Biotin: spot the regiochemistry flaw

Single question

Two-step end-game from a bicyclic thio-urea aldehyde to biotin: Wittig to install the pentyl ester chain, then ester hydrolysis. The chemistry is plausible; check where the new C-C bond lands on the ring.

Biotin connectivity (side chain on the C2 of the thiophane)

Wittig olefination regiochemistry from the correct aldehyde

Why the wrong attachment point destroys streptavidin recognition

Find the flaw

Click the step that contains the flaw, or pick “No flaw” if you think every step is correct.

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