The stereodependent effect of oxygen on the chemical shifts and vicinal coupling constants of tetrahydropyran

Abstract

The analysis of the ' H NMR spectra of two selectively deuterated derivatives of tetrahydropyran at -115 O C has provided accurate values for proton chemical shifts and vicinal coupling constants about the C(2)-C(3) bond. A comparison with the corresponding parameters of cyclohexane allows a precise definition of the stereodependent effect of oxygen (3J2a3a, 3J2e3e, 3J2a3e are reduced while 3J2e3a is increased) in this fundamental molecule. An explanation of the trends observed is given and the conformational and stereochemical implications of the results 'are discussed.The field of conformational and stereochemical analysis of basic six-membered ring systems is often considered fundamental in organic chemistry. NMR spectroscopy ranks among the most useful tools of experimental investigation in this domain and, when appropriate reference parameters are available, its application to molecules with readily analyzable spectra has become rather straightforward.l Vicinal coupling constants recently published2 for cyclohexane-d8 have provided the reference basis for many applications through an empirically determined dependence on dihedral angles.It is well known that heteroatoms affect the values of vicinal couplings according to their electronegativity, and many efforts have been aimed at providing information on the magnitude of this effect. In particular, approximate coupling values have been proposed for pyranoid derivatives from studies on carbohydrate derivative^,^ but there is a t present no knowledge of the quantitative effect of oxygen in -CH2CH20-fragments from cyclic systems and more specifically from the parent compound, tetrahydrofuran.Because of the fundamental nature of tetrahydropyran both for theoretical reasons and for practical conformational and stereochemical applications to many classes of heterocyclic organic compounds, it appeared to us that accurate chemical shift and coupling information for this basic compound should be a necessary prerequisite to quantitative deductions which go beyond merely drawing parallels between carbocyclic and heterocyclic rings.Because of the complexity of the l H NMR spectrum of tetrahydropyran and the need to obtain very accurate coupling constants we have prepared stereospecifically deuterated derivatives with simpler spectra and because ring inversion is rapid a t normal room temperature4 we have analyzed the spectra at low temperatures in order to obtain parameters characteristic of the noninverting chair conformation of this molecule. ResultsAs illustrated in Scheme I, y-pyranone (2) obtained from chelidonic acid (1)5 was deuterated with deuterium gas in the presence of palladium on carbon to give a mixture of syn and anti isomers6 of tetrahydropyranone-cis-2,3,cis-5,644 ( 3 4 ) which were reduced with lithium aluminum deuteride to give the alcohols 4-d5 which were converted to tetrahydropyran-4,4,cis-2,3,cis-5,6-d6 (5-dG). Protonic exchange of the 01 deuterium in 3-d4 gave a mixture of syn and anti isomers tetrahydropyranone-2,6-d2 (3-dz) which were reduced t...

Proton chemical shifts in NMR. Part 12.1 Steric, electric field and conformational effects in acyclic and cyclic ethers

“…Lambert et al 12 examined deuterated analogues and reported the ∆G ‡ and coupling data, but no chemical shift data. Late work by Lambert et al 13 and a detailed study by Canuel and St.-Jacques, 14 again on deuterated THP, excluded the H3 and H4 chemical shifts. These analyses were in different solvents, CD 3 OD-CHClF 2 and CS 2 , with the former only giving ∆(δeq Ϫ δax) values.…”

A series of chiral polybrominated biindoles from the marine blue-green alga Rivularia firma. Application of carbon-13 NMR spin-lattice relaxation data and carbon-13-proton coupling constants to structure elucidation

“…Br Br X 8 X:Bf, R:fV:Y:H 14 Y:OMe, R':Ac, R: X :H 9 Y:Br, R:R':X:H 15 R:OMe, R':X:Y:H 10 X:Y:Br, R:R':H 16 R:OMe,R':Ac, X:Y:H 1 3 Y:OM•, R:R':X:H dominant biindoles, together with a small amount of (+)-2,5,5,-tribromo-7,-methoxy-3,4'-bi-l//-indole (4). 2,2',5,5,-Tetrabromo-3,3,-bi-l//-indole (1).…”

Conformational dynamic investigation of 1,5-benzodioxepin and its dimethyl derivative by 1H and 13C nmr

“…Hence, the chemical shift difference A6 between geminal axial and equatorial protons P to oxygen is -0.074ppm in tetrahydropyran (11) and about -0.80ppm in 1,3-dioxan (12). Our results seem to show that the same trend exists in seven-membered rings.…”