Here, we report a synthesis of the lower half C 21 −C 40 fragment of the shellfish toxin, azaspiracid-1. The C 28 −C 40 fragment was synthesized by a coupling between the C 28 −C 35 epoxide and the C 36 −C 40 dithioacetal anion, followed by the HI-ring spiroaminal formation. An aldehyde corresponding to the C 28 −C 40 fragment was then coupled with the C 21 −C 27 allylic stannane by using InCl 3 . Finally, the FG-ring was constructed by HF‚pyridine to accomplish the synthesis of the suitably protected C 21 −C 40 fragment.
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