(2aS,4R,8bR)-1-Benzyl-5-benzyloxy-4-benzyloxymethyl-6,8-dimethoxy-7-methyl-2-oxo-2,2a,4,8b-tetrahydro-1H-1,3-diaza-cyclobuta[a]naphthalene-3-carboxylic acid tert-butyl ester