Hemiacetals, cyclic from lactones

A stable tetrahedral intermediate is more likely in the reduction of lactones, and DIBAL is most reliable in the reduction of lactones to lactols (cyclic hemiacetals), as in E.J. Corey s synthesis of the prostaglandins. The key step, the hydride transfer from Al, is shown in the green frame. 

Large cyclic hemiacetals are also formed from the intramolecular alkylation of ketal radicals of lactones, as demonstrated by Eq. 56 [84]. 

A more complex oligomer is obtained by treatment of butanal with KOH or NaOH in aqueous ethanol. This reaction has been shown to produce the tetramers (3)-(5) and trimer (6 equation 14), some in respectable yield, depending on the exact reaction conditions. Compound (3) is produced as a mixture of four diastereomers and probably arises from dimerization of the initial aldol condensation product, 2-ethylhexenal. Diol (4), also obtained as a mixture of four stereoisomers, appears to be a Cannizarro reduction product of (3). Lactones (5) and (6) can reasonably arise by mechanisms involving hydride transfer from a cyclic hemiacetal. 

A stable tetrahedral Intermediate Is more likely In the reduction of lactones, for the same reasons that cyclic hemiacetals are more stable than acyclic ones. DIBAL Is most reliable In the reduction of lactones to cyclic hemiacetals (also known as lactols), as In this reaction from E. J. Corey s synthesis of the prostaglandins. 

In the first case, it is ejected by the 3-ketoacid decarboxylation of 414, whereas in the second example the hemiacetal 416c, derived from the cyclic ether 416a via the corresponding lactone 416b, is split in a radical reaction to provide iodide 418. Immediate dehalogenation of this unstable material with tributyl tinhydride leads to the tertiary alcohol 417 [140]. 

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