✍ Scribed by Baas, Jan M. A. ;Woudenberg, Richard H. ;Maat, Leendert
No coin nor oath required. For personal study only.
Geometrically fully‐optimized transition structures are calculated for the Diels‐Alder adducts of methyl propenoate and the 3‐demethoxy derivatives of the dienes thebaine, 6‐demethoxythebaine, 5β‐methylthebaine, and 5β‐methyl‐6‐demethoxythebaine using the semiempirical PM3 method. The number of calculated forms is limited according to a search of the conformational space of methyl propenoate and of the dienes. The lengths of the incipient bonds in the transition structures vary from 2.0 to 2.4 Å, but the average value of the lengths of these two bonds in the same structure is much more constant: 2.18 ± 0.02 Å. The effects of the methoxycarbonyl group and of the 6‐methoxy group on the lengths of the incipient bonds are in agreement with the relative magnitudes of the frontier orbital coefficients. Comparison of the energetic data with experimental results shows that the prediction of the stereo‐ and regioselectivities from the Gibbs energy of activation differences is good as far as it concerns the main products. The absence of minor products and the formation of the remarkably large amount of 8β‐ethoxycarbonyl‐6β,14β‐ethenomorphinan from 5β‐methyl‐6‐demethoxythebaine does not follow from the calculations.
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