Stereoselective synthesis of 5a-ethyl-1,2,3,3a,4,5,5a,6,9a,9b-decahydro-1,3,4-trihydroxy-3a-(hydroxym ethyl)-7H-benz[e]inden-7-one derivatives

Homochiral Diels-Alder cyclodimerization of (+/-)-6-ethenyl-7-oxabicyclo[2.2.1]hept-5-en-2-endo-ol followed by oxidation gives (1RS,4RS,4aSR,4bSR,5RS,8RS,8aRS)-8a-ethenyl-1,3,4a,4b,5,6,8,8a,9-decahyd ro-11,4:5,8-diepoxyphenanthrene-2,7-dione (18). Selective hydrogenation followed by epoxidation produced (1RS,4RS,4aRS,5aRS,6aRS,7RS,10RS,10aSR,10bRS)-6a-ethyl-1,4,5a,6,6a,7,9,1 0,10a,10b-decahydro-1,4:7,10-di-epoxyphenanthro[8a,9b]oxirene-3,8-dione (21), which was solvolyzed (Me3SiOSO2CF3, Piv2O) with concomitant pinacol rearrangement involving an acyl-group migration to give a 6-oxo-7-oxabicyclo[2.2.1]hept-2-yl cation intermediate, which finally generated (1RS,3SR,3aRS,4SR,5aRS,6RS,9RS,9aSR,9bSR)-5a-ethyl-1,4,5,5a,6, 7,8,9,9a,9b-decahydro-7,10-dioxo-3H-6,9-epoxy-1,3a-ethanonaphtho[1,2-c]f uran-3.4-diyl bis(2,2-dimethylpropa noate) (24). Photo-reductive 7-oxa bridge opening of 24, followed by water elimination and silylation, provided (1RS,3SR,3aRS,4SR,5aSR,9aSR,9bSR)-7-{[(tert-butyl)dimethylsilyl]oxy}-5a- ethyl-1.4,5,5a,9a,9b-hexahydro-10-oxo-3H- 1,3-ethanonaphtho[1,2-c]furan-3,4-diyl bis(2,2-dimethylpropanoate) (34). Reduction of 34 with NaBH4 in MeOH followed by desilylation and alcohol protection produced (1RS,3RS,3aRS,4SR,5aSR,9aSR, 9bSR)-5a-ethyl-2,3,3a,4,5,5a,6,7,9a,9b-decahydro-1,3-bis(methoxymethoxy) -3a-[(methoxymethoxy)methyl]-7-oxo-1H-benz[e] inden-4-yl 2,2-dimethylpropanoate (5), a polyoxy-substituted decahydro-1H-benz[e]indene derivative with cis-transoid-trans junction for the two cyclohexane and the cyclopentane rings bearing an angular 3a-(oxymethyl) substituent.