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The 1,3-dipolar cycloaddition reaction of C,N-diphenylnitrone with methacrolein is efficiently catalyzed by the rhodium diphosphine compound (S_Rh,R_C)-[(η⁵-C₅Me₅)Rh(R-Prophos)(H₂O)](SbF₆)₂ [R-Prophos = (R)-(+)-1,2-bis(diphenylphosphino)-propane, 1·SbF₆]; the asymmetric catalytic process occurs with reversal of regioselectivity, perfect endo selectivity, and up to 92% ee. The complete (NMR and X-ray analysis) characterization of the involved intermediate (S_Rh,R_C)-[(η⁵-C₅Me₅)Rh(R-Prophos)(methacrolein)](SbF₆)₂ (7·SbF₆) allows us to interpret the observed selectivity.