New chemical constituents of Euphorbia quinquecostata and absolute configuration assignment by a convenient Mosher ester procedure carried out in NMR tubes - Su, B N; Park, E J; Mbwambo, Z H; Santarsiero, B D; Mesecar, A D; Fong, H HS; Pezzuto, J M; Kinghorn, A D (J Nat Prod, 2002)