A stereoselective and novel method for construction of the spiroketal moiety is described, as illustrated by a synthesis of (2R*, 5R*)- and (2R*, 5S*)-2-methyl-1, 6-dioxaspiro[4.5]decane (2A and 2B).The sulfide (7), prepared from a lactone (4), was oxidized with sodium metaperiodate followed by treatment with an acid to provide the two sulfoxide products (3A and 3B) in a 1 : 1 ratio. Acid-catalyzed cyclization of both compounds gave the corresponding dioxaspiro derivatives (9 and 10) without significant stereoselectivity, while their intramolecular bromohydrin formation reactions exhibited moderate stereoselection. Upon treatment with N-bromosuccinimide (NBS) followed by reductive operations, 3A gave 9a and 3B afforded 10d as major products. On the other hand, base-catalyzed intramolecular Michael reactions of 3A and 3B yielded 9b and 10c with extremely high stereoselectivity. These two dioxaspiro products were found to isomerize to the corresponding more stable isomers 9d and 10a on acidic treatment. Desulfurization of 9b or 10a afforded 2A and that of 9d or 10c gave 2B without any isomerization at the spiro center.