The Total Synthesis of Lauraceae Lactones: Obtusilactones, Litsenolides, and Mahubanolides

The total synthesis of obtusilactones (la,b, 2a,b), mahubanolides (lc, 2c), epilitsenolides (5b, 6b), and dihydromahubanolides (4c, 5c, 6c) is described. The enolates derived from the -phenylselenenyl esters (17a-c) were used as acrylate -anion synthons; aldol addition to propargylaldehyde, followed by oxidation to the selenoxide and elimination, furnished the isomeric acetylenic esters 20a-c and 21a-c. In a similar manner, aldol addition to acrolein followed by oxidation/elimination yielded olefinic esters 18b,c and 19b,c. The acetylenic esters were saponified, and the corresponding carboxylic acids were converted by either mercuric ion-catalyzed or bicarbonate-catalyzed lactonization to the obtusilactones (la,b, 2a,b) and mahubanolides (lc, 2c). The olefinic esters were saponified, and the corresponding carboxylic acids lactonized to 7-substituted lactones (30b,c, 32b,c, 32c) by treatment with either phenylselenenyl chloride or iodine. The epilitsenolides (5b, 6b) and dihydromahubanolides (5c, 6c) were then obtained by treating the substituted 7-lactones with tri-zi-butyltin hydride. Homogeneous catalytic hydrogenation (Rh(PPh3)3Cl) of isomahubanolide (2c) gave a mixture of dihydromahubanolides 4c and 6c.