Multinuclear Magnetic Resonance (1H, 11B, 13C, 15N and 205Tl) Study of Thallium Hydridotris(3,5-dimethylpyrazol-1-yl)borate

✍ Scribed by Dionisia Sanz; Rosa María Claramunt; Julius Glaser; Sviatoslaw Trofimenko; Jose Elguero

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 333 KB
• Volume: 34
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199607)34:7<564::aid-omr918>3.0.co;2-g

✦ Synopsis

An even more pronounced effect is observed in the 13C NMR spectra of spiro steroids. Signals of axial carbon atoms [(C(aJ are shifted upfield by 1.2-6.6 ppm compared with the respective signal of equatorial carbons [(C(a,,)] of the same type (Tables 2 and3). The only exception is the pair of compounds 2n and 2b. In this case, severe steric compression is produced by the two methoxyl groups attached to the axial carbon atom in the strained four-membered ring of 2n. Carbon atoms in the B position [Fig. 1 : C(g.3 and C(8,,J] show a small shift difference, in most cases below 1 ppm, of no value for configurational assignments. Introduction of an additional ring of the spirane type into the steroid framework causes sigdcant signal shifts of the steroid ring A carbon atoms in a and B positions with respect to the spirane carbon atom C-3. The downfield shifts (Ah = hC,p,,o compou~d -6C5a-cho,esranJ for signals of the a, and a2 carbons (Fig. l), ranging from 4.5 (4) to 15.5 ppm (Sa, 8b and 9n) are observed. Smaller, yet persistent, upfield shifts, ranging from 1.6 (a) to 5.8 ppm (111 are characteristic of the and B2 carbons. For all spiro compounds, the methylene carbon (C-1) exhibits a smaller upfield shift than the methine carbon (C-5)" Carbon atoms in y and 6 positions exhibited spirane-induced shifts of CLI 1 ppm or smaller.