Amplumthrin, a new dimeric 9,10-dihydrophenanthrene derivative, was isolated from the orchid Dendrobium amplum which also afforded the known 9,10-dihydrophenanthrene dimer flavanthrin and the monomeric stilbenoids gigantol, batatasin III, its 3'-O-methyl ether and 3,3'-O,O-dimethyl ether, 2,7-dihydroxy-3,4,6-trimethoxyphenanthrene and its 9,10-dihydro derivative, 2,3,7-trihydroxy-4,6-dimethoxyphenanthrene and its 9,10-dihydro derivative and coelonin. The structure of amplumthrin was established as 2,2',7,7'-tetrahydroxy-3,3',4,4',6,6'hexamethoxy-9,9',10,10'-tetrahydro-1,1'-biphenanthryl from various spectral and chemical evidence. The structure of amplumthrin was finally confirmed by regioselective biomimetic synthesis of the compound from its monomeric congener 2,7-dihydroxy-3,4,6-trimethoxy-9,10-dihydrophenanthrene by oxidative phenol-coupling reaction with CuCl(OH).TMEDA in very good yield (80%). Similar biomimetic synthesis of flavanthrin was also achieved in 82% yield from its monomeric congener coelonin using the same oxidant. The co-occurrence of amplumthrin and flavanthrin with their respective monomers in the same orchid Dendrobium amplum provides a strong circumstantial evidence in support of the proposed biogenesis of the naturally occurring biphenanthryl derivatives assumed to have arisen from their corresponding monomers through enzymatic oxidative phenol-coupling reaction.