Solvent acquisition is always a pain. Some things pop out at me:
Eucalyptol: This is a naturally occurring cyclic ether, easily available from eucalyptus oil, which might hopefully have similar dissolving power to some other ethers although it is significantly less polar. Eucalyptol's main disadvantage is its high boiling point, which makes it difficult to remove, though unlike other ethers it doesn't generate peroxides on storage.
4-methyltetrahydropyran: Rose oxide is a naturally occurring tetrahydropyran in rose oil; alkene cleavage with KMnO4 and decarboxylation via distillation with NaOH gives the relevant compound. It's a THF-lookalike, which makes it interesting; the issue is I don't actually know how much rose oxide is present in synthetic or natural rose oils.
Oxetane: THF minus a methylene group, this is the lowest-boiling polar solvent I know of. Prepared from glycerol and oxalic acid to allyl formate, HBr / H2O2 to 3-bromopropyl formate, and NaOH gives oxetane. The first and last steps are well-characterized; the second is taught in every first-year orgchem course, and an analogous reaction proceeds in one hour with an 87% yield which qualifies as not half bad in my book. Oxetane is limited mainly by its flammability and a low bp, even moreso than diethyl ether, but otherwise looks quite attractive, in reactions where THF's properties make it one of the only suitable options (e.g. strong reducing conditions, organolithium reagents).
Eucalyptol: This is a naturally occurring cyclic ether, easily available from eucalyptus oil, which might hopefully have similar dissolving power to some other ethers although it is significantly less polar. Eucalyptol's main disadvantage is its high boiling point, which makes it difficult to remove, though unlike other ethers it doesn't generate peroxides on storage.
4-methyltetrahydropyran: Rose oxide is a naturally occurring tetrahydropyran in rose oil; alkene cleavage with KMnO4 and decarboxylation via distillation with NaOH gives the relevant compound. It's a THF-lookalike, which makes it interesting; the issue is I don't actually know how much rose oxide is present in synthetic or natural rose oils.
Oxetane: THF minus a methylene group, this is the lowest-boiling polar solvent I know of. Prepared from glycerol and oxalic acid to allyl formate, HBr / H2O2 to 3-bromopropyl formate, and NaOH gives oxetane. The first and last steps are well-characterized; the second is taught in every first-year orgchem course, and an analogous reaction proceeds in one hour with an 87% yield which qualifies as not half bad in my book. Oxetane is limited mainly by its flammability and a low bp, even moreso than diethyl ether, but otherwise looks quite attractive, in reactions where THF's properties make it one of the only suitable options (e.g. strong reducing conditions, organolithium reagents).