Total Synthesis of (+)-Condylocarpine, (+)-Isocondylocarpine and (+)-Tubotaiwine
Connor L. Martin, Seiichi Nakamura, Ralf Otte, and Larry E. Overman
Total Synthesis of (±)- and (-)-Actinophyllic Acid
Connor L. Martin, Larry E. Overman, and Jason M. Rohde
This review summarizes two very interesting papers published more or less recently by the Overman group (he is still my favourite… ). I decided to combine both papers because a common intermediate was used to make all four natural products and its synthesis makes use of some uncommon in situ Umpolung chemistry.
The first schemes were reproduced from the JACS paper while the last two schemes came from the OL paper.
Starting with Boc-protected GABA 1 the free acid was transformed into the Weinreb amide and alkylated with a vinyl-Grignard to get 2. Enantioselective reduction of the resulting ketone with high ee was accomplished by using catalyst A in the presence of hydrogen (Noyori’s catalyst). Ozonolysis of the double bond and trapping of the alcohol and resulting aminal with acetic anhydride furnished piperidine 4.
The second main fragment was obtained in two steps from di-tert-butylmalonate. Deprotonation and acylation gave compound 6 which formed indole 7 after reduction of the nitro group with Pd on charcoal in the presence of vanadate.
Then it is getting more interesting: the blue and red fragment were combined by using a bit of scandium triflate to form 8 with great diastereoselectivity. Reductive removal of the acetyl protecting group and Swern oxidation of the resulting free alcohol produced ketone 9. Next my favourite reaction of the whole paper was employed: first a double deprotonation of the ketone and the malonate and then combination of the two carbanions to form the critical bicyclic ring system. Although the yield is moderate it proceeds with high dr. [Fe(DMF)3Cl2][FeCl4] was prepared from dehydrated iron(III)chloride and DMF by simply mixing the reagents. Finally addition of vinyl-Grignard under Luche conditions to the ketone forms lactone 11.
Going on with the synthesis the lactone and the remaining ester group were reduced to get bis-alcohol 12. At this stage Overman makes use of his almighty aza-Cope/Mannich reaction.
The t-Bu- and Boc-groups were cleaved off in the presence of dilute acid before formalin was added. For clarity I added the main stages of the following events:
First a Schiff base formed from formaldehyde and the secondary amine. This underwent an aza-Cope rearrangement (or some sort of Prins-reaction) with the allyl alcohol to form 13b. The newly formed enol then attacks the rearranged Schiff base in a Mannich reaction to give (-)-Actinophyllic acid 14 as its hydrochloride.
Finally to the paper mentioned first. Starting from key intermediate 10 the ketone was reduced, the Boc group removed and the malonate decarboxylated/transesterified to give amine 15. Reductive amination with the dithioacetal aldehyde shown was followed by a DMTSF mediated alkylation to give 17. Reductive desulfuration with Raney Ni and oxidation of the remaining alcohol under Albright/Goldman conditions (Swern-oxidation) furnished 18.
Wittig reaction of the keto group then produced (+)-condylocarpine (and (-)-isocondylocarpine respectively) which was reduced in the presence of platinum oxide to give (+)-tubotaiwine.
Nice… I had the schemes finished a few days ago but also had to write my last serious exam so … What do you think? Any comments?