Total Synthesis Highlights
Total Synthesis Highlights
Total Synthesis Highlights
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Debenzylation of 11 followed by acetylation delivered 12, an intermediate in the Carreira<br />
synthesis of the zaragozic acids. Following that precedent, the ring acetates of 12 were selectively<br />
removed, leaving the acetate on the side chain. Boc protection was selective for the endo ring<br />
secondary hydroxyl, leaving the exo ring secondary hydroxyl available for condensation with the<br />
enantiomerically-pure acid 13. Global deprotection then completed the synthesis of Zaragozic<br />
Acid C (3).<br />
The key to the success of this synthesis of the complex spiroketal 3 was the assembly of 7 in one<br />
step as a single diastereomer from the readily-available building blocks 4, 5, and 6. This process,<br />
reminiscent of group transfer polymerization, will be a useful complement to the cascade<br />
organocatalyzed aldol condensations that have recently been developed.<br />
24. The Carter <strong>Synthesis</strong> of (-)-Lycopodine<br />
Rich G. Carter of Oregon State University described (J. Am. Chem. Soc. 2008, 130, 9238 ) the first<br />
enantioselective synthesis of the Lycopodium alkaloid (-)-lyopodine (3). A key step in the<br />
assembly of 3 was the diastereoselective intramolecular Michael addition of the keto sulfone of 1<br />
to the enone, leading to the cyclohexanone 2.<br />
The key cyclization substrate 1 bore a single secondary methyl group. While that could have been<br />
derived from a natural product, it was operationally easier to effect chiral auxiliary controlled<br />
conjugate addition to the crotonyl amide 4, leading, after methoxide exchange, to the ester 5. The<br />
authors reported that double deprotonation with LiTMP gave superior results, vs. LDA or BuLi, in<br />
the condensation of 6 with 5 to give 7. Metathesis with pentenone 8 gave the intramolecular<br />
Michael substrate 1.