The NMR spectra and conformations of cyclic compounds—VII: The conformations of β‐pinene, pinocarvone and the cis‐ and trans‐pinocarveols

Abstract

Abstract-The proton magnetic resonance spectra of P-pinene pinocarvone and the cis-and trans-pinocarveols have been completely assigned at 220 and 300 MHz. On the basis of the proton-proton spin-spin couplings derived, conformations have been deduced for these molecules with greater certainty than has hitherto been possible. Pinocarvone is Y-shaped, while in all the other compounds the conformation is intermediate between a Y-shape and a bridged chair, with the C, atom bent away from the gem dimethyl groups. These conformations are discussed in terms of the steric interactions in these systems and are compared with related molecules.THE DEDUCTION of the precise conformations in solution by NMR of a-pinene and some derivatives was given in the previous part of this series.l It was therefore of interest to investigate similarly the conformation of /3-pinene, particularly in view of the need to interpret conformationally the recent results of the kinetic study of the hydration of a-and P-pinene reported by some of US.^ Complete spectral analyses have also been given for similar s y~t e m s . 3 *~*~ P-pinene itself (1) presents a formidable spectral analysis problem, and to aid in the interpretation, the cis-and trans-pinocarveols (2) and (3) and pinocarvone (4) were also studied. The related system nopinone (5) had previously been studied by US,^ while ( 5 ) R, = H (6) R1 = CH3 brief details of the PMR spectra of (2) and (3) have been given by other workers,, although no complete analysis was presented.

A molecular mechanics and ab initio prediction of the 1H chemical shifts of pinanes

“…The Y‐shaped geometry is also present in cis ‐pin‐3‐en‐2‐ol ( 5 ) (ф = 0.1°) and cis ‐ δ ‐pinene ( 8 ) (ф = 0.5°); thus, the interaction of the 9‐methyl and 4‐methyl groups does not significantly affect the conformation. This is also consistent with the results from previous NMR studies; although in the absence of any quantitative relationship at that time between the vicinal 3 J HH couplings and the H.C.C.H dihedral angle, only qualitative information was obtained.…”

“…An important terpene subgroup is the pinanes with a fused four‐membered ring. We have previously undertaken extensive studies of the 1 H‐NMR spectra of these molecules, confirming their structures and in several cases suggesting their conformations. However, since this early work, there has been no comprehensive attempt to calculate the 1 H chemical shifts of these molecules.…”

“…Our earlier work suggested that in β ‐pinene ( 13 ) and the pinocarveols ( 15 and 16 ), C 3 was displaced away from the 9‐Me group to give a pseudo chair. PCMODEL confirms this for β ‐pinene with ф calculated as 21.1°.…”

“…Also, the NMR data should be complete, and the spectrum fully assigned in non‐polar solvents. The analysis of the 1 H‐NMR spectra of 19 derivatives of pinane in non‐polar solvents has given earlier, and we will use this data to parameterize the HSPEC model.…”

Conformation of Pinan-3-ol Derivatives

“…Abraham et af. 9) were able to read coupling constants between vicinal protons of the pinicarveols (5 and 6) by the use of 220 and 300 MHz PMR apparatuses. Our coupling constants for the protons of 5 and 6 agreed with Abraham's data.~) Consequently, the conformations of 5 and 6 were half bridged chair forms* as described by them.…”