in high to quantitative yields via the 1,5-CH insertion reaction of generated magnesium carbenoid intermediates. When this procedure was commenced with acyclic ketones, multi-substituted tetrahydrofurans were obtained in up to a 96% yield. This procedure provides a new and good way for the synthesis of 1-oxaspiro[4.n]alkanes and tetrahydrofurans with the formation of a carbon–carbon bond between a carbenoid