Include cis/trans isomers. There should be about 15 or so. The parent isomers are pyran, furan, oxytane, and epoxide.
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Six-membered ring: pyran, or tetrahydropyran
Five-membered ring: 2-methyltetrahydrofuran, 3-methyltetrahydrofuran
Four-membered ring: 2,2-dimethyloxetane, cis- and trans-2,3-dimethyloxetane, cis- and trans-2,4-dimethyloxetane, 3,3-dimethyloxetane, 2-ethyloxetane, 3-ethyloxetane
Three-membered ring: n-propylepoxide, i-propylepoxide, 2-ethyl-2-methylepoxide, cis- and trans-2-ethyl-3-methylepoxide, 2,2,3-trimethylepoxide
17 total.
Five-membered ring: 2-methyltetrahydrofuran, 3-methyltetrahydrofuran
Four-membered ring: 2,2-dimethyloxetane, cis- and trans-2,3-dimethyloxetane, cis- and trans-2,4-dimethyloxetane, 3,3-dimethyloxetane, 2-ethyloxetane, 3-ethyloxetane
Three-membered ring: n-propylepoxide, i-propylepoxide, 2-ethyl-2-methylepoxide, cis- and trans-2-ethyl-3-methylepoxide, 2,2,3-trimethylepoxide
17 total.