Ether formation cyclodehydration

We have systematically examined the facility with which DTPP promotes the cyclodehydration of simple diols to cyclic ethers 1,3-propanediol (1) - oxetane (2) (2-5%) 1,4-butanediol (3) te-trahydrofuran (4) (85%) 1,5-pentanediol (5) - tetrahydropyran (6) (72%) 1,6-hexanediol (7) - oxepane (8) (55-68%). Increased alkyl substitution at the carbinol carbon s gnificantly diminishes the facility for cyclic ether formation. For example, a mixture of meso- and d, 1 —2, 6-heptanediol gave only 6-10% of the cis- and trans-2,6-dimethyltetrahydropyrans when treated with DTPP. While diol 1 resists cyclodehydration with DTPP to oxetane, some 2,2-di-substituted 1,3-propanediols are readily converted to the appropriate oxetanes [e.g., 2-ethyl-2-phenyl-l,3-propanediol -> 3-ethyl-3-phenyloxetane (78%)]. 

In this subsection, two types of cyclodehydrations will be discussed, namely the formation of cyclic Schiff bases, and the formation of cyclic enol ethers and cyclic enamines. 

Cyclodehydration. The reaction of this reagent with 1,4-diols results mainly in cyclodehydration to tetrahydrofuranes. Extent of formation of cyclic ethers decreases in order of the ring sizes 5 > 6 > 4 = 7. The reaction with 1,2-diols or 1,3-diols results in chlorohydrins as the major products. If the reaction is conducted in the presence of solid K,CO, in CCI4 (reflux), epoxides become the major products formed from 1,2-diols. 

In contrast, operating in the presence of palladium catalysts at 250°C under 80 bar hydrogen pressure cyclodehydration reactions of sorbitol and mannitol occurred with formation of cyclic ethers (isosorbide 5, 2,5-anhydromannitol, 2,5-anhydroiditol, and 1,4-anhydrosorbitol) (6), Up to 50% and 90% yield of isosorbide were obtained from sorbitol and mannitol, respectively. These... 

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