Studies of alicyclic systems

During the last decade the synthesis and spectroscopic characteristics of nitro compounds have aroused much interest. New synthetic methods have been developed, and the excellence of the nitro group as a chromophore in spectroscopy has been demonstrated. Previous work in this Department1 has been concerned with the synthesis and spectroscopic properties of secondary nitrosteroids and related compounds. In the present work, this investigation was extended to nitro compounds in the bornane series, and to α-nitroketosteroids and related compounds. Many oximes and ketones were handled as starting materials, and other ketosteroids were available from microbiological studies. These provided a sufficiently wide coverage of the positions on the steroid nucleus to enable studies to be made of the solvent shifts of angular methyl groups in the n.m.r. spectra of the ketones, and of the low field signals and chemical shifts of the angular methyl groups in the n.m.r, spectra of the oximes. Accordingly, the present investigation is divided into four sections.
In the first Section, general methods for the preparation of secondary nitro compounds are reviewed, and the preparation, and elucidation of the structures of the 2- and 3-nitrobornanes (l, 2) are described. 3-Nitrocamphor (3) was prepared and its behaviour as a non-enolic nitro-ketone studied. Preparation of the 3-nitrobornanes involved the conversion of camphor (4) into bornan-3-one (5). Both the methods available for this conversion, the classical route2 via the sodium amalgam reduction of the ketol (6), prepared from bornan-2,3-dione (7), and the reported sequence3
involving Huang-Minlon reduction of 3-hydroxyirainocamphor hydrazone (8), were studied. The former route was found to be satisfactory and bornan-3-one (5) was prepared from camphor in an overall yield of 10%. The latter route was not satisfactory: attempts to prepare 3-hydroxyiminocamphor hydrazone (8) failed, and reduction of the reaction product from treatment of the isomeric mixture of syn- and anti-3-hydroxyiminocamphor (9) with hydrazine hydrate, under a variety of conditions, did not yield either of the reported products,3 3-hydroxyiminobornane (10) and bornano-[2,3,v]-triazole (11). 2-Nitrobornane (l) was prepared both by N-bromosuccinimide, and nitric acid treatment of camphor oxime (12), with reduction of the bromo-nitro-, and gem-dinitro-intermediates. 3-Nitrobornane (2) was prepared by the N-bromosuccinimide route.
2-Nitrobornane (l) was shown to be a single compound, and 3-nitro-bornane (2) to be a 50:50 epimeric mixture. The literature on the n.m.r. spectra of the bicyclo-[2,2,l]-heptane system was summarised, and the structures of these compounds were deduced by n.m.r. examination to be 2-endo-nitrobornane (13), and 3-exo-, and 3-endo-nitrobornane (l4, 15). A study of these spectra, those of their respective intermediates and, in some cases, of their deuterated analogues, confirmed previously reported features and revealed some unexpected differences between 2- and 3- substituted bornanes.
3-Nitrocamphor (3) was prepared from camphor by the classical route,4 reactions involving less vigorous conditions, and base catalysed nitration with ethyl nitrate having failed. The mutarotation5 of this compound was studied by n.m.r. spectroscopy and polarimetry. The configuration of the
nitro group in the only isolable nitrocamphor was shown to be endo-, and Bell's postulate6 that mutarotation involved the interconversion of stereoisomers was confirmed.
In Section II methods for preparing α-nitroketones and related compounds are described. Base catalysed nitration of 5αcholestan-3-one, 5α-cholestan-6-one and 5α-cholestan-7-one was carried out. The conditions for this reaction, originally used for the nitration of monocyclic7 and steroid8 ketones, and modified by Bull,1 were studied in detail. The experimental conditions finally evolved afforded 2-nitro-5α-cholestan-3-one (16) in reproducible yields of 52%. The proportion of byproduct, 2,4-seco-2,4-dinitro-5α-cholestane (17) formed from the bis-condensation product (18), was substantially reduced by working at - 30°. The nitroketone (16) was shown, spectroscopically, to be almost totally enolic, and was smoothly brominated to give 2α-bromo-2β-nitro-5α-cholestan-3-one (19), which could be decomposed to give the parent nitroketone (16) in 75% yield. The nitroketone formed an oxime (20) which was used to prepare the furoxan (21).
5α-Cholestan-7-one was nitrated under the same conditions as the 3-ketone and afforded a mixture of nitroketones (44%). Crystallisation of the mixture gave 6α-nitro-5α-cholestan-7-one (22), and equilibration of the mixture by treatment with NaHCO3, and acidification yielded 6β-nitro-5α-cholestan-7-one (23). The structures of these compounds were determined by n.m.r. spectroscopy, and were confirmed by the instability of the 6β-nitroketone (23) due to interaction with C-19. Both nitroketones, on treatment with a trace of NaHCO3, without acidification in the working up,
afforded the 6A7alpha;-nitroketone (22), and a second compound which was not investigated further. Nitration of 5α-cholestan-6-one occurred only in low yield (<5%), the major product being unchanged ketone. This is attributed to hindrance of approach to C-7 by the C-15 methylene group.
A second route to steroid nitroketones was investigated, but not carried to its conclusion. This route led, instead, to a study of compounds containing vicinal nitrogen substituents. 5α-Cholestan-3-one was hydroxy-iminated9 under basic conditions: with one mol. of amyl nitrite, 2-hydroxy-imino-5α-cholestan-3-one (24) was obtained in 34% yield. With 2 mols. of nitrite, the 2,4-disubstituted ketone (25) was obtained (22%). Under acid catalysed conditions,10 the hydroxyiminoketone (24) was obtained in 70% yield, and was converted to the 2,3-dioxime (26) in 99% yield. This dioxime was also prepared by oxygenating 5α-cholestan-3-one under Barton's conditions,11 and oximating the resulting dionediosphenol mixture (27). This dioxime was dehydrated to the furazan (28), and oxidised with sodium hypochlorite to the furoxan (29). The properties of these compounds were in agreement with analogous compounds previously reported.10 The furoxan (29) was shown to have the structure (29) by comparison with the furoxan (21) prepared from the 2-nitro-3-oxime (20). The dioxime was oxidised under conditions (buffered peracid) where steroid monoximes give good yields of secondary nitre-compounds,1 but the only identifiable product (18%) was a furoxan whose properties were similar to those of the product prepared above. A small amount of material (6%) was isolated which exhibited strong nitro absorption in its i.r. spectrum (l540, 1580 cm.-1).
During the preparation of the starting materials used in this section, it was found that the hydroboration of cholest-5-ene in the preparation of the 6-ketone, was not as stereospecific as had been reported.12 The mixture of 5α- and 5β-6-ketones produced was subjected to an alkaline epimerisation without any change in composition. In view of the fact that the ketone would be used under equilibrating conditions, the material prepared, containing approximately 15% of 5β-cholestan-6-one, was used without further purification.
The signals of the angular methyl groups in twenty-six mono-, di- and tri-oxo-steroids , free from other substituents were examined in carbon tetrachloride, deuterochloroform, benzene and pyridine (Section III). Benzene was found, as expected,13 to give rise to the largest shifts, and the Δ31 values (ΓC6H6 - ΓCCl4) of the methyl groups varied characteristically with the position of the ketonic groups. With the 6-ketones, the Δ31 value appeared to be greater for the C-18 than for the C-19 protons: unexpectedly, changes in stereochemistry at C-5 had little effect on the shifts in the 6- and 17-ketones. The methyl resonances of deuterochloroform solutions of di- and tri-oxo-5α-androstanes showed remarkably good agreement with the figures predicted from Zürcher's survey,14 and the observed Δ31 values were close to those calculated from the shifts of the corresponding monoketones.
In Section IV, a study of the n.m.r. spectra of eight monocyclic mono- and dioximes, eight bicyclic monoterpene oximes and steroid monoximes at the 1, 2, 3, 4, 6, 7 and 17 positions is described. The features of the spectra of the monocyclic oximes were found to be in agreement with those previously reported.15 In the spectra of the isomeric mixture of syn- and anti-3- hydroxyimino camphor (9) and of pure anti-3-hydroxyimino camphor (30), a clear indication of the deshielding effect of the hydroxyl group of the oxime function on the C-4 proton was observed. The low field signals in the spectra of the steroid oximes were shown to be equatorial in nature because of their appearance as doublets. Comparison of the shifts of the C-18 and C-19 protons' signals in these compounds, relative to the parent hydrocarbons, with those of compounds with ketone groups in the same positions indicated that the general effect of the oxime function is similar to that of the carbonyl group, since the observed shifts were of the same magnitude and sign.
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T.M. Lowry and V. Steele, J., 1915, 1038.
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