A new synthesis of spiroketals via sulfonyl-substituted dihydropyrans

6,6- and 6,5-Spiroketals were prepared in good overall yields by alkylation and epoxide-opening reactions of 3,4-dihydro-6-[(para-toluenesulfonyl)methyl]-2H-pyran (1) and subsequent acid catalysed cyclisation. The diastereoselectivity of the cyclisations was determined by a combination of 2D-NMR spectroscopy and X-ray crystallography. Attempted preparation of a 7,6-spiroketal using the same methodology was unsuccessful.