Asymmetric synthesis of complicated bicyclic and tricyclic polypropanoatesvia the double Diels-Alder addition of 2,2 '-ethyiidenebis[3,5-dimethylfuran]

A new, non-iterative method for the asymmetric synthesis of long-chain and polycyclic polypropanoate fragments starting from 2,2'-ethylidenebis[3,5-di methylfuran] (2) has been developed. Diethyl (2E,5E)-4-oxohepta-2,5-dienoat e (6) added to 2 to give a single meso-adduct 7 containing nine stereogenic centers. Its desymmetrization was realized by hydroboration with (+)-IpcBH (2) (isopinocampheylborane), leading to diethyl (1S,2R,3S,4S,4aS,7R,8R,8aR, 9aS,10R,10aR)-1,3,4,7,8,8a,9,9a-octahydro-3-hydroxy-2,4,5,7,10-pentamethyl- 9-oxo-2H,10H-2,4a : 7,10a-diepoxyanthracene-1,8-dicarboxylate ((+)-8; 78% e .e.). Alternatively, 7 was converted to meso-(1R,2R,4R,4aR,5S,7S,8S,8aR,9aS ,10s,10aS)-1,8-bis(acetoxymethyl)-1,8,8a,9a-tetrahydro-2,4,5,7,10-pentameth ly-2H-10H-2,4a :7,10a-diepoxyanthracene-3,6,9(4H,5H,7H)-trione (32) that wa s reduced enantioselectively by BH3 catalyzed by methyloxazaborolidine 19 d erived form L-diphenylprolinol giving (1S,2S,4S,4aS,5S,6R,7R,8R,8aS,9aR,10R ,10aS)-1,8-bis(acetoxymethyl)-1,8,8a,9a-tetrahydro-6-hydroxy-2,4,5,7,10-pen tamethyl-2H,10H-2,4a :7,10a-diepoxyanthracene-3,9(4H,7H)-dione ((-)-33; 90% e.e.). Chemistry was explored to carry out chemoselective 7-oxabicylo[2.2. 1]heptanone oxa-ring openings and intra-ring C-C bond cleavage. Polycclic p olypropanoates as (1R,2S,3R,4R,4aR,5S,6R,7S,8R,9R,10R,11S,12aR)-1-(ethoxy-c arbonyl)-1,3,4,7,8,9,10,11,12,12a-decahydro-3,11-dihydroxy-2,4,5,7,9-pentam ethyl-12-oxo-2H,5H-2,4a :6,9 :6,11-triepoxybenzocyclodecene-10,8-carbolacto ne (51), (1S,2R,3R,4R,4aS,5S,7S,8R,9R,10R,12S,12aS)-1,10-bis(acet-oxymethyl )tetradecahydro-8-(methoxymethoxy)-2,4,5,7,9-pentamethyl-3,9-bis[[2-(trimet hylsilyl)ethoxy]methoxy)-6,11-epoxycyclodecene-4a,6,11,12-tetrol((+)-83), a nd (1R,2R,3R,4aR,4bR,6S,6R,7R,8R,8aS,9S,10aR)-3,5-bis(acetoxymethyl)-4a,8a- dihydroxy-1-(methoxymethoxy)-2,6,8,9,10a-pentamethyl-2,7-bis([2-(trimethyls ilyl)-ethoxy]methoxy)dodecahydrophenanthrene-4,10-dione (85) were obtained in few synthetic steps.