Double Diastereoselection in Aldol Reactions Mediated by Dicyclohexylchloroborane between Chiral Aldehydes and a Chiral Ethyl Ketone Derived from l-Erythrulose. Synthesis of a C<sub>1</sub>−C<sub>9</sub> Fragment of the Structure of the Antifungal Metabolite Soraphen A<sub>1</sub><sub>α</sub>

Both matched and mismatched diastereoselections have been observed in the aldol reactions of a range of chiral aldehydes with the dicyclohexylboron enolate of a chiral ethyl ketone related to l-erythrulose. As was previously observed in the corresponding aldol reactions with l-erythrulose derivatives, the Felkin−Anh model provides an adequate explanation for the stereochemical outcome of reactions with chiral α-methyl aldehydes. However, a satisfactory account of the results observed with α-oxygenated aldehydes was only possible with the Cornforth model. As a practical application of the methodology described herein, a C<sub>1</sub>−C<sub>9</sub> fragment of the structure of the antifungal macrolide soraphen A<sub>1</sub><sub>α</sub> has been prepared in a convergent and stereoselective way.