Pyridiniums derived from amino alcohols cyclise to ethers or rearrange to aldehydes on heating. Monopyridiniums from diamines can be acylated or converted into ureas or thioureas: these products cyclise on heating in solution to give dihydro-thiazoles,-4H- thiazines,-oxazoles,-4H-oxazines, or tetrahydro-3H-thiazepines.