N-Butyl tin hydride

The synthesis of the key intermediate aldehyde 68 is outlined in Schemes 19-21. The two hydroxyls of butyne-l,4-diol (74, Scheme 19), a cheap intermediate in the industrial synthesis of THF, can be protected as 4-methoxybenzyl (PMB) ethers in 94% yield. The triple bond is then m-hydrostannylated with tri-n-butyl-tin hydride and a catalytic amount of Pd(PPh3)2Cl238 to give the vinylstannane 76 in 98 % yield. Note that the stereospecific nature of the m-hydrostannylation absolutely guarantees the correct relative stereochemistry of C-3 and C-4 in the natural product. The other partner for the Stille coupling, vinyl iodide 78, is prepared by... 

The photoreduction of cyclobutanone, cyclopentanone, and cyclohexanone by tri-n-butyl tin hydride was reported by Turro and McDaniel.<83c> Quantum yields for the formation of the corresponding alcohols were 0.01, 0.31, and 0.82, respectively. Although the results for cyclopentanone and cyclohexanone quenching were not clear-cut (deviations from linearity of the Stem-Volmer plots were noted at quencher concentrations >0.6 M), all three ketone photoreductions were quenched by 1,3-pentadiene, again indicating that triplets are involved in the photoreduction. 

Preliminary work with tri-n-butyl tin hydride and other reducing reagents has demonstrated that the addition product can be readily converted to the a,a-difluoroester. 

Essentially complete reductive dechlorination of the precursor poly(chlorofluoroethylene)s to PVF was achieved after 24 h at 60"C in tetrahydrofuran with a molar excess of tri(n-butyl)tin hydride and 1 mol% of azo6i s(isobutyronitrile) (11). The reaction mixture was homogeneous until dechlorination was almost complete, after which the mixture turned cloudy owing to the insolubility of PVF. 

The second Matsuo synthesis (233) was initiated with methyl 3-(2,2-dimethoxyethyl)-4-methyl-4-pentenoate (357) prepared (234) from carvone (239). Hydrolysis with lithium hydroxide followed by iodolactonization (I2-KI-NaHC03) and reduction with tri-n-butyl tin hydride in the presence of azobisisobutyronitrile in benzene under reflux afforded 358. The carbo-methoxy group was introduced with methyl chloroformate and LDA in quantitative yield, and reductive animation of 359 with methylamine and sodium cyanoborohydride gave the lactone 360. Heating of 360 with 10% Pd/C in decalin gave cerpegin (118) in 81% yield (Scheme 42) (233). 

Figure 1. 25-MHz 13C NMR spectrum of FVC reduced with tri-n-butyl-tin hydride (see text for details)...

R = Me, CH2Ph) with tri-n-butyl tin hydride followed by NaBHjCN reduction affords the 7-exo cyclization products (165 R = Me, CHzPh), even if in only poor to moderate yield <9ITL2829>. In a similar reaction, 2,2-dichloro-iV-methyl-iV-(o-allylphenyl)-acetamide furnishes the 1-exo product, 4-methyl-2,3,4,5-tetrahydro-l/7-benz[Z)]azepine (166) in 49% yield <91JCS(P1)353>. The established photo-induced cyclization of chloracetamides has been applied to a synthesis of the azepino[3,4,5-c,ring system isomeric to that of the clavicipitic acids <88SC671>. 

The ring expansion of substituted piperidines by means of free-radical-induced rearrangements has been used to prepare fully saturated azepines <91T4847>. Ethyl l-benzyl-3-oxo-4-phenyl-selenomethylpiperidine-4-carboxylate (259) is converted into the azepinone (260) by tri-n-butyl tin hydride in benzene under reflux in the presence of AIBN (Equation (21)). The reaction works also with ethyl 3-phenylselenomethyl-4-oxo-piperidine-3-carboxylate, preferably with the benzyl group replaced by a trityl group. 

Methodology similar to Pschorr and Motherwell reactions, was employed in the synthesis of phenanthrene-type structures, where an intramolecular free-radical arylation was accomplished by the reaction of bromo-c/s-stilbene 428 with tri-n-butyl tin hydride, giving 429 in 85% yield [6], respectively. Scheme 4. 

Most of the elegant synthetic studies of Ley and co-workers involve either additions to the enol-ether double bond, or reduction of this bond, giving dihydroazadirachtin (42) prior to any other synthetic steps. Thus, acetic acid (but no other carboxylic acids) can be added across the double bond to give 44 as a 2 1 (a b) mixture 46, 47). Addition of bromine in an alcohol gives the bromo ethers, 45, 46, or 47 as C-23 epimers 48, 49). Tri-n-butyl tin hydride reduction of the bromo ethers affords the ethers, 48, 49, and 50. The natural product 22,23-dihydro-P-methoxyazadirachtin (51) was synthesized by separating the P-isomer from the mixture 45 prior to reduction. 

Any concern that the displacement product originates in an excited state reaction or in competitive photolysis of the S-R bond is mitigated by the observation that reaction of iodosulfides (I) with tri-n-butyltin radicals, via the BusSnH/BusSn chain, also gives the reduction and displacement products (II and III) the distribution of products is a function of the concentration of the hydrogen donor, tri-n-butyl-tin hydride (Table III). ... 

When (519) is reduced with tri(n-butyl)tin hydride, one product is that of an allylmethyl-cyclopropylmethyl radical rearrangement. Reduction of (520) and (521) clearly shows that a bicyclic cyclopropylmethyl radical undergoes preferential opening of an external bond to give the (nominally) primary allylmethyl radical (Scheme 71). This is in agreement with earlier results. 

It was reported last year that the stereospecificity of tri(n-butyl)tin hydride reduction of cyclopropyl halides is dependent upon the other substituent present at the reaction site, i.e. the ability of the substituent to stabilize the radical or affect its geometry. Norcarane derivatives (618) have been made by standard methods and reduced with HSnBu3 under a variety of experimental... 

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