 C 2 Et 3, C (86%) C 2 Bailey, P.; ollinshead, S. P.; Dauter, Z. Chem. Commun. 1985, 1507-1509 Shen, L.; Zhang, M.; Wu, Y.; Qin, Y.*, Angew. Chem. nt. Ed. 2008, Early View Adam oye @ Wipf Group Page 16 of 19 6/15/2008
Cyclopropanation rearrangement Ts Ts Ts R R 1 3 R 3 R 1 2 R 3 () R 2 R 3 R 2 R 2 R 2 Ts R 1 () ab 4 C 2 R 2 Ts (76%) R 2 Ts 9a Adam oye @ Wipf Group Page 17 of 19 6/15/2008
End game Déjà vu? verman-like intermediate Ts 2 5 mol % CuTf C 2 () LiCl (2 eq.) C 2 C 2 Cl 2, rt (81%) Ts 2, DMS, 130 C, 7 h (87%) Ts ab 4 (98%) Ts (7:4 d.r.) a/naphthalenide (10 eq.) TF, -78 C, 1 h (95%) Ts K 2 C 3, C, 70 C (82%) Dess-Martin C 2 Cl 2 (90%) 5 mol %Pd(Ac) 2, PPh 3 (0.5 eq.), TBAB K 2 C 3, DMF, 2, 70 C (60%) amds, Comins' reagent TF, -78 C (88%) Tf 10 mol % Pd(PPh 3 ) 4, Bu 3 SnC 2 (4 eq.) LiCl, dioxane, µw (85%) TMSTf C 2 Cl 2 (83%) (±)-Minfiensine 14 steps, 9.2% overall yield Adam oye @ Wipf Group Page 18 of 19 6/15/2008
Conclusion Arguably, a case of: Target-driven synthesis (verman) vs. thodology-driven synthesis (Qin) Regardless, each group accessed very similar advanced intermediates completely independent of one another-- the chemistry guides them! Combine end game strategies of each group to maximize efficiency of synthesis: (verman protecting groups and Pd-vinylation conditions with Qin stille coupling) Adam oye @ Wipf Group Page 19 of 19 6/15/2008