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
