2.4- Pentanediol reduction

Trimethylene dibromide (Section 111,35) is easily prepared from commercial trimethj lene glycol, whilst hexamethylene dibromide (1 O dibromohexane) is obtained by the red P - Br reaction upon the glycol 1 6-hexanediol is prepared by the reduction of diethyl adipate (sodium and alcohol lithium aluminium hydride or copper-chromium oxide and hydrogen under pressure). Penta-methylene dibromide (1 5-dibromopentane) is readily produced by the red P-Brj method from the commercially available 1 5 pentanediol or tetra-hydropyran (Section 111,37). Pentamethylene dibromide is also formed by the action of phosphorus pentabromide upon benzoyl piperidine (I) (from benzoyl chloride and piperidine) ... 

The method of Ito et at. 50) as applied by Bakos et at. 12) to the reduction of acctylacetone to either ( —)-(2R,4J )- or ( + H25,45)-2,4-pentanediol will serve to illustrate how a chiral heterogeneous catalyst has been used to prepare a chiral homogeneous ligand precursor. 

The combination of the enantiomerically pure 7V-methylephedrine derived silylketene acetal l-[(l/ ,2S)-2-dimethylamino-1-phenylpropoxy]-l-triniethylsilyloxy-l-propene with the chiral aldehyde (,R)-3-benzyloxy-2-methylpropanal leads, after reduction with lithium aluminum hydride, to the formation of a single 1,3-pentanediol 9 ( matched pair ). 

Reduction of diketones such as pentane-2,4-dione using (RfBiNAP-RuCF under hydrogen (75 100 atm) gives the corresponding diol, in this case (R),(R)-2,4-pentanediol with an excellent diastereomer ratio (98 %) and optical purity (>99 %)[48]. 

Reductive cleavage of the eight-membered ring of the cycloheptatriene-fused dioxocins 615 and 656 was achieved by treatment of the substrates with LiAlH4 in ether at low temperature to give in nearly quantitative yields the pentanediol derivatives 657 and 658, respectively (Scheme 131) <2003CL128, 2004OL4439>. 

Reaction with chiral acetals. The chiral ketals derived from (2R,4R)-(-)-2,4-pentanediol (1) can be cleaved with high diastercoselectivity by aluminum hydride reagents, in particular DIBAH, CI2AIH, and Br,AlH. Oxidative removal of the chiral auxiliary affords optically active alcohols. This process provides a useful method for highly asymmetric reduction of dialkyl ketones. ... 

Aldol coupling of chiral acetals. The acetals (2) prepared from an aldehyde and (2R,4R)-pentanediol react with a-silyl ketones orenol silyl ethers in the presence of TiCI, to form aldol ethers 3 and 4 with high diastereoselectivity (>95 5). Removal of the chiral auxiliary usually results in decomposition of the aldol, but can be effected after reduction... 

Mixed y-Ga203-Al203 oxides of different stoichiometry were prepared by the solvothermal method from Ga(acac)3 and Al(OPr-i)3 as starting materials and were used as catalysts for selective reduction of NO with methane. The initial formation of gallium oxide nuclei controls the crystal structure of the mixed gallium-aluminum oxides. It is found that the acid density per surface area is independent of the Al Ga feed ratio but depends on the reaction medium (diethylenetriamine, 2-methylaminoethanol, toluene, 1,5-pentanediol etc.), whereby in diethylenetriamine the catalyst had lower densities of acid sites and showed a higher methane efficiency. 

Reduction of Acetals. Reductions of acetals of 2,4-pentanediol can provide (after removal of the chiral auxiliary by oxidation and 3 elimination) secondary alcohols with good enan-tioselectivity. The choice of reagents dictates the configuration of the final product. Use of Dibromoalane gives products from selective syn cleavage of the acetal while Triethylsilane/Titanium(IV) Chloride gives the more usual anti cleavage products (eq 2). ... 

The reduction of available solvent by addition of precipitant is, in principle, no different than the crystallization of sea salt from tidal pools as the heat of the sun slowly drives the evaporation of water. It is a form of dehydration but without physical removal of water. A similar effect may be achieved by the slow addition to the mother liquor of certain organic solvents such as ethanol or methyl pentanediol. The only essential requirement for the precipitant is that at the specific temperature and pH of the experiment, the additive does not adversely affect the structure and integrity of the protein. This is often a very stringent requirement and deserves more than a little consideration. Organic solvent competes, to some extent, like salt for water molecules, but it also reduces the dielectric screening capacity of the intervening solvent. Reduction of the bulk dielectric increases the effective strength of the electrostatic forces that allow one protein molecule to be attracted to another. 

This volume begins with two procedures in the area of catalytic asymmetric synthesis. The first procedure describes the synthesis of (R)-2-Dl PH ENYLPHOSPHI NO-2 -METHOXY-1,1 -BINAPHTHYL (MOP), a chiral ligand that has proven very useful in palladium-catalyzed hydrosilylation of olefins and palladium-catalyzed reduction of allylic esters by formic acid. The next procedure describes the catalytic asymmetric synthesis of nitroaldols using a chiral LANTHANUM-LITHIUM-BINOL COMPLEX, illustrated by the synthesis of (2S,3S)-2-NITRO-5-PHENYL-1,3-PENTANEDIOL. 

Neopentyl glycol is used in the preparation of polyesters. Because there are no /3-hydrogen atoms, the polymers are more stable. The self-condensation of isobutyraldehyde followed by reduction leads to 2,2,4-trimethyl-1,3-pentanediol, the monoisobutyrate of which is the most common coalescing agent (used at 0.5-2 vol%) in latex paints. Isobutyl acetate is used as a solvent for nitrocellulose coatings. 

An attempt to obtain evidence for this hypothesis was made. Reduction of l-hydroxyl-4-pentanone might proceed via a stereochemically less favorable, seven-membered ring intermediate, and the product (1,4-pentanediol) of much less than 30% optically purity might result. Although optically active diol was obtained, no assignment of optical purity could be made since the diol could not be transformed stereo-specifically, despite several attempts, into 2-methyl-tetrahydrothiophene-1-dioxide, whose maximum rotation is known (18). The validity of the above hypothesis thus remains moot. 

The Tishchenko reaction (see page 334) may take place as a side reaction to reduction of aldehydes, but for aliphatic aldehydes it is suppressed by using an excess of isopropoxide. Also the aldehyde may undergo an aldol condensation (see page 868) with itself or with the acetone produced. Recently the formation of 2-methyl-2,4-pentanediol as by-product was observed.361 These side reactions, however, occur only occasionally and in no way detract from the value of this method of reduction. 

Pentanediol [(R,/ )-28] has been used for the formation of chiral acetals and as a precursor for chiral alkenes (Sections D.1.5.1. and 1.6.1.5.). The original procedure for the resolution of the phenylboronic acid derivative with brucine39 was impractical, but recently a kinetic resolution by lipase-catalyzed esterification and hydrolysis has been developed40. In addition, a good method for catalytic reduction of 1,3-diketones with Raney nickel modified with sodium bromide and tartaric acid (for a procedure, see Section D.2,3.1.) allowed the production for commercial purposes41. Similarly, sterically more hindered and less water-soluble 2,6-dimethyl-3,5-heptanediol (29) has been introduced for the same purpose. It is obtained in the same way from the diketone and separated from the meio-compound by simple recrystallization42. 

Treatment of a diol with chlorodiphenylphosphine in the presence of triethylainine or pyridine leads to diphosphinites. By this easy method, (S,S)-1,2-bis(diphenylphosphinoxy)cyclohexane (CHDPPO, 59) was obtained from (S,S)-l,2-cyclohexanediol (Section 4.1.2)23 58 and (5,S)-2,4-bis(diphenylphosphinoxy)pentane [(S,S)-BDPOP, (S.SV60] from (S,S)-2,4-pentanediol (Section 4.I.2.)59. In ref 59, the synthesis is described in detail from the beginning, i.e., the asymmetric synthesis of the diol by catalytic reduction of the diketone with Raney nickel modified with tartaric acid/sodium bromide. 

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