PHOTOINDUCED MOLECULAR-TRANSFORMATIONS .157. A NEW STEREOSELECTIVE AND REGIOSELECTIVE SYNTHESIS OF 2,6-DIARYL-3,7-DIOXABICYCLO[3.3.0]OCTANE LIGNANS INVOLVING A BETA-SCISSION OF ALKOXYL RADICALS AS THE KEY STEP - NEW TOTAL SYNTHESES OF (+/-)-SESAMIN, (+/-)-EUDESMIN, AND (+/-)-YANGAMBIN

New total syntheses of naturally occurring 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane lignans, (+/-)-sesamin and (+/-)-eudesmin, and the first total synthesis of (+/-)-yangambin were achieved according to a general method devised by Suginome and colleagues for replacing the carbonyl group of the cyclopentanone ring with an oxygen atom to give a corresponding tetrahydrofuran ring involving a regioselective beta-scission of alkoxyl radicals; arylation of dimethyl, diallyl, or dibenzyl 3,7-dioxobicyclo[3.3.0]octane-2,6-dicarbonate (18 and 19a,b) with aryllead triacetate 9a-c, followed by dealkoxycarbonylation of the resulting arylated product 20a-f, gave 2,6-diaryl-3,7-dioxobicyclo-[3.3.0]octane 21a-c. A regioselective Baeyer-Villiger oxidation of 21a-e with m-CPBA-NaHCO3 or -K2CO3 gave the corresponding delta-lactone 22a-c, which was reduced with DIBAL to give the corresponding lactol 23a-c. The irradiation of a solution of the hypoiodite of 23a-c, generated in situ with mercury(II) oxide-iodine, in benzene with Pyrex-filtered light resulted in a regioselective beta-scission of the corresponding alkoxyl radical to give iodo formate 24a-c. Heating 24a-c in MeOH with NaBH4 gave (+/-)-sesamin (25a), (+/-)-eudesmin (25b), or (+/-)-yangambin (25c).