This short review summarizes the syntheses of various types of chrom-3-ene
derivatives by hydration of acetylenic linkage and polyheterocycles such as
benzofuro [3,2-c]benzopyrans, benzofuro[3,2-b]benzofurans, benzofuro[2,3-b
]benzofurans and a number of other heterocyclic systems by [3,3] sigmatropi
c rearrangements from 1,4-dichlorobut-2-yne derivatives.