Reversed selectivity: 1,3-Dioxan-5-ol templated epoxy alcohols undergo a remarkably selective cyclization, which occurs with near complete endo selectivity that is opposite to that predicted by Baldwin’s rules. This endo preference in water near pH 7.0 is an order of magnitude larger relative to that of a previously disclosed THP-templated cyclization (see scheme), suggesting that “templates” may be important in the biosynthesis of marine ladder polyethers