Polyethers epoxide-opening reactions

Inspired by the postulated biogenesis of polyether compounds, a valuable cascade cyclization approach was developed on the predictable carbocation formation of the Nicholas reaction using very mild acidic conditions (silica gel as promoter) with excellent yields via very often inefficient epoxide opening cascade (13AGE3659). 

Terpene polyethers have also been proposed to arise through epoxide-opening cascade reactions. These structures are distinct from the other compound classes in this chapter because their carbon frameworks are not derived from polyketide synthase pathways. However, epoxide hydrolases are commonly encountered in terpene biosynthetic pathways [18], indicating that epoxides are viable intermediates in the construction of these compounds. 

Propylene oxide and other epoxides polymerize by ring opening to form polyether stmctures. Either the methine, CH—O, or the methylene, CH2—O, bonds ate broken in this reaction. If the epoxide is unsymmetrical (as is PO) then three regioisomers are possible head-to-tad (H—T), head-to-head (H—H), and tad-to-tad (T—T) dyads, ie, two monomer units shown as a sequence. The anionic and... 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

Opening of epoxides has also been employed in the preparation of tetrahydrofuran frameworks <05OL2945 05OL4867 05SL1630>. The reaction illustrated below is a relevant example <05JA5806>. Moreover, electrophiles generated in an oxidative manner have been shown to serve as initiators in epoxide cascade cyclization routes to form polyethers <05TA3570>. 

In the presence of a photogenerated protonic acid, the ring-opening polymerization of epoxides proceeds efficiently through the chain carrier 0x0-nium ion, to ultimately produce a polyether according to the following reaction scheme ... 

---