Dedicated to Albert Eschenmoser on the occasion of his 70th birthday (12.VI.95)A concept of first choice for the synthesis of the title compounds had been proposed by Dane in the late 1930s. It was soon turned down, because the opening move -a chirogenic Diels-Alder reaction -did not work. With Lewis acids as mediators, however, a successful start has been achieved now. With Ti complexes of chelating ligands (Seebach 's TADDOLs ( = a,a,a',a'-tetraaryl-1,3-dioxolane-4,5-dimethanols)), enantioselective formation of the desired adducts does occur. Efficient total syntheses of 2 and 3a have been accomplished.

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