Synthesis of Siphonazole B Through Domino Cycloisomerization‐Oxazolonium Ion Rearrangements

4-Alkenyl- and 4-acyloxazoles represent important substructures across a diverse array of bioactive molecules. Practical methods to synthesize these motifs are, therefore, of considerable interest. Here, we develop a novel domino process wherein a cycloisomerization is followed by a 1,2-rearrangement via an oxazolonium ion to yield 4-alkenyloxazoles from an abundant β-chloro-N-benzyl propargylamine and acyl chlorides. The synthetic utility of the method is highlighted by its application to both oxazole units of siphonazole B, leading to a concise convergent total synthesis of this natural product.