Rhodium-mediated cyclization of the alpha-diazo ester 1 proceeds with high diastereoselectivity, to give the trisubstituted cyclopentane 5. A computational model (ZINDO and Molecular Mechanics) based on our current mechanistic understanding of the reaction is presented. This model correctly predicts the dominant diastereomer from the cyclization of 1 and the eight (Table 2) other alpha-diazo esters studied thus far.