Studies on electrophilic interaction of 2,3-allenols with electrophilic halogen reagents: Selective synthesis of 2,5-dihydrofurans, 3-halo-3-alkenals, or 2-halo-2-alkenyl ketones

The reaction of primary 2,3-allenols with iodine (I-2) afforded 2,5-dihydrofurans while that of readily available 1-aryl or 1-methyl substituted 2,3-allenols with bromine (Br-2), N-bromosuccinimide (NBS), I-2 or N-iodosuccinimide (NIS) formed the not easily available but synthetically useful 3-halo-3-alkenals and 2-halo-2-alkenyl ketones with good selectivity and yields via a sequential electrophilic interaction of X+ with the allene moiety, 1,2-aryl or 1,2-proton shift, and H+ elimination process.