Stereoselective Synthesis of Dihydropyrone-Containing Marine Natural Products. Total Synthesis and Structural Elucidation of (−)-Membrenone-C

[figure: see text] Three diastereomers of membrenone-C were separately prepared using a common two directional chain extending synthetic strategy. This has established the absolute and relative configuration of the natural product to be as shown in the foregoing graphic. Key steps in the synthesis of all the isomers are a stereoselective aldol coupling and reduction giving the C7-C9 stereocenters, a two direction chain extending double titanium aldol coupling, and the trifluoroacetic acid promoted double cyclization/dehydration giving the two dihydropyrone rings.