Acetals, acid catalyzed reduction

Clemmensen-type reduction.1 Aromatic ketones can be reduced to the corresponding methylene compounds with ammonium formate on transfer hydrogenation in acetic acid catalyzed by 10% Pd/C. The reduction is usually complete in 10-30 minutes at 110°. Halo and nitro substituents can be reduced under these conditions, and a,p-unsaturated carbonyl groups are reduced to saturated carbonyl groups. 

Cyclic diene ether 93 underwent oxidative acetalization to produce corresponding 3-substituted acetals 100 and 101 (Scheme 17) <1995TL8263>. Further Lewis acid-catalyzed reduction with triethylsilane afforded corresponding 3-bromo- and 3-hydroxy-oxonenes (102 R = Br (68%) 103 R = OH (49%), respectively) together with 1 1 diastereomeric mixture of acyclic methyl ethers 104 (R = Br (18%) R = OH (13%)). 

Solomon et al. [203] developed a technique allowing the terminal aminoxyl to be replaced by a hydroxyl function. With this aim, they reacted the terminal aminoxyl containing oligomer with acetic acid catalyzed by zinc. Pradel et al. [218] achieved the synthesis of hydroxy-telechelic polybutadiene by applying the methodology of Solomon et al. to a-hydroxyl/o-aminoxyl polybutadiene (the synthesis was presented earlier) at 80 °C. After 2 h, they obtained a quantitative reduction of the aminoxyl functions evidenced by ll NMR (Scheme 38). The average hydroxyl functionality of oligobutadiene was 2.06. 

Pyrrole 38 was generated from nitrobenzene and a 1,4-diketone with indium metal in acetic acid. The reductive conditions convert nitrobenzene to aniline in situ cyclization then occurs with the dione in moderate to excellent yield (40-98%, 21 examples) (13T6698). A Pd(OCOCF3)2-catalyzed cascade furnished tetrasubstituted pyrrole 39 from 1,3-dicarbonyl compounds and primary amines in good yield (13CC4667). 

We also explored an alternate synthesis of Mono(F). Figure 3 outlines the four step synthetic procedure used to prepare Mono(F). The first step consisted of the rhodium catalyzed hydrosilation reaction of tetramethyldisiloxane with one equivalent of allyloxytrimethylsilane (77). The next step consisted of the platinum catalyzed hydrosilation of the disiloxane silicone hydride intermediate with allyloxyoctafluoropentane. Both hydrosilation reactions were monitored for extent of reaction (loss of Si-H) by NMR spectroscopy. The third step in the reaction consisted of an acetic acid catalyzed deprotection of the trimethylsilyl group using a 10% solution of acetic acid in methanol. The deprotection was quantitative with no apparent degradation of the siloxane linkage. The final step consists of the reaction of the deprotected disiloxane (used as is) with methacryloyl chloride. The final purified product, as expected, is fi ee of the methacrylate reduction by-products. 

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

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. 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

In the same year, Evans and coworkers reported the electrochemical reduction of protons to H2 catalyzed by the sulfur-bridged dinuclear iron complex 25 as a hydrogenase mimic in which acetic acid was used as a proton source [201]. The proposed mechanism for this reaction is shown in Scheme 60. The reduction of 25 readily affords 25 via a one electron reduction product 25. Protonation... 

It was envisioned that hydrindanone 83 and cyclopentene 85 could be used as intermediates in the synthesis of e f-retigeranic acid A (1) and e f-retigeranic acid B (2), respectively. To prepare the building block 90, cyclopentene 85 was reduced with diimide (93 %) in order to prevent isomerization and subsequently deprotected with PPTS to yield hydrindanone 90 (quant.), which could provide access to <77/-retigeranic acid B (2) (Scheme 10.7). Hydrindanone 83 was reduced via an enol triflate and then subjected to Pd-catalyzed reduction to provide cyclopentene 91 (87 % from 83). Upon hydrogenation of 91 with Pd/C and cleavage of the acetal with iodine, protected hydrindanone 92 (95 % from 91) was obtained. The deprotection of 92 provided ent-60, whose enantiomer was used in previous syntheses of retigeranic acid A (1) by Corey [14] and Hudlicky [46, 47]. 

Hoye and Richardson have published an ingeneous synthesis of the tricyclic iridoid sarracenin (170) which relied on the Paterno-Buchi cycloaddition between acetaldehyde and cyclopentadiene as the intial step (Scheme 38)79. This reaction provided a 5 1 mixture of adducts 166a and 166b. The major adduct was opened with camphor-10-sulfonic acid (CSA) in methanol and the alcohol was tosylated to give 167. Displacement with malonate 168 and decarboalkoxylation/demethylation steps gave 169. Ozonolysis, reductive workup and acid-catalyzed acetalization then furnished 170. 

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

Crich and Rumthao reported a new synthesis of carbazomycin B using a benzeneselenol-catalyzed, stannane-mediated addition of an aryl radical to the functionalized iodocarbamate 835, followed by cyclization and dehydrogenative aromatization (622). The iodocarbamate 835 required for the key radical reaction was obtained from the nitrophenol 784 (609) (see Scheme 5.85). lodination of 784, followed by acetylation, afforded 3,4-dimethyl-6-iodo-2-methoxy-5-nitrophenyl acetate 834. Reduction of 834 with iron and ferric chloride in acetic acid, followed by reaction with methyl chloroformate, led to the iodocarbamate 835. Reaction of 835 and diphenyl diselenide in refluxing benzene with tributyltin hydride and azobisisobutyronitrile (AIBN) gave the adduct 836 in 40% yield, along with 8% of the recovered substrate and 12% of the deiodinated carbamate 837. Treatment of 836 with phenylselenenyl bromide in dichloromethane afforded the phenylselenenyltetrahydrocarbazole 838. Oxidative... 

Pattenden and Teague have prepared tricyclic diol 684 which is epimeric to the naturally occurring A < -capnellene-8p,10a-diol (68S) Their strategy, which is summarized in Scheme LXXI, encompasses two critical cyclization steps. The first is the Lewis acid-catalyzed ring closure of enol acetate 686 and the second involves reductive closure of acetylenic ketone 687. It is of interest that the oxidation of 688 proved to be stereospecific. 

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