Prostaglandins Dieckmann reaction

Problem 24.20 One synthesis of prostaglandin PGAj involved a Dieckmann reaction as a key step. What is the structure of X in the reactions drawn below ... 

In our last look at prostaglandins, we can see this put to use in an approach to some prostaglandin derivatives. The plan was to construct PG derivatives from an intermediate of type 216. This j8-ketoester was to be derived from a Dieckmann condensation (construction of the C9-C10 bond). The Dieckmann precursor was to be derived from the reaction of malonate 215 with geminally activated cyclopropane 214. Note that control of absolute stereochemistry in this route leads back to 209, which was resolved as its brucine salt. It is also notable that 214 reacts selectively with malonate 215 at the more hindered secondary (but allylic) carbon of the cyclopropane with clean inversion of configuration. 

When the Claisen ester condensation is intramolecular, it goes by the name of the Dieckmann condensation. As with the intramolecular aldol reactions, only processes that give rise to five- or six-membered rings are favorable (Figure 17.33). This process has been used in the total synthesis of prostaglandin A2 (Figure 17.34). 

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