Studies of heterocyclic compounds. Part 27. Routes to 1,6-dioxa-6aλ4-thia-2-azapentalenes, 1,6-dioxa-6aλ4-selena-2-azapentalenes, and 1-oxa-6aλ4-thia-2,5,6-triazapentalenes
Studies of heterocyclic compounds. Part 27. Routes to 1,6-dioxa-6aλ<sup>4</sup>-thia-2-azapentalenes, 1,6-dioxa-6aλ<sup>4</sup>-selena-2-azapentalenes, and 1-oxa-6aλ<sup>4</sup>-thia-2,5,6-triazapentalenes