Synthetic and biosynthetic methods for selective cyclisations of 4,5-epoxy alcohols to tetrahydropyrans

In this review, methods for the selective intramolecular epoxide ring opening (IERO) of 4,5-epoxy-alcohols are discussed as well as biosynthetic pathways to tetrahydropyran-containing natural products which utilise IERO reactions.

Influence of the Substituents on the Opening of Silylepoxy Alcohols: 5-exo-Cyclization towards Tetrahydrofurans vs. Unexpected Side Reaction Leading to Tetrahydropyrans

The regioselective ring opening of epoxy alcohols is an effective method for the synthesis of different types of oxacycles. The 5-exo opening being preferred vs. the 6-endo mode, according to Baldwin rules, the use of silyl-substituted oxiranes has been reported as a possible method to favor the 6-endo cyclization. However, there is a need for a detailed study on the different factors (structural factors, catalyst nature or conditions) that influence this process. In this paper, the acid-catalyzed cyclization of epoxysilyl alcohols was studied, focusing on the effect of substituents and reaction conditions on the outcome of the process. Two types of heterocycles (tetrahydrofurans or tetrahydropyrans) were selectively obtained depending on the structure of the initial epoxysilyl alcohol. Interestingly, cyclization of hindered epoxysilyl alcohols mainly proceeds through an unexpected side reaction, which implies a previous isomerization to an aldehyde. A mechanistic proposal for the formation of the different products is presented.

Synthesis of Resolvin D6 and the Silyl Ether of the Resolvin E2 Methyl Ester via trans-Enynyl Alcohols

Two trans-enynyl alcohol intermediates corresponding to the C1–C8 and C13–C22 parts of resolvin D6 (RvD6) were prepared through the Hudrlik–Peterson reaction of the TMS-substituted trans-epoxy alcohols with TMS-acetylide and subsequent TMS-desilylation. These intermediates were coupled with a 1,4-dihalo-2-butyne derivative under copper catalysis, and the resulting acetylene was reduced with Zn(Cu/Ag) to afford the TBS ether of RvD6 methyl ester. Desilylation with TBAF yielded the γ-lactone of RvD6, which was hydrolyzed to RvD6. The total yield of RvD6 was 1.9% in 19 steps from (3-trimethylsilyl)propargyl alcohol. The TBS ether of RvE2 methyl ester was also synthesized.