A NEW METHOD OF GENERATION OF ALPHA-SELENOCARBENIUM IONS FROM SE,O-HETEROACETALS AND THEIR REACTIONS

Various Se,O-heteroacetals were prepared by the LiAlH4 reduction of di selenides 1 followed by alkylation with methoxymethyl chloride or (2-m ethoxyethoxy)methyl (MEM) chloride. Olefinic and acetylenic alpha-sele no carbenium ions were generated by the selective C-O bond cleavage of O-(2-methoxyethyl)-Se,O-heteroacetals with titanium(IV) chloride and cyclized to give the seleno heterocyclic compounds. Olefinic MEM-selen ides 3a,b,d-f,h underwent the endo-mode cyclization to afford 4-chloro selenacycloalkanes 4a,b,d-f,h in good yields, whereas acetylenic MEM-s elenides 12b-e,g-j underwent the exo-mode cyclization to give 3-(1-chl oroalkylidene)selenacycloalkanes 13b-e,g-j. This new utilization of al pha-seleno carbenium ions was also applied to the intramolecular and i ntermolecular Friedel-Crafts reactions.