(2R,4S,4bS,9aR,10aR)-4-[(E)-3-(tert-Butyl-dimethyl-silanyloxy)-propenyl]-5-isopropyl-1,3,4,9,9a,10a-hexahydro-2H,4bH-10-oxa-4a-aza-indeno[1,2-a]indene-2-carboxylic acid ethyl ester