Blanchette, A locally written computer program, NLLSQ, based on the Marquardt algo-with the values predicted on the basis of the curve shown in Figure 3, e.g., r i t h~n ,~~ was used. We thank Dr. Eric Enwall for kindly obtaining for us the computer-drawn plot shown in Figure 3. CH values for torsion angles, $6, of 60 and 180' are predicted to have (42) The observed values of J14wcH measured in 15N-enriched amino acids opposite signs "due to a substituent effect involving the lone pair electrons measured approximately at their isoelectric points agree remarkably well on the nitrogen and the *-electrons of the carbonyl groups".z8b = 3.6 and flAh& = 1.3 H z .~~ Abstract: The conformational and dynamic properties of 1,3-dioxa-5,6-benzocycloheptene (5,2,4-benzodioxepin) and three of its 2,2-disubstituted and two of its 2-monosubstituted derivatives have been investigated by IH and I3C DNMR methods.Analysis of the spectra of low temperatures (below coalescence) indicates that the most stable seven-membered ring conformations detected for solutions in CHFzCl are C (79%) and T B (21%) for 5, TB for 6, 7, and 8 (the disubstituted derivatives), C for the 2-methyl-derivative (9), and T B for the 2-methoxy derivative (10). Thus the polar nature of the single methoxy substituent causes a change in the ring conformation as a consequence of the anomeric effect. Free-energy barriers were determined for the chair inversion of 5 (8.0 kcal/mol) and for the twist-boat pseudorotation of 6,7,8, and 10 (10.0,9.0,9.9, and 6.7 kcal/ mol, respectively). The substituent effects on both the conformational and dynamic properties of the seven-membered rings are discussed and explained

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FRANCOISE SAURIOL-LORD and MAURICE ST-JACQUES. Can. J. Chem. 53, 3768 (1975). A dynamic n.m.r. study of a deuterated derivative of dibenzo-1,s-cyclooctadiene (2-d4), N,N'-dimethyl-1,4-diazocine 3, and N,N'-diacetyl-1,4-diazocine (4) is reported. Two spectral changes of different origin are observed for 2-d4, one for 3 and three for 4. The resultsreveal the presence of a mixture of chair and twist-boat conformations for 2-d4 and 4, while the single spectral change observed for 3 is interpreted in terms of a single twist-boat form. Interconversion pathways are suggested to explain the characteristic behavior of each compound investigated. Important aspects of conformational analysis involve the assessment of the nature of the most stable conformation and of the rates of various types of stereomutations characterizing different classes of molecules.Recently, dynamic nuclear magnetic resonance (d.n.m.r.) spectroscopy (1) has been used to study 1,5-cyclooctadiene (2), dibenzo-1,5-cyclooctadiene (3), syn-3,7-dibromo-cis,cis-1,5-cyclooctadiene (4), as well as derivatives of 5,6,11,12-tetrahydrodibenzo[b,fl[l,4]diazocine (5-8) and has revealed the complexity of the stereodynamic properties of this fundamental ring system. Specifically, the conformational properties of 1,5-cyclooctadiene (1) are significantly different from that of dibenzo-l,5-cyclooctadiene (2). The stable conformation of 1 has been re~orted to be a twist boat which can undergo two dynamic processes (2) while 2 exists as a mixture of chair and boat conformations of varying proportion depending on the nature of the solvent (3). For this latter molecule, owing to spectral complexities and solubility problems, only one of the possible dynamic processes has been identified for the boat form. In order to define more precisely the stereodynamic properties of 2, the deuterated derivative 2-d, was prepared and studied by the d.n.m.r. method.

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F. S~L R I O L -L O R Dand hf. SF-JACQUES. Can. J . Chern. 57.3221 (1979). The conformational properties of 1,3-dithia-5,6-benzocycloheptene (7) (2,4-benzodithiepin), three 2,2-dialkyl derivatives, and two monosubstituted derivatives (2-CH, and 2-OCH,) have been investigated by 'H and I3C dnmr methods. It is reported that the most stable conformation of the seven-membered ring is a chair on which, in the case of the monosubstituted compounds, the 2-CH, group adopts an equatorial position while the 2-OCH, group takes the axial position. Only for the 2,2-pentamethylene derivative (10) was a minor amount (13%) of TB form detected. These results are markedly different from those found previously for the oxygen analogs and the conformational effect of the cyclic heteroatonis will be discussed in terms of the anomeric and steric effects. Chern. 57,3221 (1979). On a etudie les proprietes conformationnelles du dithia-1,3 benzo-5,6 cycloheptene (benzo-2,4 dithikpine) (71, de trois dkrives dialkyl-2,2 et de deux derives monosubstitues (CH3-2 et OCH,-2) en utilisant les methodes de la rmn dynamique du 'H et du I3C. Dans le cas des coniposCs monosubstitues on note que la conformation la plus stable des cycles a 7 est la forme chaise dans laquelle le groupe CH, en position 2 occupe la position Cquatoriale tandis que le groupe OCH, est axial. On a detecte de faibles quantites (13%) de forme bateau tordu seulement dans le cas du derive pentamethylene-2,2 (10). Ces resultats sont tres differents de ceux obtenus anttrieurement pour les analogues oxygenes et I'effet conformationnel des heteroatomes dans ces molkcules sera discute en termes d'effets anomkriques et stiriques.[ Traduit par le journal]Recently results from 'H and 13C nmr studies (1) of seven-membered heterocyclic compounds containing the -0-CR2-0-and -0-CHR-Omoieties such as in 2,4-benzodioxepin, its monosubstituted, and its disubstituted derivatives (i.e. 1-6), have revealed that the ring conformations were determined by the nature and pattern of substitution. Thus were observed: a mixture of chair (C) and twistboat (TB) forms for 1, a TB conformation for the dialkyl derivatives 2, 3, and 4, a C form for the 2-methyl derivative 5, and a TB form for the 2-methoxy derivative 6; that is, two different ring conformations were observed for the monosubstituted compounds.It is of obvious interest to extend such observations to ring systems containing other heteroatoms, such as S and N, in order to delineate all of the factors governing the conformational features of sevenmembered heterocycles.The current interest in the properties of 1,3-dithianes (2) suggests that a study of seven-membered heterocycles containing the -S-CR2-Sand -S-CHR-Sfragments such as for compounds 7-62 ought to yield valuable complementary information useful in characterizing the fundamental differences between the conformational properties of similar families of six and seven-membered heterocycles.Compounds 7-12 were consequently prepared and studied by 'H and 13C nmr undcr experimental conditions s...