1H and 13C NMR Studies of conformational substituent effect in 4- and 5-monosubstituted derivatives of benzocycloheptene

Structure and stereochemistry of (±)-demethoxycarbonyl-3α- and -3β-nauclechine

“…2). This conformation is analogous to that assigned to benzocycloheptene and a number of its simple derivatives (M~nard & St-Jacques, 1983). The slight distortion from C~ symmetry can be seen in the C(14)-C(15)-C(20)-C(19) torsion angles of 5.0 (8) ° in (la) and 3.5 (7) ° in (2a) (despite the annulated pyridine ring) and in the torsion angles about bonds C(3)-C(14) and C(14)-C(15) which differ by 6.1 and 6.7 o between the two isomers.…”

“…The strong preference of a benzocycloheptene for the C s chair conformation (M+nard & St-Jacques, 1983) has important consequences for structural studies, and it is significant that the present work shows that N-containing analogs show the same preference even when substituted and incorporated into a larger molecular framework. Although the conformations established for crystalline (la) and (2a) are not necessarily those of their acetates (lb) and (2b) in solution, the 1H NMR spectra (Shariff & McLean, 1983) of (lb) and (2b) provide vicinal coupling constants compatible with the dihedral angles associated with the Cs chair conformation.…”

“…Our synthesis of demethoxycarbonylnauclechine provided both diastereoisomers in crystalline racemic form (Shariff & McLean, 1983); one isomer (labelled HC) predominated when hydrogen chloride was used for the final cyclization, and the other (labelled FA) predominated when formic acid was used. Although the isomers could be clearly distinguished by 1H NMR spectroscopy, their relative configurations could not be assigned on this basis because the conformation of their seven-membered rings was uncertain; conformational analysis of flexible seven-membered rings continues to receive attention (Toromanoff, 1980;DeClercq, 1981;Esteban, Galiano, Diez & Bermejo, 1982;M~nard & St-Jacques, 1983) but remains a much more complex problem than that of six-membered rings. Furthermore, in the molecules of interest, the presence of the annulated pyridine ring and an N atom susceptible to inversion adds to the complexity; for ~H NMR purposes, the two three-spin systems are effectively isolated, and the configurational relationship between C(3) and C(19) can be deduced from coupling constants only if the conformation of the sevenmembered ring can be assigned with confidence.…”

One hundred years of benzotropone chemistry

“…2.2.3. Reduction-based studies: To define the stereochemical, conformational, and dynamic properties of both benzo[7]annulenones (and related compounds) and monosubstituted tetrahydro-7 H -benzo[7]annulenone, St-Jacques' group benefited extensively from the use of dynamic nuclear magnetic resonance (DNMR) techniques [ 126 – 127 ]. A catalytic deuterogenation of 4,5-benzotropone ( 11 ) followed by deuteration led to deuterated 5,6,8,9-tetrahydro-7 H -benzo[7]annulen-7-one 119- d 6 with the presence of appreciable quantities of d 4 and d 5 species ( Scheme 21 ).…”

Repulsive gauche conformational effect. Analysis of seven-membered rings containing endo and exo sulfur atoms

“…For example, the data in Table 2 (6,(14)(15)(16) show that the equatorial preference of cyclohexylmethyl sulfide is higher than that of the heavy halogen and methoxy substituents. But when the SCH3 substituent interacts with a vicinal polar 0CH3 substituent, as in trans-1 -methylthio-2-methoxycyclohexane, a conformational population change occurs that is larger than predicted on steric grounds (32% of axial conformation in CC14, -AGO = 0.4 kcal/mol) (6).…”