Unexpected Prins cyclization: iron-promoted cyclization/hydration of alkynyl-dimethyl acetals

Iron Catalysis in Organic Synthesis

“…Prins cyclizations of alkynyl acetals with stoichiometric amounts of iron(III) chloride have been reported by Li, Xie and co-workers. , An iron-catalyzed variant has been presented by the groups of Li and Yu . They cyclized alkynyl acetals in the presence of catalytic amounts of iron(III) chloride hexahydrate or iron(III) bromide with concomitant halogenation to afford halobenzylidene-substituted five-membered carbo- and heterocycles (Scheme , eqs a and b).…”

2.03 Prins Reactions and Carbonyl, Imine, and Thiocarbonyl Ene Reactions

“…192 Intramolecular Prins reactions have also been explored with alkynes. [194][195][196] Stirring 350 in 60% sulfuric acid hydrolyzes the enol ether to give the trans-fused bicyclic ketone. The protonated ketone adds to the triple bond to give a vinylic cation that reacts with water as the nucleophile to form 7-hydroxylycopodine intermediate 351 in 48-70% yield.…”

Atom Economical, One-Pot, Three-Reaction Cascade to Novel Tricyclic 2,4-Dihydro-1H-benzo[f]isochromenes

“…Based on our previous work, we envisioned that a tricyclic 2,4,5,6-tetrahydro-1 H -benzo[ f ]isochromen-5-ol product might be accessible utilizing a cascade sequence involving an alkynyl-Prins reaction 17,18 and Friedel-Crafts cyclization 19 from an unsymmetrical alkynediol and an aldehyde (R-CHO) under Lewis acid mediated conditions (Scheme 2). …”