Interest in naturally occurring materials containing pyran rings continues with particular emphasis on the marine macrolides, the spongistatins that contain two spiroketal units and two tetrahydropyran moieties. Other marine products that have been synthesized include mycalamide B, pederin, phorboxazole A, and bryostatins. Total syntheses have been reported for the spiroketals okadaic acid, pinnatoxin, rutamycin, and palmarumycins, and for the polycyclic ether, hemibrevitoxin B. Syntheses of bis-spiroacetal units similar to that present in the salinomycins incorporate functionalization designed to assist in elaboration of the antibiotic itself. The synthesis of polyether frameworks can be accomplished by reiterative procedures, involving ring closing metathesis reactions and by a W(CO)5-promoted alkynol cyclization, and by a convergent approach utilizing a Pd-catalyzed Suzuki cross coupling reaction. Trisubstituted 2H-pyrans are formed via an intramolecular Wittig reaction when acetylene dicarboxylates and Ph3P are mixed with triformylmethane.
Hepworth, J. D., & Mark Heron, B. (1999). Chapter 6.4 Six-Membered Ring Systems: With O and/or S Atoms. Progress in Heterocyclic Chemistry, 11(C), 299-318. https://doi.org/10.1016/S0959-6380(99)80018-X