Efficient hydrochlorination of 2-ethynylpyridines was achieved without the use of special reagents. Ethynylpyridine readily reacts with hydrochloric acid to form a pyridinium salt. The salt formation considerably enhances the electrophilicity of the ethynyl group and attracts a chloride ion as the counteranion. The spatial proximity facilitates the nucleophilic addition of the halide anion to the ethynyl group, producing 2-(2-chloroethenyl)pyridine in high yields. This protocol could also be applied for hydrobromination and hydroiodination using hydrobromic and hydroiodic acids, respectively. In the case of acetic acid, the reaction did not proceed because of the low acidity and lack of salt formation. This problem was overcome by exchanging the counteranion using silver acetate; the resultant pyridinium acetate underwent hydroacetoxylation.

Dedicated to Ryoma on the occasion of his birthday.A new metal-free hydrohalogenation method for alkynes has been developed, which proceeds through a pseudo-intramolecular process. In this reaction, ethynylaniline serves as a substrate to quantitatively form an anilinium salt upon treatment with hydrochloric acid. The spatial proximity facilitates the efficient electrophilic addition of HCl to the ethynyl group in syn-mode, affording the corresponding chloroalkene without overaddition. This protocol was applied to HBr and HI, and the corresponding bromo-and iodoalkenes were obtained, respectively. The obtained chloroalkene was converted to tri-substituted alkenes possessing different aryl/alkynyl groups through Pd-catalyzed cross-coupling reactions.

An indolizine-based oligomer was synthesized by stirring a solution of 1-(α-alkoxybenzyl)indolizine derivative in chloroform at room temperature. The oligomer was characterized as an octamer; it was the main component in the obtained mixture of oligomers. The oligomerization was accelerated by both acid and light. The green color of the oligomer under acidic conditions turned yellowish brown under basic conditions; this color change could be repeated by changing the acidic/basic conditions.

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