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The diastereoselectivity of [3+2] cycloaddition of tricarbonyl[(4,5,6,7-η)-2-methyltropone]iron (2) with an E,Z isomeric mixture of η1-(crotyl)Fp (4E/4Z) [Fp: C5H5Fe(CO)2 or CpFe(CO)2] has been studied. By this cyclopentaanulation, four stereogenic centres are formed. The reaction occurs regioselectively and stereoselectively. The relative configurations of the cycloadducts 5a, 5b and 5c were assigned on the basis of 2D and NOE NMR experiments. A mechanism for the stereoselective [3+2] cycloaddition has been proposed. The selectivity of the cycloaddition depends on the difference in steric discrimination approaches of the η1-(crotyl)Fp 4E/4Z to the tricarbonyl[(3,4,5,6,7-η)-1-trimethylsilyloxy-2-methyltropylium]iron 3 (repulsion of the methyl and methylene groups of 4 and the planar skeleton of 3) and also upon the isomeric structure (E or Z) of the η1-(crotyl)Fp reactant. The geometrical isomerism of the reagent (4E/4Z) is preserved in the products 5a–c. [Copyright &y& Elsevier]