MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2001; 39: 251–258 NMR and quantum-chemical study of the stereochemistry of spiroepoxides obtained by oxidation of (Z)-3-arylidene-1-thioflavan-4-ones † G´ abor T ´ oth, 1* J´ ozsef Kov ´ acs, 1 Albert L ´ evai, 2 Erich Kleinpeter 3 and Andreas Koch 3 1 Technical Analytical Research Group of the Hungarian Academy of Sciences, Institute for General and Analytical Chemistry, Technical University of Budapest, Szent Gell ´ ert t ´ er 4, H-1111 Budapest, Hungary 2 Department of Organic Chemistry, Debrecen University, P.O. Box 20, H-4010 Debrecen, Hungary 3 Universit ¨ at Potsdam, Institut f ¨ ur Organische Chemie und Strukturanalytik, P.O. Box 691553, D-14415 Potsdam, Germany Received 29 September 2000; Revised 19 January 2001; Accepted 22 January 2001 Epoxidation of (Z)-3-arylidene-1-thioflavan-4-ones (1) yielded trans,cis (2) and trans,trans (3) isomers. The structure and signal assignments were elucidated by extensive application of one- and two-dimensional 1 H and 13 C NMR spectroscopy. The conformational analysis was achieved by the application of 3 J(C,H) couplings and ab initio MO calculations. Both the preferred ground-state conformers (envelope-A conformations) obtained as global minima of the HF ab initio structures and the 13 C chemical shifts calculated by the GIAO method from the global minima structures of the trans,cis and trans,trans isomers are in agreement with the experimentally obtained NMR results. Copyright  2001 John Wiley & Sons, Ltd. KEYWORDS: NMR; 1 H NMR; 13 C NMR; spiroepoxides, long-range 3 J(C,H) couplings, conformational analysis; ab initio MO study INTRODUCTION We have recently reported on the stereoselective epoxidation of exocyclic ˛,ˇ-unsaturated ketones including 2-arylidene- 1-tetralones, 1 -1-indanones 2,3 and -1-benzosuberones, 2,3 3- arylidenechromanones 4 and 3-arylideneflavanones 5 by di- methyldioxirane. Unfortunately, this convenient oxidizing agent proved to be inadequate for the epoxidation of 3- arylidene-1-thioflavan-4-ones; only sulfoxides or sulfones were obtained depending on the amount of the oxidant, instead of epoxides. 6 For this reason, we have now con- ducted the epoxidation of (Z)-3-arylidene-1-thioflavan-4- ones using nucleophilic oxidants, viz. alkaline hydrogen per- oxide (Weitz–Scheffer reaction) and sodium hypochlorite. In principle, these procedures may provide four diastereomeric spiroepoxides, namely the trans,cis (2), trans,trans (3), cis,cis (4) and cis,trans (5) isomers. The first prefix refers to the rela- tive position of the carbonyl and the aryl groups of the epox- ide ring, and the second prefix describes the relative position of the phenyl group at C-2 and the epoxide oxygen connected to the C-3 atom. All of these compounds are racemates. Ł Correspondence to: G. T ´ oth, Technical and Analytical Research Group of the Hungarian Academy of Sciences, Institute for General and Analytical Chemistry, Technical University of Budapest, Szent Gell´ ert t´ er 4, H-1111 Budapest, Hungary. E-mail: g-toth@ tki.aak.bme.hu † This paper is dedicated to Prof. Dr L´ aszl ´ o-Szil´ agyi on the occasion of his 60th birthday. Contract/grant sponsor: Hungarian Scientific Research Fund; Contract/grant number: OTKA T026264; OTKA T029171. RESULTS AND DISCUSSION The oxidation reactions of the (Z)-3-arylidene-1-thioflavan- 4-ones (1) always yielded two diastereomers (2 and 3) in ratios of ca 3 : 2 (Scheme 1). The separation of diastereomers was achieved by column chromatography. 7 Beirne and O’Sullivan also obtained a mixture of two isomers from (Z)-3-benzyliden-6-methyl-1-thioflavan-4-ones on treatment with alkaline hydrogen peroxide or sodium hypochlorite. 8 Complete 1 H and 13 C NMR signal assignments were achieved using gs-COSY, gs-HSQC and gs-HMBC measure- ments. The chemical shifts of 2 and 3 are summarized in Tables 1 and 2. The 13 C-coupled HSQC measurement not only proved to be very powerful for the assignment of the direct coupled carbon– proton pairs but also allowed the dif- ferentiation between the methine groups in positions 2 and 3 0 . It is well known 9,10 that the value of the one-bond CH cou- pling constant of epoxides is extremely high, around 180 Hz. The cross peaks at 5.3/64 and 4.4/65 ppm for 2 and 3 show a 1 J(C,H) coupling of 180–183 Hz, unambiguously proving the existence of the oxirane ring and allowing differentiation of the C-2 and C-3 0 methine groups. For the evaluation of the interprotonic proximities, we measured selective 1D NOESY and phase-sensitive 2D NOESY spectra. These revealed that H-2 and the aryl group at C-3 0 are in spatial proximity in the products 2 and 3, whereas no NOE responses were observed among H-3 0 /H-2 and H-3 0 /H-2 00 ,6 00 protons. These observations support the trans,cis and trans,trans type of structures of the products DOI: 10.1002/mrc.840 Copyright  2001 John Wiley & Sons, Ltd.