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In this reaction taken from Snyder's total syntheses of Coccinellid alkaloids,1 how does the Boc deprotection lead to ring closure? I understand that the $\ce{PBr3}$ step simply converts the alcohol to a bromide.

TFA and PBr3 step in synthesis

  1. Sherwood, T. C.; Trotta, A. H.; Snyder, S. A. A strategy for complex dimer formation when biomimicry fails: total synthesis of ten coccinellid alkaloids. J. Am. Chem. Soc. 2014, 136 (27), 9743–9753. DOI: 10.1021/ja5045852.
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Those conditions just give the amine. One you have an amine and an aldehyde or ketone, you should form an imine or iminium ion. Imines are relatively stable, but secondary amines, such as the one in this question, can only form the iminium ion. But these iminium ions frequently generate enamines instead by tautomerization.

Brief outline of mechanism

You should consult your study materials on imine and iminium ion formation. The mechanisms there are very good to know in general because they reinforce the general reactivity pattern for carbonyl addition and elimination from the tetrahedral intermediate.

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