Stereoisomere Aromastoffe, XVI. Struktur und Eigenschaften optisch reiner 2‐Methyl‐4‐propyl‐1,3‐oxathian‐3‐oxide

Abstract

Durch Oxidation von cis‐2‐Methyl‐4‐propyl‐1,3‐oxathian (1) mit NaIO4 entstehen diastereomere Sulfoxide (äquatorial/axial‐Verhältnis 3:1), die flüssigkeitschromatographisch rein erhalten werden. Aus optisch reinem (2S,4R)‐1 werden die Sulfoxide 2 (75%) und 3 (25%), aus (2R,4S)‐1 die Sulfoxide 2′ (75%) und 3′ (25%) erhalten. Für 3 wurde eine Röntgenstrukturanalyse durchgeführt.

Simultaneous gauche and Anomeric Effects in α-Substituted Sulfoxides

“…1,3-Oxathiane 3-oxide (1, Figure 1) was computationally (B3LYP/aug-cc-pVDZ) found to be more stable in the SO ax conformation than in the SO eq form by 0.8 kcal mol −1 in the gas phase. The SO eq conformation agrees with the X-ray crystal structure of 2-phenyl-1,3-oxathiane 3-oxide 8 but not with the more structurally similar compounds trans-5-methyl-3-oxo-1,3oxathiane 9 and (2S C ,3R S ,4R C )-2-methyl-4-propyl-1,3-oxathiane 3-oxide (Figure 1); 10 Indeed, both experiment 8−10 and theory suggest that conformers SO ax and SO eq must be populated very similarly in the equilibrium. In the gas phase, the SO ax conformer of 1 is clearly disfavored because of higher steric/electrostatic repulsion, particularly due to gauche repulsion between the oxygen atoms.…”

“…1,3-Oxathiane 3-oxide ( 1 , Figure ) was computationally (B3LYP/aug-cc-pVDZ) found to be more stable in the SO ax conformation than in the SO eq form by 0.8 kcal mol –1 in the gas phase. The SO eq conformation agrees with the X-ray crystal structure of 2-phenyl-1,3-oxathiane 3-oxide but not with the more structurally similar compounds trans -5-methyl-3-oxo-1,3-oxathiane and (2 S C ,3 R S ,4 R C )-2-methyl-4-propyl-1,3-oxathiane 3-oxide (Figure ); other examples of the preferred gauche orientation along the OCSO fragment can be found in the Cambridge Structural Database (43 hits from the SOCH 2 O substructure). Actually, a more general representation of 1,3-oxathio 3-oxide compounds is the model (methoxymethyl)methyl sulfoxide (CH 3 SOCH 2 OCH 3 ); a conformational screening around the C–O–C–S and O–C–S–O dihedral angles gives the energy hypersurface of Figure , in which the global minimum A and a local minimum G correspond to gauche conformers relative to the O–C–S–O dihedral angle.…”

“…Conformational isomerism in 1,3-oxathiane 3-oxide ( 1 ) and 2-fluorothiane 1-oxide ( 2 ) and compounds obtained from the literature − using X-ray crystallography.…”

“…Indeed, both experiment − and theory suggest that conformers SO ax and SO eq must be populated very similarly in the equilibrium. In the gas phase, the SO ax conformer of 1 is clearly disfavored because of higher steric/electrostatic repulsion, particularly due to gauche repulsion between the oxygen atoms.…”