A concise approach to the core structures of pinnaic acid and halichlorine.

An efficient and flexible synthetic approach to the core structures of pinnaic acid and halichlorine is described using spirocyclic nitrone 4 as a key intermediate. 1,3-Dipolar cycloaddition of 4 with dipolarophile 8 provides access to the azaspirocyclic core of pinnaic acid 5 while the spiroquinolizidine core of halichlorine 6 has been synthesised via cycloaddition of 4 with dipolarophile 29. Nitrone 4 is accessed by oxidative ring opening of isoxazolidine 9. The utility of this synthetic strategy in the synthesis of C5 substituted analogues of pinnaic acid is also demonstrated.