Camphor, reduction isolation

Diketimines can be prepared by condensation of 1,2-diketones with 2 equiv of an amine, or 1 equiv of a 1,2-diamine, by azeotropic removal of water. Either a chiral diketone or a chiral amine/diamine can be used in order to obtain a chiral diimine. In both cases, the use of 1,2-diamines is expected to provide better stereocontrol, because of the rigidity of the derived cyclic diimines. For example, the reaction of camphor 1,2-diketone 275 and racemic 1,2-diphenylethylenediamine (d,l)-26 gave the diimine 276 as a mixture of two diastereomers (Scheme 45) [138]. Reduction of 276 with sodium borohydride followed by hydrogenolysis of the N substituents afforded the camphordiamine, which was isolated as the dihydrochloride... 

Electrochemical reduction of camphor-and norcamphoroxime at a Hg cathode proceeds with a high degree of stereoselectivity to give products of opposite stereochemistry to those formed in the dissolving metal (Na-alcohol) reduction of the oximes. The electrolyses are proposed to proceed by a kinetically controlled attack by the electrode on each oxime from the less hindered side (Fig. 62) [348]. In contrast, the corresponding N-phenyl imines yield products of the same stereochemistry as those isolated from a dissolving metal reduction. Cyclic voltammetry and polarographic data point to RH and intermediates in this case that are proto-nated from the least hindered side [349]. 

In the presence of excess monoalkylamine, carbonyl compounds in aqueous solution are in equilibrium with the corresponding imine. In most cases these imines cannot be isolated but they are reduced at a less negative potential than the carbonyl compound. Selective reduction of such equilibrium mixtures is a useful route to alkylamines from ketones in yields of 70-90%. The process fails with hindered ketones such as camphor and with bulky amines such as fert.-butyl amine. Overall the reaction has advantages of lower costs and simpler work-up compared to the use of cyanoborohydride reducing agents. In the electrochemical reaction, protonation of carbanion intermediates occurs from the more hindered side and where two isomeric products are fomied, the least hindered amine predominates [193]. 

A -Substituted bornyl- and isobornylamines 17-19 are best obtained by reduction of the imines formed from camphor and a suitable amine4, either by isolation of the imine or in situ formation. The substituted amines were used as the lithium salt for enantioselective deprotonation and elimination reactions (Section C ). In almost all cases, the exo-amines (isobornylamines) are formed however, using (S)-l-phenylethylamine as the starting amine, the selectivity is reversed and the enob-amine (bornylamine) is obtained. 

The enzymic reduction of geraniol and nerol to citronellol was mentioned in Vol. 1, p. 8 Dunphy and Allcock have now isolated a solubilized enzyme reductase from rose petals that is specific for the reduction of primary terpene alcohols with either a cis- or a trans-aWylk double bond. A pseudomonade has been found that converts linalool into camphor and 2,6-dimethyl-6-hydroxyocta-2,7-dienoic acid. ... 

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)." ... 

Carbonyl Reactivity—The reagent lithium dimesitylborohydride bis(dimethoxy-ethane), which has been isolated and whose crystal structure has been reported, may be employed as a stereoselective reducing agent Thus after 3 h at 0°C in di-methoxyethane 2-, 3-, and 4-methylcyclohexanones were reduced respectively to the cis- (99 %), trans- (99 %), and cis- (94 %) alcohols. However, 3 days, at 25°C, were required for complete reduction of camphor to give 99.8 % isoborneol. 

Di-w-butyltin dihydride reduces halides somewhat more easily than tri-n-butyltin hydride. Yields of reduction products isolated from typical halides are 2-bromooctane, 99% 1-bromooctane, 84% bromocyclo-hexane, 82% benzyl bromide, 60% benzyl chloride, 83% 3-bromo-camphor, 79%. 

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