Total Syntheses of Borolithochromes A, D and G.

Total syntheses of borolithochromes A, D and G, red pigments isolated from fossils of Jurassic putative red alga Solenopora jurassica, have been achieved. The benzo[gh]tetraphene skeletons of the borate ligands in these substances were constructed using Diels-Alder reactions of aryl dienes with naphthoquinone, followed by intramolecular Corey-Chaykovsky reactions. Complexation of these ligands with trimethyl borate generated homocomplexes, which upon sequential O-demethylation produced borolithochromes A and G. In the route to borolithochrome D, a heterocomplex was prepared by stepwise complexation of the ligands with 2-(dimethylamino)ethyl dimethyl borate. The strategy devised to accomplish the first total synthesis of borolithochromes A, D and G should be applicable to the preparation of other borolithochromes as well as spiroborates possessing two fused polycyclic aromatic ligands.

Total Synthesis of Alcyonolide.

The total synthesis of alcyonolide, an antitumor xenicn diterpenoid, has been achieved. The inverse electron demand hetero-Diels-Alder reaction using a dienophile possessing an electron-withdrawing group provided the endo adduct which included a condensed lactone and dihydropyran rings with the desired three stereogenic centers. After introduction of the side chain by the Negishi coupling, the lactone ring was opened to form a Weinreb amide. The sequential transformation including conversion of Weinreb amide to aldehyde, PMB to acetate, and allylation of the aldehyde gave a mixture of separable four diastereomers. The desired stereoisomer was submitted to the 2,2,6,6-tetramethylpiperidine 1-oxyl oxidation, which afforded the δ-lactone and the methyl ketone side chain. Finally, the olefin metathesis of the desired isomer gave racemic alcyonolide.