Total Syntheses of ominine i Literature t Group eting Lizzie Bryan ctober 17, 2007
Aconite Alkaloids Atidane V eatchane Cycloveatchane Aconitane etisan Muratake,. M.; atsume, M.; akai,. Tetrahedron, 2006, 62, 7093-7112. Muratake,. M.; atsume, M. Tetrahedron, 2006, 62, 7056-7070
Background ominine is the simplest hetisine-type alkaloid. Aconite alkaloids are isolated from Aconitum, Delphinium, Consolida, Thalictrum, and Spiraea. ominine was originally isolated from Aconitum sanyoense by chiai and coworkers in 1956. This family of natural products has been widely used in herbal medicine. Many of these alkaloids exhibit potent antiarrhythmic, immunomodulating, and analgesic activities in vivo. Muratake and atsume s synthesis, completed in 2004, was the first total synthesis of a hetisine i alkaloid. l hetisine core ominine Peese, K. M. and D. Y. Gin. J. Am. Chem. Soc, 2006, 128, 8734-8735. Muratake,. M.; atsume, M.; akai,. Tetrahedron, 2006, 62, 7093-7112. Muratake,. M.; atsume, M. Tetrahedron, 2006, 62, 7056-7070.
Muratake and atsume s Synthetic Approach MM Ac C C Piv Br I Muratake,. M.; atsume, M.; akai,. Tt Tetrahedron, hd 2006, 62, 7093-7112. 7112 Muratake,. M.; atsume, M. Tetrahedron, 2006, 62, 7056-7070. Muratake,. M.; atsume, M. Agnew. Chem. Int. Ed. 2004, 43, 4646-4649. Muratake,. M.; atsume, M. Tet. Lett. 2002, 43, 2913-2917.
Synthesis of the Tetracycle Br 5 steps 2 Br 1. K, then KMn 4, MgS 4, / 2, 75% C I 2. PdCl 2 (Ph 3 P) 2,CsC 3, TF, 65% 1. (C 2 ) 2, pts 2. Li, Et, liq. 3 /TF; 0.5% Cl in TF/ 2, 0 o C 87% over 2 steps 94% ab 4 CeCl 3 7 2 2 toluene, 170 o C 2 BF 3 Et 2 68%
Formation of the Tetracycle via an Acetal-Ene Reaction 2 BF 3 Et 2 2 BF 3 R BF 3 5-exo-trig 2 F 3 B BF BF 3 2 3 6-endo-trig R R= C 2 R=C 2 R= C 2 R 2 F 3 B 2
Elaboration of the tetracyclic intermediate Piv 7steps MM Ac 1. Et 2 AlC, 94% 2. TMSCl, LDA, TF; LiAl 4 3. CbzCl, Et 3, C 2 Cl 2, 63% over 2 steps MM MM 1. ab 3 C, Cl (aq),, 90% K 2 C 3, CBz Ac 2. K 2 C 3, 3. PCC/Al 2 3, 84% over 2 steps CBz P 2 98% MM CBz 1. 5% Cl, DME/ 2 2. Bu 3 Sn, AIB, Ph; Si 2,C 2 Cl 2 1. MsCl, Et 3 2. LiBr, acetone, reflux, 3. tbu, Se 2 55% over 2 steps C 2 Bz 67% over 3 steps Br CBz
Completion of the Synthesis Br CBz 1. ab 4 CeCl 3 7 2, 0 o C, quant. 2. Ac 2,pyridine 3. Zn, 4 Cl, ipr 90% over 2 steps CBz Ac 1. Et 3 Si, cat. Pd(Ac) 2,cat. Et 3, C 2 Cl 2, 2 SCl 2, pyridine, C 2 Cl 2, 80% over 2 steps 3. K 2 C 3,,95% Completed the first total synthesis a hetisine type alkaloid in 40 steps in 0.15% overall yield. Utilized Pd-catalyzed intramolecular α arylation reaction developed in their laboratories.
Gin s Approach Gin envisioned using two cycloadditions to gain access to the hetisine core in an efficient manner. An aza-1,3-dipolar cycloaddition would provide the bridged pyrrolidine ring. X A Diels-Alder reaction would be used to assemble the [2.2.2]- bicyclic structure. Peese, K. M. and D. Y. Gin. J. Am. Chem. Soc, 2006, 128, 8734-8735.
Synthesis of the Aza-1 1,3-Dipole 1. t BuLi, Et 2, -23 o C then Cl 3 AcCl, 2. a 3, acetone 99% 49% over 2 steps 3 1. PBu 3, 1. AlEt 2 C, C 6 6 ; TBAT, Tf 2 C 1. DIBAL, Ph, 92% C 2. ab(ac) 3 79% 81% 2. Zn(C) 2,Pd(PPh 3 ) 4 Tf DMF, 60 o C, 85% C C 10% TFA in C 2 Cl 2 93% C
1, 3-Dipolar Cycloaddition TF, 180 o C C TF, 180 o C C C TF, 180 o C C (1:3.6) C 1: 3.6 ratio of desired: undesired Reaction is reversible, could recycle undesired product to desired by subjecting to initial cyclization conditions.
End Game C 1. ab 4,Et 2. SCl 2,C 2 Cl 2 3. Bu 3 Sn, AIB, C 6 6 reflux 68% over 3 steps C 1. DIBAL, Ph, 85% 2. Ph 3 P C 2, 96% a 0, 2 C, TF; Cl, 97% 9:1 : pyrrolidine 78% 1. Ph 3 P C 2, 77% 2. Se 2, t Bu, C 2 Cl 2,66%
Summary of Gin s Synthesis Completed synthesis of nominine in 15 steps Completed synthesis of nominine in 15 steps nly one protecting group manipulation in the synthesis Key steps include intramolecular 1,3-dipolar cycloaddition and Diels-Alder reactions.