NMR and quantum‐chemical study of the stereochemistry of spiroepoxides obtained by oxidation of (Z)‐3‐arylidene‐1‐thioflavan‐4‐ones

Abstract

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.

Dimethyldioxirane oxidation of the epoxides of (Z)‐3‐arylidene‐1‐thioflavanones

“…In summary, it can be concluded that, as results of our previous [24][25][26] and present investigations, we managed to oxidize both the carbon-carbon double bond and the sulfur atom of the (Z)-3-arylidene-1-thioflavanones by using either nucleophilic or electrophilic oxidants. All oxidized derivatives of the (Z)-3-arylidene-1-thioflavanones are now easily accessible.…”

“…Recently we have reported the stereochemistry and 1 H and 13 C nmr data of the starting trans,cis-and trans,trans-2,3'-diarylspiro[2H-1-benzothiopyran-3(4H),2'-oxiran]-4-ones 1 [26]. Comparison of the corresponding nmr data of the starting materials 1 with the sulfoxides 2 and sulfones 3 (δH-3' = 4.70-5.12, δC-3' = 58.9-67.9 and 1 J C-3',H-3' ≈ 181-182 Hz) unequivocally proves that the epoxide moiety has remained intact on the dimethyldioxirane oxidation of compounds 1.…”

“…Moreover, in the case of the sulfoxidation of the 2-substituted 1,5-benzothiazepine derivatives a high trans diastereoselectivity was observed. All these results prompted us to study the dimethyldioxirane oxidation of the (Z)-3-arylidene-1-thioflavanone epoxides described in our previous papers [25,26]. It should also be mentioned that no example has hitherto been published concerning the oxidation of the sulfur atom of 3-arylidene-1-thioflavanone epoxides.…”

“…Thus, these electrophilic oxidants oxidize only the sulfur atom of the 3-arylidene-1-thioflavanones in a chemoselective reaction which may be a consequence of the strongly electron deficient character of the carbon-carbon double bond. For this reason, the nucleophilic oxidants alkaline hydrogen peroxide and sodium hypochlorite proved to be the reagents of choice for the epoxidation of 3-arylidene-1-thioflavanones [17,25,26]. Although the use of these oxidants afforded a diastereomeric mixture of the trans,cis-and trans,trans-epoxides, the two diastereomeric epoxides could easily be separated by silica gel column chromatography.…”

“…Their (E)-isomers, where the aryl and carbonyl groups are on the same side of the carbon-carbon double bond were prepared by the photoisomerization of the appropriate (Z)-3-arylidene-1-thioflavanones [19,20]. Previously, we have studied several chemical transformations of the 3-arylidene-1-thioflavanones including their oxidation with dioxiranes [10,[21][22][23][24][25][26].It has been found that, depending on the amount of the oxidant, dimethyldioxirane oxidation of (Z)-3-arylidene-1-thioflavanones afforded sulfoxides and/or sulfones which could not be epoxidized to their corresponding epoxides either with a high excess of dimethyldioxirane or with the much more effective methyl(trifluoromethyl)dioxirane [24]. 3-Arylidene-1-thioflavanones gave sulfones with mchloroperbenzoic acid as another electrophilic oxidant [17].…”