Tributylstannane reduction

A related convenient and mild method for the preparation of aliphatic and aromatic aldehydes in high yield from carboxylic acids is the reductive cleavage of l-acyl-3-methylimidazole-2-thiones by tributylstannane. 

In order to establish the correct absolute stereochemistry in cyclopentanoid 123 (Scheme 10.11), a chirality transfer strategy was employed with aldehyde 117, obtained from (S)-(-)-limonene (Scheme 10.11). A modified procedure for the conversion of (S)-(-)-limonene to cyclopentene 117 (58 % from limonene) was used [58], and aldehyde 117 was reduced with diisobutylaluminium hydride (DIBAL) (quant.) and alkylated to provide tributylstannane ether 118. This compound underwent a Still-Wittig rearrangement upon treatment with n-butyl lithium (n-BuLi) to yield 119 (75 %, two steps) [59]. The extent to which the chirality transfer was successful was deemed quantitative on the basis of conversion of alcohol 119 to its (+)-(9-methyI mande I ic acid ester and subsequent analysis of optical purity. The ozonolysis (70 %) of 119, protection of the free alcohol as the silyl ether (85 %), and reduction of the ketone with DIBAL (quant.) gave alcohol 120. Elimination of the alcohol in 120 with phosphorus oxychloride-pyridine... 

Geminal dihalides undergo partial or total reduction. The latter can be achieved by catalytic hydrogenation over platinum oxide [512], palladium [512] or Raney nickel [63, 512], Both partial and total reduction can be accomplished with lithium aluminum hydride [513], with sodium bis(2-meth-oxyethoxy)aluminum hydride [514], with tributylstannane [503, 514], electro-lytically [515], with sodium in alcohol [516] and with chromous sulfate [193, 197]. For partial reduction only, sodium arsenite [220] or sodium sulfite [254] are used. 

Chemical reduction of aromatic aldehydes to alcohols was accomplished with lithium aluminum hydride [5i], alane [770], lithium borohydride [750], sodium borohydride [757], sodium trimethoxyborohydride [99], tetrabutylam-monium borohydride [777], tetrabutylammonium cyanoborohydride [757], B-3-pinanyl-9-borabicyclo[3.3.1]nonane [709], tributylstannane [756], diphenylstan-nane [114], sodium dithionite [262], isopropyl alcohol [755], formaldehyde (crossed Cannizzaro reaction) [i7i] and others. 

Further research by the same group [229] involved conversion of the lactosamine derivative (337) into the 2 -deoxy- (338) or 3-deoxy- (339) derivatives by conversion of the corresponding alcohols into the phenylthiocarbonate derivatives followed by reduction with tributylstannane. 

Hi) Reduction of Glycosyl Halides with Radical Rearrangement. The reduction of acetylated or benzoylated halides or selenides with a low concentration of tributylstannane leads to 2-deoxy sugar derivatives (Scheme 11). The driving force of this radical reaction is the 1,2-cA-selective migration of an ester group because of the stabilization of the radical at C-2.46... 

Sometimes special structural features render possible a reduction of the porphyrin 7r-system. Chlorins 98a were obtained from the Cu complex 97 by nucleophilic addition to a peripheral double bond activated by a nitro group. Reductive denitration occurred on treatment of 98a with tributylstannane. The chlorins 98b can be easily transformed into alkoxyporphyrins 99, which are enol ethers of oxohydroporphyrins and are treated in Section IV,C (84CC1537). 

Reductive cleavage of Aloe groups can be accomplished under Pd 0)-catalysis using various hydride donors such as formic acid,190 tributylstannane,191 phenyl-silane,192 sodium borohydride193 or various borane-amine complexes.194 The reaction gives metal carbamate derivatives and propene as the primary products With tributylstannane the reaction is usually conducted in the presence of acetic... 

In reduction of arenediazonium fluoroborate with tributylstannane, formation of the arene takes place not only by hydrogen atom abstraction from the stannane, but also by reaction of the radical interm iate with the solvent. Reduction of 4-methoxybenzenediazonium fluoroborate with tributyltin deuteride in te-trahydrofuran or acetonitrile affords not only deuterated but also deuterium-free arenes (equation 89). There are also small amounts of biphenyl by-products. ... 

For diphosphiranes belonging to series 3, the exchange reactions of P-chloro derivatives with halosilanes lead to the corresponding P-bromo- and P-iododiphosphiranes <90CB1245>. In the same diphosphirane series, the reduction of the P-chlorodiphosphirane to P—H analogous to the tributylstannane method was reported (Equation (6)) <93CB645>. 

The reduction by tributylstannane is a chain reaction which must be initiated by azaisobutyronitrile. The overall result is given by reaction (5.21). The tin product decomposes rapidly and an excellent yield of the reduced product is obtained. 

Primary tosylates are reduced by LiAlH4. The diacetal of D-fucose 5.39, an enantiomer of L-fucose, is thus prepared by reduction of tosylate 5.38 derived from the diacetal of D-galactose 5.11. A hydroxyl group can also be replaced by a halogen (Cl, Br, I) at any position of a protected sugar. The halide is easily reduced by a radical mechanism with tributylstannane, Bu3SnH (reaction 5.19). 

Deoxykanamycin B has also been prepared by reduction of the corresponding dichloro derivative with tributylstannane, showing improved activity against Staphylococcus epidermidis ATCC 12228 and Klebsiella pneumoniae 602. It was slightly more active than kanamycin B against strains of S. aureus Apo-1, K. pneu-... 

A further use of Barton esters has been described as a path to enol ether radicals. The reaction involves the photochemical decomposition at 355 nm of the derivative (85). As part of an approach to the synthesis of a series of Kopsia alkaloids, the reductive decarboxylation of the derivative (86) was carried out. This involved irradiation of the Barton ester (86a) in the presence of t-BuSH. This affords the product (86b). The photochemical decomposition of the Barton ester (87) provides a path to the silyl derivatives (88). The nature of the trapping agent X is dependent on the conditions under which the reaction is carried out. Thus a variety of derivatives can be obtained using alcohols to afford ethers, or using ethanesulfonyl azide to give azides. The Barton esters (89) undergo the usual photochemical decarboxylation to afford ethenoyloxy radicals. Cyclization within these, in the presence of tributylstannane yields the lactones (90). ... 

The same issue is addressed in Table 8, in which reactions of nonracemic substrates are described. Regioselectivity is under steric control. All four acetates 49-52 provide mainly the ( -enantiomer82-83, probably through a common syn,syrt-n-a] yl complex. In most cases, almost 100% chirality transfer is observed. The a-isocyanocarboxylates (Table 8, entries 9, 10)84 are interesting nucleophiles for the preparation of amino acids easily obtained from the alkylated products after acidic hydrolysis. This sequence constitutes an alternative to the Schiff bases of glycine already discussed (see Table 4). Moreover, radical reduction of the isocyano group with tributylstannane yields the a-unsubstituted carboxylic esters. 

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