Conformation of ring A in triterpenoid and 4,4-dimethylsteroid 3-ketones. Chemical shifts of methyl protons and lanthanide and benzene induced shifts

Abstract

Chemical shifts of signals due to methyl groups in position 10β (in CDCl3) and 4α and 4β (in C6D6) in 1H NMR spectra of pentacyclic triterpene 3-oxo derivatives (V, VIII, IX, and XII) are suitable for estimation of chair-boat equilibrium in the ring A. Benzene and lanthanide induced shifts of 4α and 4β-methyl protons were also used for this purpose. The results obtained with 2α-methyl-3-ketones (III, X) and 2β-methyl-3-ketones (IV, XI) as the respective chair and boat models agree well with those derived from other physical data (about 40% boat). The same methods were applied to 4,4-dimethylsteroid 3-ketones XV-XVII.

E-Homolupane Derivatives Substituted in Position 17 and 22a. 1H NMR, 13C NMR and IR Spectra

“…The signals of the 23-, 24-and 25methyl groups on the ring A were only little sensitive to substitution on the ring E and their shifts corresponded to the values described for other triterpenic "3-deoxy" derivatives (see ref. 7 and references therein), including the characteristic fine splitting of the 25-methyl signal. The 26-and 27-methyl signals were located downfield, the 27-methyl signal usually being broadened or split by long-range interaction with the H-15β proton.…”

Oleanane Triterpenoids Functionalized at C-25 and C-26

“…ref. 7 ), to δ 3.60. Nitrimines 10 and 11 changed neither on boiling in aqueous dioxane under conditions that should convert them into aldehydes 5,6 nor on reflux with pyridine.…”