Citronellol asymmetric synthesis

The procedure for the synthesis of the title compound is a representative example of asymmetric hydrogenation in the presence of BINAP-Ru(ll) diacetate.5 The method is based on the synthesis of BINAP-Ru(ll) dicarboxylate complexes and enantioselective hydrogenation of geraniol.7 The present method provides the first practical means for asymmetric synthesis of (S)- and (R)-citronellol. (S)-(-)-Citronellol of optical purity up to 92% can be obtained in a limited quantity from rose oil. A microbiological reduction of geraniol was reported to give enantiomerically pure (R)-(+)-citronellol. ... 

Oppolzer has developed a method of asymmetric synthesis based on the use of the chiral auxiliaries 39A and 39B derived respectively from (+ )-camphor [(+ )-40] and (- )-camphor [(- )-40]. Crotonylation of 39A gave the ester that was attacked by 4-methyl-3-pentenyllithium in the presence of copper iodide tributylphosphine and boron trifluoride from only one side of the molecule, the product 41 having the (S)-configuration (enantioselectivity 98.5%). The ester 42—similarly obtainable from 39B—was methylated under similar conditions, also yielding 41 with 92% enantioselectivity. (S)-Citronellic acid [(S)-36] or (S)-citronellol [(S)-33] were then obtained from 41 by the action of sodium hydroxide or lithium aluminum hydride (Scheme 6). Reduction of potassium... 

An attempt from citronellol using carbene chemistry Asymmetric Synthesis... 

Asymmetric synthesis of citronellol has been reported with Pd-dialkyl-menthylphosphonites (Men P(OR)2) as catalyst [48]. 

Before leaving asymmetric hydrogenation reactions, we should mention one other related process that has acquired immense importance, again because of its industrial application. You have come across citronellol a couple of times in this chapter already the corresponding aldehyde dtroneUal is even more important because it is an intermediate in the a synthesis of L-menthol by the Japanese chemical company Takasago. Takasago manufacture about 30% of the 3500 ton annual worldwide demand for L-menthol from dtroneUal by using an intramolecular ene reaction (a cycloaddition you met in Chapter 35). 

Synthesis from Citronellal. Citronellal can be hydrogenated to citronellol by the use of special catalysts and/or special hydrogenation techniques, e.g. [47]. The citronellal which is used as starting material may originate from synthetic production or from isolation of essential oils. Citronellal from citronella oil yields (+(-citronellol the corresponding material from citronellal from Eucalyptus citriodora oil is racemic. Pure (+(-citronellol is also obtained from (+)-citronellal which is produced as an intermediate of (-(-menthol (see p. 55-58). By this asymmetric technology, pure (-(-citronellal and therefore pure (-(-citronellol is also available. 

A synthesis of vitamin E (129) [91] developed at Hoffmann-La Roche was based on highly efficient asymmetric C=C reductions in the presence of chiral diphosphine ligands 125 and 127 (Scheme 8.11) [95]. Large-scale hydrogenations of 122 and 126 afforded citronellol (123) in 98.5% ee and 128 in 98% ee, respectively. 

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