Triethylsilane, reduction with

A variety of para-substituted 2-phenyl-2-butanols undergo quick and efficient reductions to the corresponding 2-phenylbutanes when they are dissolved in dichloromethane and a 2-10% excess of phenylmethylneopentylsilane and boron trifluoride is introduced at 0° (Eq. 30).126 Several reactions deserve mention. For example, when R = CF3, use of trifluoroacetic acid produces no hydrocarbon product, even after two hours of reaction time. In contrast, addition of boron trifluoride catalyst provides an 80% yield of product after only two minutes. When R = MeO, both trifluoroacetic acid and boron trifluoride produce a quantitative yield of the hydrocarbon within two minutes. However, when R = NO2, attempts to promote the reduction with either trifluoroacetic acid or even methanesulfonic acid fail even after reaction periods of up to eight hours, only recovered starting alcohol is obtained. Use of boron trifluoride provides a quantitative conversion into 2-(/ -nitrophenyl)butane after only ten minutes. It is significant that the normally easily reducible nitro group survives these conditions entirely intact.126129 Triethylsilane may be used as the silane.143... 

Alkyl iodides, benzyl chlorides, benzyl bromides, and adamantyl bromides and iodides undergo reduction with triethylsilane/palladium chloride.195 The reduction of a /3-chloro ether occurs in excellent yield with this system (Eq. 56).195... 

The TFA-catalyzed triethylsilane reductive condensation of an aldehyde with indoles provides a convenient route to 3-substituted indoles in modest to good yields (Eq. 197).355... 

For the reduction of aliphatic ketones to hydrocarbons several methods are available reduction with triethylsilane and boron trifluoride [772], Clemmensen reduction [160, 758] (p. 28), Wolff-Kizhner reduction [280, 281, 759] (p. 34), reduction of p-toluenesulfonylhydrazones with sodium borohydride [785], sodium cyanoborohydride [57i] or borane [786] (p. 134), desulfurization of dithioketals (jaeicaipioles) [799,823] (pp. 130,131) and electroreduction [824]. 

Reduction of the keto group in naphtho derivative 115 with sodium borohydride results in 69% of the alcohol 116 (Scheme 23, Section 2.1.3.3 (1999PHA645)). Further triethylsilane reduction gives 117 in 67% yield. Synthesis of a series of pyrrolo-benzazepine and pyrrolo-benzothiazepine acetic acids (Scheme 77, Section 5.1.1 (1994MI385)) includes reduction of ketoesters 380 into corresponding hydroxyl esters, subsequent deoxygenation with iodine/PPhs and hydrolysis. 

Bennasar et al. reported a new radical-based route for the synthesis of calothrixin B (378) (869). This synthesis starts from the 2,3-disubstituted N-Boc indole 1558 and uses a regioselective intramolecular acylation of a quinoline ring as the key step for the construction of the calothrixin pentacyclic framework. Chemoselective reaction of in s/fM-generated 3-lithio-2-bromoquinoline [from 2-bromoquinoline 1559 with LDA] with the 3-formylindole 1558 followed by triethylsilane reduction of the... 

In addition to the alkylation of benzyl alcohols with silyl enol ethers, the hydroxyl group could be removed in a reduction employing triethylsilane Et3SiH as the reductant. With 1 mol% of Bi(OTf)3 as the catalyst, the desired (5-arylester 34 could be isolated in 75% yield (Scheme 26). 

For benzo[Z ]thiophene the heterocycle is rather more resistant to ring opening and oxidation with hydrogen peroxide in acetic acid at 95 C, for example, gives the 1,1-dioxide (Scheme 7.22) reduction with either sodium and ethanol or triethylsilane in trifluoroacetic acid affords 2,3-dihydrobenzo[Z)]thiophene. Electrophiles give mainly 3-substituted benzo[Z ]thiophenes, although these products are often accompanied by smaller amounts of the 2-isomers. 

A reduction with triethylsilane, which is a pure SN1 reaction, is familiar from Figure 17.62, namely as the second step of a two-step reduction of a carboxylic acid ester to a simple ether via an oc-acetoxyether. 

If desired, the mixture of triethylsilane and triethylfluorosi-lane can be reconverted into triethylsilane by reduction with lithium aluminum hydride.5... 

A neat way of preparing96 the system (215) (useful in bufadienolide synthesis) from (214) is illustrated for compound (216). Bromination to (217) followed by dehydrobromination with lithium bromide in DMF gave the dienone (218), which on triethylsilane reduction produced (219) and thence, by condensation with diethyl oxalate, (220). Methylthiotoluene-p-sulphonate in ethanol-potassium acetate now produced (221) whose oxidation with N-chlorosuccinimide in 2% methanolic sulphuric acid gave (223). A previous route to such compounds was by way of the a-acetoxy-ketones (219) but suffers from a low yield at the acetoxylation step, (219) —> (222). 

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%)). 

Oxidative fluorodesulfurization can also be achieved by the action of nitrosonium tetrafluo-roborate, as oxidant, and hydrogen fluoridc/pyridine, as a source of fluoride ions, on aryl sulfides.249 The starting compounds are easily prepared from ketones or aldehydes and ben-zenethio) using boron trifluoride monohydrate as catalyst, and subsequent reduction with triethylsilane.249 250... 

Conversion to the Aldehyde. This transformation is accomplished through a two-step procedure. One such variant requires reduction to the alcohol (e.g. LiAllTt, H2O) and subsequent oxidation (e.g. Swem conditions). Alternatively, Wein-reb transamination " followed by Diisobutylaluminum Hydride or conversion to the thioester (see below) and subsequent Triethylsilane reduction, afford the desired aldehyde in excellent yields. Weinreb transamination proceeds with minimal endocyclic cleavage when there is a p-hydroxy moiety free for internal direction of the aluminum species. 

Reduction with Triethylsilane allows for the formation of enantiomerically pure 5-substituted pyrrolidinones and 2-substituted pyrrolidines in the same manner. ... 

Hydrogenolysis of aryl chlorides can also be achieved by treatment with triethylsilane in the presence of palladium, and by reduction with isopropyl alcohol under UV irradiation (equation 53). ... 

The conversion of oc,p-unsaturated esters into saturated esters proceeds by selective reduction with triethylsilane in the presence of Wilkinson catalyst - or by hydrogenation using 5-10% Pd/C in... 

Benzyl ethers can be prepared from THP ethers by the Me3SiOTf-catalyzed reaction with PhCHO and EtiSiH." Analogously, amides, carbamates, and ureas are A -alkylated via condensation with aldehydes followed by in situ reduction with Et SiH. The aldehyde may be replaced by a thioester that is subject to reduction in situ. Thus, a mixture of sodium triacetoxyborohydride, triethylsilane, and Pd-C catalyst is employed. ... 

Dihydroxypyrrolidinones, which can be considered as cyclic GABA derivatives, are potential nootropic drug candidates. All four possible diastereomers 794—797, as shown in Scheme 174, can be prepared from tartaric acid. Treating L-tartaric acid sequentially with acetyl chloride, methyl glycinate, and then acetyl chloride provides in 81% overall yield the C2-symmetric succinimide 790. In order not to reduce the methyl ester, the very mild treatment with sodium borohydride at —40 °C is employed to prepare the cw-hydroxylactam 791 in an 80% isolated yield. Esterification of 791 with trifiuoroacetic anhydride followed by triethylsilane reduction yields to the extent of 79% the pyrrolidinone 792. This is deprotected with sodium methoxide to provide in 97% yield (3i ,45)-3,4-dihydroxy-A -methoxy-carbonylmethyl-2-pyrrolidinone (793). Ammonolysis of 793 affords (3i ,4 S)-3,4-dihydroxy-2-oxopyrrolidine-A -acetamide (794) in 60% yield. Subsequent modifications to 793 allow for the preparation of (35, 4S)-3,4-dihydroxy-2-oxopyrrolidine-A/-acetamide (795), (3R,4R)-3,4-dihydroxy-2-oxopyrrolidine-A -acetamide (796) and (35, 47 )-3,4-dihydroxy-2-oxopyrrolidine-7V-acetamide (797) [234]. 

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