The crystal structure of RU60358, C20H21NO3, has been determined using X-ray diffraction to establish the configuration and stereochemistry of the molecule around the C15-C16 triple bond. The compound crystallises in the orthorhombic space group P2(1)2(1)2(1), a = 7.7575, b = 11.3182, c = 21.3529 angstrom, V = 1874.80 angstrom 3 and Z = 4. The structure has been refined to a final R = 0.068 for the observed structure factors with I >= 3 sigma ( I). The refined structure was found to be significantly non planar. A comparative study, using the ab initio calculations of the structure at B3LYP/6-31G** levels of theory, shows good geometrical agreement with the X-ray diffraction data. Standard deviations between the calculated and experimental bond values have been shown to be 0.01 angstrom and 0.5 degrees for bond angles. Vibrational wavenumbers were obtained from a normal mode analysis using the ab initio calculations.