([a]D = +22.8, c = 0.89, CHCl3, lit.19 [a]D = +26.8,
c = 0.85, CHCl3).
In summary, a total synthesis of (À)-mitragynine was
realised starting from 4-methoxytryptamine in nine steps by
using as the key step an enantioselective Pictet–Spengler
reaction catalysed by a chiral thiourea. This route also
provided the first total syntheses of the related natural
products paynantheine and speciogynine. Our work once
again emphasises the power of asymmetric organocatalysis
in an appropriate combination with transition-metal catalysis
for the total synthesis of natural products.22
This research was partly supported by the National
Research School Combination-Catalysis (NRSC-C).
Notes and references
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Scheme 4 Total syntheses of the title alkaloids.
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Both 19 and 20 were then separately treated with the ylide
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´ ´
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20 In this process a small amount of TFAA was added to the TFA to
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 12243–12245 12245