Neral hydrogenation

Citral from ALDRICH was a mixture the E-isomer (geranial) and the Z-isomer (neral), the initial ratio E/Z = 0.51 remained almost constant during hydrogenation, therefore we only referred to the concentration of citral. 

With the catalysts derived from (S,S)-l,2-bis(diphenylphosphinomethyl)cyclobutane and [RhH(CO)(PPh3)3] or rhodium carbonyls, the a,3-unsaturated aldehydes, neral and geranial, are hydrogenated to (E)- and (S)-citronellal in 79% and 60% ee, respectively. Cyclic a,3-unsaturated ketones such as isophorone and 2-methyl-2-cyclohexenone have been hydrogenated using ruthenium hydrides coordinated with chiral diphosphines in up to 62% ee to give chiral ketones, though conversions are not satisfactory. ... 

More advantageous, due to fewer reaction steps, is the direct enantioselective hydrogenation of geranial or neral to (+)-(J )-citronellal with an enantiomeri-cally pure rhodium-Chiraphos ligand. It is also attractive to apply the carbonyl-ene reaction in the presence of a sterically demanding aluminium complex, in order to suppress largely the formation of isopulegol diastereo-mers. [129,131]... 

Most recently, Takasago published the synthesis of (/ )-citronellal by enantioselective hydrogenation of the mixture of geranial and neral in the presence of a dual catalyst (Pd/BaS04 and an enantiopure 2-diaryl-methylpyrrolidine). [132]... 

Cyanobacterium converted geraniol (271) to geranic acid (278) via geranial (276), followed by hydrogenation to give citronellic acid (262) via citronellal (261). Furthermore, the substrate 271 was isomerized to nerol (272), followed by oxidation, reduction, and further oxidation to afford neral (275), citronellal (261), citronellic acid (262), and nerolic acid (277) (Kaji et al., 2002 Hamada et al., 2004) (Figure 19.12). 

Citral (a mixture of geranial (276) and neral (275), 56 44 peak area in GC) is easily transformed to geraniol (271) and nerol (272), respectively, of which geraniol (32) is further hydrogenated to (+)-citronellol (258) and (-)-citronellol (258 ). Geranic acid (278) and neric acid (277) as the minor products are also formed from 276 and 275, respectively. On the other hand, when either 271 or 272 is used as a substrate, both compounds are isomerized to each other, and then 271 is transformed to citronellol (258 or 258 ). These results showed the Euglena could distinguish between... 

Geranial on reduction with Baker s yeast gave (R)-citronellol. However, reduction of the Z isomer (neral) gave a 6 4 R S mixture probably due to isomerisation of the double bond in neral prior to the delivery of hydrogen. ... 

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