LOUIS CANUEL and MAURICE ST-JACQUES. Can. J. Chem. 52, 3581 (1974). The computer analysis (LAOCN-3 program) of the p.m.r. spectrum of 5-oxabenzocycloheptene (3) at -105' has provided values for all coupling constants about the C3-C4 bond, while the complete line-shape simulation (DNMR program) has provided the following activation parameters: AG* = 9.5 kcal/rnol (at -57"), AH' = 9.9 kcal~mol, and AS* = 1.5 e.u. A novcl long distance (transannular) deuterium isotope effect has been discovered during the analysis of the spectrum of a partially deuterated derivative of 3. A comparison of the p.m.r. parameters for 5-oxabenzocycloheptene and benzocycloheptene provides the basic data from which stereochemical and conformational inferences can be made for more complex derivatives in this series.LOUIS CANUEL et MAURICE ST-JACQUES. Can. J. Chem. 52, 3581 (1974). La simulation a l'ordinateur (programme LAOCN-3) du spectre r.m.p. de I'oxa-5 benzocycloheptene (3) a -105 ' C a permis d'obtenir toutes les valeurs des constantes de couplage pour la liaison C3-C,; de p!us la simulation des formes raies (programme DNMR) a permis d'obtenir les parametres d'activation suivants: AG' = 9.5 kcal/mol (a -57'C), AH* = 9.9 kcal/mol et AS* = 1.5 e.u. Un nouvel effet isotopique (transannulaire) a ete mis en evidence durant l'analyse du spectre d'un derive partiellement deuterie de 3. Une cornparaison des parametres r.ni.p. entre I'oxa-5 benzocycloheptene et le benzocycloheptene etablit les donnees de base permettant de faire des deductions stereochimiques et confortnationnelles pour des derives plus complexes dans cette serie.[Traduit par le journal]Nuclear magnetic resonance spectroscopy is a particularly well-suited technique to investigate the conformational and stereochemical properties of cyclic organic molecules. Whereas five-and six-membered rings have been investigated extensively in the last 10 years (1-3), relatively little information is available on basic seven-membered cyclic compounds (2-6). In fact, it was only recently that the conformation of benzocycloheptene (1) was identified and fully characterized through a p.m.r, study (7) at -120"; that work provided accurate vicinal coupling constants for this basic molecule from which reliable dihedral angles were determined for all vicinal protons about the C,-C, (and C,-C,) bond. Our continuing systematic investigation of the cycloheptene-benzocycloheptene system (7-9) has led us to study the effect of oxygen on both the static and dynamic conformational properties of this system and possible inferences on the future stereochemical applications of the Karplus theory (10, 11) to derivatives containing heteroatoms. Although much has been written about the effect of heteroatoms on vicinal coupling constants (11, 12) and significant improvements have been made to account for deviations caused by oxygen (13)(14)(15), it now appears that generalizations are limited to well-defined sets of compounds and cannot be transplanted confidently from one set to another.Our first objective the...

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...

2 mm wide and would have been clearly visible. (The retention time was well separated from that of other peaks and the noise level was on the order of 0.2 mm.) Analogous results were obtained with tetramethyl orthocarbonate. With trimethyl orthoacetate there appeared to be a peak about 0.2 mm high at the retention time for dimethyl ether. The reaction mixture from the hydrolysis of triethyl orthoformate showed no peak at the retention time for diethyl ether although addition of enough diethyl ether to correspond to a 1 % yield gave a peak 4.5 mm high and 3 mm wide. On the basis of this evidence the hydrolysis of all the ortho esters was assumed to proceed without side reactions.The extent of the hydrolysis to formic acid that took place during the hydrolysis of trimethyl orthoformate was determined by potentiometric hydrolysis.

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