1-Methyl-norbornene ester 9 and 1-methyl-2,3-diazabicyclo[2.2.1]heptene ester 10 were employed in intermolecular Pauson-Khand reactions with various terminal alkynes 11af to give the dimethyl 1-methyltricyclo[5.2.1.0 5,9 ]dec-7-en-6one 2,3-dicarboxylates 12 and 13, and diethyl 2,3-diaza-1methyltricyclo[5.2.1.0 5,9 ]dec-7-en-6-one 2,3-dicarboxylates 14 and 15, respectively. Whereas the co-cyclization of norbornene 9 with alkynes 11 bearing small substituents R resulted in the preferred formation of 12 (12:13 Յ 85:15), regioisomer 13 was obtained as the major product when sterically bulky alkynes were employed (12:13 Ն 6:94). For 2-methyl-3-bu-Transition-metal-catalysed co-cyclizations, in particular the Pauson-Khand reaction, can be considered as valuable tools in organic synthesis. The Pauson-Khand reaction, a cobalt-mediated co-cyclization of an alkyne, an alkene, and CO to give a cyclopentenone, has been elaborated in detail, and many applications have been reported, [1] especially in the field of natural product synthesis. It has also been found that other transition metals such as Fe, [2] Ru, [3] Rh, [4] Ni, [5] Cr, [6] Mo, [7] W, [8] Ti, [8] and Zr [9] can be used for the same purpose. According to the commonly accepted mechanism, [10] the co-cyclization is initiated by the formation of cobalt alkyne complex 1 with a tetrahedral Co 2 C 2 core (Scheme 1). Under thermal conditions, or in the presence of amine N-oxide promoters, [11] it is assumed that complex 1 undergoes decarbonylation at the basal carbon monoxide, which is orientated anti relative to R 1 , followed by coordination of an alkene to give alkene complexes 2a, and b. The regioselectivity with respect to the alkene is due to steric hindrance in the insertion step 2a Ǟ 3 versus 2b Ǟ 4. The less hindered face of the alkene is inserted into the less hindered Co-C bond. For alkenes with sufficiently large substituents R 2 , conformation 2b, and thus cyclopentenone 4, is preferred. However, with most alkenes mixtures of regioisomers 3 and 4 are obtained. The sequence is completed by the insertion of carbon monoxide and extrusion of