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20.6.1.4.4 Variation 4: The Steliou Method

DOI: 10.1055/sos-SD-020-01369

Maier, M. E.Science of Synthesis, (2007201460.

This macrocyclization is based on a tin-mediated template-driven esterification process.[‌170‌] Thus, with dibutyltin(IV) oxide the hydroxy acid reacts to give the ω-hydroxystannylenealkoxy carboxylic acid intermediate 132. It is believed that under high dilution (102 M) a double internal hydrogen bond leads to the formation of a cyclic alkoxystannylene carboxylate 133. Lactone formation is favored because of the nucleophilic tin alkoxide and the leaving ability of dibutyltin(IV) oxide (Scheme 57). Since the hydroxy acids may decompose under the required high temperature (165°C), stoichiometric amounts of dibutyltin(IV) oxide or more effective agents such as dibutyldiimidazolylstannane are used. However, this method does not compare well with other activation agents. If a chelating group is next to the carboxylic function, intramolecular chelation can interfere with the macrolactonization. A successful example is the 13-membered ricinelaidic lactone 134, which can be obtained in 44% yield (plus 17% dimer, 19% starting material, and polymer).

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