Steerochemistry of heterocycles.

Conformational analysis of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes

“…The value of ΔG 0 of the chair-chair inversion of dithiane I coincides with the experimentally observed for 2,5-dimethyl-1,3-dithiane [3]. For the dithiane II (ketal) they are naturally reduced.…”

“…This is due to the above noted inadequate accounting for the stereoelectronic interactions in the 1,3-dithiane ring, and the influence of the environment. The data on the configurational isomerization were obtained for solutions in CHCl 3 [2,3,5], while NMR spectra were recorded for solutions in CCl 4 , C 6 H 6 , and C 6 H 5 NO 2 [2,3]. We suggest that a good agreement of the method of spinspin coupling and the data on the configurational isomerization evidences the adequacy in general of the calculated and actual geometric characteristics of molecules I and II.…”

“…for the alkyl group at the C 5 atom in 1,3-dithianes [2,3,6]. A possible reason for the large difference in energy of the C e and C a is the existence of notable stereoelectronic interactions in 1,3-dithiane ring, causing, in particular, the lengthening of the equatorial C 5 -H bond [26].…”

“…Earlier using 1 H and 13 C NMR spectroscopy it has been shown that the main minimum on the potential energy surface (PES) of 5-alkyl-1,3-dithianes is the equatorial chair conformer (C e ) [2,3,[5][6][7][8][9]. Computer simulation applied to the conformational analysis of unsubstituted 1,3-dithiane has revealed the main conformer, the chair (C), and the local minima, 1,4-twist (1,4-T) and 2,5-twist (2,5-T), as well as transition states (TS): the semi-chair, symmetric boat and asymmetric boat conformations [20,21].…”

“…Based on the experimental ( 1 H NMR) and theoretical vicinal coupling constants the ΔG 0 values were determined of the methyl group at the ring C 5 atom. The reasons for the distinctions between the values of this parameter from the ΔG 0 , obtained by energy minimizations of the equatorial and axial chair conformers are discussed.The interest in substituted 1,3-dithianes are due both to the peculiarities of their structure [1][2][3][4][5][6][7][8][9][10][11] and to their use as reagents in fine organic synthesis [12][13][14][15][16][17][18][19]. Earlier using 1 H and 13 C NMR spectroscopy it has been shown that the main minimum on the potential energy surface (PES) of 5-alkyl-1,3-dithianes is the equatorial chair conformer (C e ) [2, 3, 5-9].…”