PHOTOINDUCED MOLECULAR-TRANSFORMATIONS .157. A NEW STEREOSELECTIVE AND REGIOSELECTIVE SYNTHESIS OF 2,6-DIARYL-3,7-DIOXABICYCLO[3.3.0]OCTANELIGNANS 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 gener al method devised by Suginome and colleagues for replacing the carbony l group of the cyclopentanone ring with an oxygen atom to give a corre sponding 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 a rylated 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 wa s reduced with DIBAL to give the corresponding lactol 23a-c. The irrad iation of a solution of the hypoiodite of 23a-c, generated in situ wit h mercury(II) oxide-iodine, in benzene with Pyrex-filtered light resul ted in a regioselective beta-scission of the corresponding alkoxyl rad ical to give iodo formate 24a-c. Heating 24a-c in MeOH with NaBH4 gave (+/-)-sesamin (25a), (+/-)-eudesmin (25b), or (+/-)-yangambin (25c).