General Papers ARKIVOC 2013 (iv) 107-125 Unambiguous assignment of 13C NMR signals in epimeric 4,5-epoxy3- oxo-steroids assisted by X-ray diffraction and gauge invariant atomic orbitals calculation of absolute isotropic shieldings Pablo Labra-Vázquez,a Annia Galano, b Margarita Romero-Ávila,a Marcos Flores-Álamo a and Martín A. Iglesias-Arteagaa * a Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México, D.F., México b Departamento de Química, División de Ciencias Básicas e Ingeniería, Universidad Autónoma Metropolitana Iztapalapa, 09340 México D.F., México E-mail: martin.iglesias@unam.mx Abstract Complete assignments of the 13C signals of diastereomeric 4,5-epoxy-3-oxo steroids based on a combination of 1D and 2D NMR techniques are described The assignments were corroborated or corrected by calculation of the absolute isotropic 13C NMR shieldings using the Gauge Invariant Atomic Orbitals (GIAO) method at B3LYP/6-31+G(d,p) level. 13C Keywords: 4,5-Epoxy-3-oxo-steroids, NMR, Absolute isotropic NMR shielding calculations, X-ray structure Introduction Steroids play different roles in living organisms from both animal and vegetal kingdoms. In a wide variety of steroids, the coexistence of different functionality in the steroidal nucleus confers several properties that are interesting from both the biological and the synthetic points of view. In particular, steroids containing the 4,5-epoxy-3-oxo moiety are of special utility as synthetic precursors of a wide variety of polyfunctional derivatives or rearranged compounds.1-5 4,5-Epoxy-3-oxo-steroids can be prepared in moderate to good yield and varying diastereoselectivity by treatment with different reagents that include peracids,4 H2O2 in alkaline media,5 dioxiranes6 and more recently magnesium bis(monoperoxyphthalate) hexahydrate.7 In spite that such compounds have been extensively employed as synthetic precursors for different polyfunctional or rearranged steroids, no complete and unambiguous assignment of the 13C NMR signals of such compounds is available. This may obey to the fact that in most of the synthetic Page 107 ©ARKAT-USA, Inc
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