Fenchone reduction

Pickard, Lewcock and Yates have prepared fenchyl alcohol by the reduction of laevo-rotatory. On conversion into its hydrogen phthalate and fractionally crystallising the magnesium and cinchonine salts, they obtained a fraction, which on saponification yielded Za w-fenchyl alcohol, having a specific rotation of - 15 5°, which is probably the correct value for this figure. 

By reduction fenchone is converted into fenchyl alcohol, melting at 45°. The alcohol, however, has the opposite optical rotation to that of the ketone from which it is prepared. 

As Table 10 illustrates, using this approach the authors were able to rapidly optimize the reaction conditions, obtaining the target 82 in 91% yield when employing 2 eq. of fenchone (80) and n-BuLi 74. In all cases only a single diastereomer was observed and the authors found that conducting the reaction at 0 °C resulted in a mere 3% reduction in yield. Furthermore, the reaction conditions were found to be suitable for a range of aliphatic/aromatic ketones and brominated compounds. 

The condensation of 1,2-diamino-4-nitrobenzene with bornane-2,3-dione has been investigated.290 Catalytic hydrogenation of fenchone oxime (203 X = NOH) gives the imine (203 X = NH) exclusively, whereas camphor oxime gives only the exo-amine (204) reduction with di-isobutylaluminium hydride yields predominantly heterocyclic secondary amines thus (203 X = NOH) gives (205) as the major product together with (206) and (204) and its endo-isomer.291... 

In addition to camphor (1), the metal-NHa-proton donor reduction of four other bicyclo[2.2.1]heptan-2-ones and bicyclo[2.2.1 ]hept-5-en-2-one (54) have been studied systematically. The other ketones include the parent bicyclo[2.2.1]heptan-2-one (55), fenchone (56), l-methylbicyclo[2.2.1]heptan-2-one (57) and 7,7-dimethylbicycloheptan-2-one (58). The results of these reductions and, where known, the equilibrium ratio of the alcohols are summarized in Table 1. In all cases and under the conditions noted, the endo-alcohol is produced stereoselectively. 

Camphene is a solid terpene. The dextro variety d-camphene is found in camphor, ginger and spike oils, and the levo variety, 1-camphene is in citrondla and valerian oil and in French and American turpentine. Bornylene does not occur in nature but has been prepared from the alcohol corresponding to it known as Borneol or Borneo camphor. This, as previously stated, may be prepared from pinene so that Bornylene itself may be made from pinene. Fenchene, also, is not found in nature but is obtained by reduction of fenchone a terpene ketone found in fennel oil and in Thuja oil. 

Japanese fennel fruits somewhat resemble those of anise in appearance, but are more oblong and do not taper to the apex. Umney Pharm. J. 1896, 57, 91) obtained 2.7 per cent of pale yellow oil with the odour of sweet fennel. The solidifying-point was 7° C., melting-point 10°. When fractionated, the following fractions were obtained below 220°, 26 per cent 220-225°, 32 per cent 225-230°, 34 per cent residue, 8 per cent. According to Umney, this oil probably contains 75 per cent of anethole 10 per cent of fenchone (by reduction and acetylisation), with some pinene and dipentene it is not stated how the last two were identified. 

A similar observation has been made in the reduction of fenchone a-fenchol [d.r. (exojendo) 5 9518S] is formed in the Meerwein-Ponndorf-Verley reduction, and /f-fenchol (exo) is formed in the lithium aluminum hydride reduction187. 

A vast array of chiral auxiliaries have been derived from naturally occurring compounds containing the bicyclo[2.2.1]heptane unit (for review articles, see refs 1 -3). In all cases, the ultimate sources of these auxiliaries are the ketones camphor and fenchone, and the alcohols borneol and fenchol, as at least one enantiomer of each compound is provided in enantiomericaUy pure form by nature. Thus. ( + )-camphor [( + )-2], (-)-borneol [(-)- ], and (+)-fenchonc [( + )-5] are enan-tiomerically pure, convenient and inexpensive starting materials for organic synthesis and deriva-tization to give chiral auxiliaries. Most other compounds of this series are also commercially available, but can be prepared by oxidation or reduction of inexpensive precursors by standard methods. The evo-alcohols, such as the enantiomeric isoborneols, are accessible by standard complex hydride reductions of the ketones. The interconnection between these compounds is shown diagrammatically. 

An informative and amusing background to that unique material, camphor, has appeared. Its preparation by Oppenauer oxidation of the epimeric borneols occurs without epimerization. Epimerization does not occur in the presence of potassium t-butoxide in t-butyl alcohol, but it does with potassium isopropoxide in propan-2-ol." The reaction of camphor with phosphoric acid yields a complex mixture of m- and p-cymenes, 3,4-dimethylethylbenzene, 1,2,3,4-and 1,2,3,5-tetramethylbenzene, fenchone, carvenone, and carvacrol. A very detailed examination of the metal-ammonia reduction has revealed an intermediate camphor analogue of pinacol formed by association of a camphor anion radical with the metal cation. This intermediate was isolated and characterized. Other effects are discussed, such as that of adding a large excess of metal salt (LiBr, KBr, or NH Cl)." ... 

Dehydrogenation. Reetz and Eibach have developed a new method for dehydrogenation of dihydroarenes based on deprotonation-hydride elimination. Potassium fencholate (formed by reduction of fenchone with KH) serves as base and fenchone (1) as hydride acceptor. Since potassium fencholate is regenerated in the aromatization, only catalytic quantities are required. Yields of arenes are 70-85% (isolated). The method is not useful for dehydrogenation to form alkenes. Fenchone is particularly suitable since it is reduced by hydride transfer because of steric reasons,... 

The bulk of the oil boiled between 190 and 220. The following ter-penes were identified in the oil o-pinene, y8-pinene, 1-limonene, dipen-tene, and X-terpinene. A considerable amount of a-terpinene was found (this had previously been identified by Teeple), as well as borneol and methyl-chavicol. Fenchyl alcohol was also found as a constituent of the oil, and traces of camphor and cineol. The presence of fenchyl alcohol is of particular interest as it occurs in the inactive variety. It is well known that this body results from the reduction of fenchone, but it has never before been discovered as a natural constituent of an essential oil The table below shows the character of the fenchyl alcohol occurring in this oil and of those prepared by the reduction of active and inactive fenchone. 

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