Benzylic acetals reduction

Phenyl-propyl alcohol, CgH. CHj. CH.2. CHj. OH, is the next highest homologue of phenyl-ethyl alcohol, and is also known as hydro-cinnamyl alcohol. Like the last described bodies it has been known for many years, its first preparation being described in the Aivnalen (188, 202). It occurs as a cinnamic acid ester in storax, and as an acetic ester in cassia oil. It is prepared synthetically by the reduction of cinnamyl alcohol with sodium amalgam and water, or by the reduction of cinnamic or benzyl acetic esters with sodium and absolute alcohol. It has the following characters —... 

Benzyl Bromide [Reductive Bromination of an Acetal].506 To a suspension of tin(II) bromide (5.1 mg, 0.02 mmol) and benzaldehyde dimethyl acetal (54.8 mg, 0.36 mmol) in CH2C12 (2.5 mL) were added successively Et3SiH (65.0 mg, 0.56 mmol) and acetyl bromide (96.8 mg, 0.79 mmol) in CH2C12 (1 mL) at room temperature under an argon atmosphere. The mixture was stirred for 3 hours at room temperature and quenched with a phosphate buffer (pH 7). 

The procedure given here is essentially that described previously by the submitters2 and is based on the early work of Knoevenagel.8 2-Phenylindazole has been prepared by reduction of N-(o-nitrobenzyl)aniline with tin and hydrochloric acid,4 by reduction of N-(o-nitrobenzyl) -N-nitrosoaniline with tin and hydrochloric acid,5 by dehydration of 2-(phenylazo)benzyl alcohol,6 by elimination of acetic acid from 2-(phenylazo)benzyl acetate,7 by dehydrogenation of 3,3a,4,5,6,7-hexahydro-2-phenyl-indazole with sulfur,8 and by thermal decomposition of o-azido-benzalaniline.9... 

The total synthesis of (+)-Macbecin I 78 [39] began with aldehyde 73, prepared via the addition of optically pure crotylsilane onto a benzylic acetal, which underwent an SMS reaction to give ester 75 in a 12 1 syn/anti ratio. Oxidative cleavage of the double bond, Wittig olefination of the resulting aldehyde and a reduction-oxidation sequence yielded a,/ -unsaturated aldehyde 76. A second SMS reaction was then performed leading to polyether 77 (dr > 20 1) that contains all the chiral centers of (+)-Macbecin I 78, Scheme 13.31. 

OH free Here, for all the three monosaccharides, a derivative with the 4-OH free can be obtained from the 4,6-O-benzylidene derivative through regioselective reductive opening. Formation of the 4,6-acetal with consecutive 2,3-protection (acetylation, benzoylation, benzylation) and reductive opening using various reagents yields the desired compounds. In mannosides, the selective formation of the 4,6-6>-acetal is not trivial, but can be accomplished with a 50 to 70% yield. 

The electron affinity of the acetate radical was determined from the -EDEA obtained from ECD data for ethyl acetate, benzyl acetate, and acetic anhydride. At the same time data were obtained for ethyl trifluoroacetate and ethyl trichlor-acetate, but were not analyzed or reported. These data are revisited with two states and new bond dissociation energies. This gives another example of how CURES-EC can be used to support experimental results [1, 69]. Figure 10.10 is a typical ECD plot of In KTia versus 1,000/7 for these compounds. In Table 10.11 the kinetic and thermodynamic parameters obtained from the data are shown. The values of I) E (acetate) available in the literature were used in the data reduction procedure, and the rate constant and Qan values are obtained from the ECD data. The, 41 values are close to the DeBroglie, 41. The Qan values are less than 1.0. The E values are less than 0.15 eV. The Ea are thus typical of the ECD, although in the lower range of Ea measured in the ECD for ethyl acetate, benzyl acetate, and acetic... 

Partial Cleavage of Benzylidene Acetals to Give Benzyl Ethers Reductive Methods... 

Benzyl acetates. Aryl carbonyl compounds (but not others) undergo reductive acetylation at room temperature. 

LiAlHa is normally unreactive toward ethers. Unsaturated acetals undergo reduction with double bond migration (Sn2 ) in cyclic systems, the usual stereoelectronic factors often apply (eq 5). Orthoesters are amenable to attack, giving acetals in good yield (eq 6) The susceptibility of benzylic acetals to reduction can be enhanced by the co-addition of a Lewis acid (eq 7). ... 

Reductive Etherifications and Acetal Reductions. Additional applications of triethylsilane in the reduction of C-0 bonds also continue to surface. The Kusanov-Pames dehydrative reduction of hemiacetals and acetals with trifluorosulfonic acid/EtsSiH has proven especially valuable. Under such conditions, 4,6-O-benzyli-dene acetal glucose derivatives can be asymmetrically deprotected to 6-0-benzyl-4-hydroxy derivatives (eq 28) and thioketone derivatives can be converted to syn-2,3-bisaryl (or heteroaryl) di-hydrobenzoxanthins with excellent stereo- and chemoselectivity (eq 29). Triethylsilane is also useful in a number of related acetal reductions, including those used for the formation of C-glycosides. For example, EtsSiH reductively opens 1,3-dioxolan-4-ones to 2-alkoxy carboxylic acids when catalyzed by HCU. Furthermore, functionalized tetrahydrofurans are generated in good yield from 1,2-0-isopropylidenefuranose derivatives with boron trifluoride etherate and EtsSiH (eq 30). These same conditions lead to 1,4- or 1,5-anhydroalditols when applied to methyl furanosides or pyranosides. ... 

However in aprotic media the reactions of R are more varied. Thus in the presence of carbon dioxide, benzyl halide reduction in aprotic solvents produces some phenyl acetate. In aqueous solutions only toluene is formed at negative potentials. Rifi has reported the synthesis of bicyclobutane derivatives from the corresponding 1,3-dihalo-cyclo butanes at a mercury pool cathode in DMF containing LiBr e.g. 

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of stmcture. The most useful compounds do not necessarily have the simplest stmctures, but are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hy-drogenolysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and ally], readily cleaved by Pd-catalyzed isomerisation. 

The use of benzyl alcohol as solvent produces the dibenzyl acetal at C-21, and this can be reductively removed to produce the 21-hydroxy compound... 

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