The reductive decyanation reaction : chemical methods and synthetic applications

Several sequences involving the nitrile group require the removal of this function in a convenient and clean procedure. This transformation, called reductive decyanation, has received much attention over the years. In this review, we describe the chemical methods leading to reductive decyanation as well as the proposed mechanisms. Much of these methodologies proceed with good yields and stereoselectivities allowing their exploitation in organic synthesis


Introduction
2][3][4] Especially, deprotonation of the α-carbon and alkylation is an important reaction. 5The sequences involving the nitrile function usually requires the subsequent removal of the nitrile group in a convenient and clean manner.The reduction of the nitrile group (RCN) is observed with many reducing agents.Depending on the nature of the reducing agent and experimental conditions, the reaction can produce amines, aldehydes, primary alcohol, imines or alkanes (RCH 3 or RH). 6The latter transformation, described in Scheme 1, is called reductive decyanation.

Metal dissolving conditions
2][13] The mechanism proposed involves a two electrons transfer with formation of a carbanion via a radical anion.The latter is then protonated by ammonia in situ or by another proton donor depending on the reaction conditions (Scheme 2).

Scheme 2
From the mechanism described above, the stereochemical outcome of these reductions should depend on the behavior of radical and carbanion intermediates as well as on the approach of the proton or, possibly, an hydrogen atom donor.More specific studies show that experimental conditions, additives, nature of the metal play an important role. 14,15,17,18Good stereoselectivities are obtained in many cases.

Scheme 3
During the synthesis of the Galbulimima alkaloid GB 13 7, the nitrile group appears as a dienophile activating group in 4 and is removed from 5 with lithium in NH 3 /THF.Addition of ethanol resulted in the Birch reduction of the aromatic ring (Scheme 4).The drawback of such reaction conditions is that primary and secondary nitriles give the expected amine as well as decyanation products. 11,12,35Several methods were developed to improve the yields.It is possible to break the C-CN bond in good yields in HMPA (hexamethylphosphoramide) with K in the presence of tert-butyl alcohol as protic cosolvent [36][37][38] or with high-surface-area potassium on neutral alumina (K/Al 2 O 3 ) in hexane. 39The radical anion generated from potassium metal/dicyclohexano-18-crown-6/toluene system is also highly effective for the reductive decyanation of primary, secondary and tertiary cyanides. 40,41This method was used in the preparation of ent-cholesterol 9 after alkylation at the α-carbon (Scheme 5).

Scheme 5
The cyano group appears to be a good activating group for dienophiles like 4,4disubstituted 2-cyano-2,5-cyclohexadienones.Lithium naphthalenide was used as electron donor to remove the cyano group of Diels-Alder adducts providing an approach to the angularly substituted cis-decalin system. 43Scheme 6 describes the reaction of cyclohexanone 10 with penta-1,3-diene.The reduction of adduct 11 with lithium naphthalenide followed by protonation using methanol gives 12.

Scheme 6
It is noteworthy that the ensuring organolithium can be trapped, in the absence of a proton source, by an alkylating agent.In the same way, the reaction of different nitriles with lithium and a catalytic amount of 4,4'-di-tert-butylbiphenyl (LiDBB : lithium di-tert-butylbiphenylide) in THF at low temperature (-30 or -78°C) generates an organolithium intermediate resulting from a reductive decyanation.In the presence of an electrophile (Barbier-type reaction conditions), the coupling product is obtained. 44A previous review focused on such non-deprotonating methodologies for obtaining organolithium reagents starting from non-halogenated materials. 45][48] The reduction of α-aminonitriles with metallic borohydrides leads to decyanation and proceeds through dissociation to iminium and cyanide ions (see section 2-1).Yamada et al. investigated the reductive decyanation of α-aminonitriles with sodium in liquid ammonia-THF. 49n this case, the hydride reduction was unsuccessful probably due to the failure to form the corresponding iminium ion.The mechanism of decyanation of α-aminonitriles with alkali metals is probably similar to the one described in Scheme 2 and does not involve an iminium ion.Initial electron transfer leads to a radical anion followed by C-CN bond cleavage and formation of a radical.1][52] Both α-amino radicals 53 and carbanions were trapped by intramolecular cyclisation. 54,55tudies have shown that reductive decyanation of α-aminonitriles proceeds with retention of configuration [56][57][58][59][60] or leads to the thermodynamically favored product. 52,61The decyanation of α-aminonitriles with the Na/NH 3 system was used to perform the decyanation of 2-oxoazetidine-4-carbonitriles 62 or during the asymmetric synthesis of natural ant defense alkaloids. 60,63,64cheme 7 describes the two last steps of the preparation of (+)-tetraponerine-8 15.Alkylation of the α-aminonitrile 13 leads to a single isomer 14.The stereoselective decyanation using sodium in liquid ammonia yields exclusively all-cis tetraponerine-8 15.Scheme 7

Alumino-and borohydrides
6][67][68][69][70][71][72] The mechanism is a S N 1 process with formation of an iminium ion followed by an addition of the hydride ion (Scheme 8). 73Several facts support this mechanism.The LiAlD 4 reduction leads to the α-deuterated amine.Formation of the iminium ions favors also the decyanation over the reduction to primary amine.So Lewis acids, such as AlCl 3 , are able to promote decyanation in unfavorable cases. 73Moreover, the decyanation pathway is favored with disubstituted α-aminonitriles. 74

Scheme 8
The reaction product also depends on the stereoelectronic effects and the strain of the molecule. 73,75So the stereoelectronic effects are expected to favor the decyanation process when the nitrile group is antiparallel to the lone-pair electrons of nitrogen. 76n anomalous decyanation of monosubstituted nitrile 16 has been observed (Scheme 9). 77Anion 17 is first formed and expels the cyanide ion.The resulting intermediate 18 is then reduced to the decyanation product 19.Excellent yields are obtained.When the H atom borne by the nitrogen is replaced by a methyl group the expected primary amine is obtained.Liu and Liang used the Stevens rearrangement for ring enlargement and reductive decyanation to prepare benzazepine derivatives such as 22. 78 Cyanomethyl quaternary ammonium salt 20 undergoes ring enlargement and affords 21 as a mixture of diastereoisomers (syn : trans = 9 : 1).

Scheme 12
It is important to notice that α-aminonitriles appears as "masked" iminium ions and the reaction of Grignard reagents appears as a valuable method for carbon -carbon bond formation (Bruylants reaction). 91This method has been widely used in organic synthesis. 63,75,85,88,89

Other cases
Alumino-and borohydrides promote reductive decyanation for substrates other than αaminonitriles.Potassium hydride was also described but the yield obtained was poor. 92Black and Doyle found that the treatment of 9-allylfluorene-9-carbonitrile 28 and 2,2,4-triphenylpent-3enenitrile 31 with LiAlH 4 in Et 2 O (slurry) yield the decyanation product together with the expected amine (Scheme 13).

Scheme 13
In contrast, no decyanation was observed in reactions of 2,2-diphenylpent-4-enenitrile 32 and similar compounds. 93The authors proposed an attack on the cyano group followed by elimination of a "hydrogen cyanide complex" and formation of a carbanion : the nitriles yielding the decyanation product are usually precursors of the most stabilized carbanions.No mechanism was proposed, 93 Scheme 14 could account for the reported products:

Scheme 14
We investigated the LiAlH 4 reduction of 2,2-diphenylpropionitrile 33 (Scheme 15). 94The solvents THF or HMPA favor the decyanation process in comparison with Et 2 O.We suggested that, in Et 2 O, the imine salt intermediate is stabilized by the cation Li + while in more basic and dissociative medium the C-C bond cleavage is favored.The LiAlD 4 reduction leads to the deuterated hydrocarbon and could be consistent with an "internal" protonation in a solvent cage.There was no evidence for radical intermediates in this reaction.We deduced that our observations were consistent with the mechanism proposed by Black and Doyle.However a S N pathway cannot be totally excluded since carbon atoms bearing two or three aryl groups exhibit a great reactivity toward the S N 1 and S N 2 processes. 95 H

Scheme 15
An anomalous reduction of α-tosyloxy-α-(trifluoromethyl)phenylacetonitrile 36 with NaBH 4 in DMSO was described by Németh et al. 96 Since S N was disfavored by steric (S N 2) and electronic (S N 1) factors, the authors proposed a single electron-transfer (SET) process (Scheme 16).

Scheme 17
Later, experiments based on a free radical-clock and isotopic labelling suggested the intervention of radical intermediates. 97Electrochemical data were not consistent with an electron transfer from the hydride (E °H°/H -≈ -0.2 V vs NHE) to the nitrile 38 (E p < -1.96 V vs NHE for 38).We realized, however, that this kind of trapping experiment, performed in heterogeneous medium, was not compelling.Independently, Ashby studied the hydride reduction of hindered alkyl iodides.He concluded that unfiltered heterogenous solutions of LiAlH 4 or LiAlD 4 lead to inconsistent results. 98We are now working on a series of substituted 2-phenylsulfonylnitriles in order to obtain a better reproducibility.
The mechanism of the reductive decyanation promoted by LiAlH 4 for substrates other than α-aminonitriles remains a puzzle.Different pathways have been proposed depending on the nature of the substrate.It is noteworthy that all substrates undergoing the decyanation reaction are α-substituted by a carbanion stabilizing group.The addition-elimination process first proposed by Black and Doyle would be similar to the mechanism proposed for reductive decyanation induced by the hydroxide ion or organometallic reagents (see sections 3 and 5-1).

Acid or base promoted decyanation reactions
The decyanation reaction is also observed under harsh conditions.Old methods describe sodamide in boiling toluene 99 or polyphosphoric acid at 200 °C. 100Berkoff et al. have developed the reductive decyanation of sufficiently activated tertiary and secondary nitriles using alkali fusion (150 °C, KOH 85%, 4 eq.). 101They proposed a process involving the initial attack of hydroxide ion on the nitrile carbon followed by a C-C bond cleavage.Trapping of potassium cyanate supports this mechanism (Scheme 18).More recently, Bendale et al. reported the reductive decyanation of alkyldiphenylacetonitriles using sodium hydroxide-PEGs in a domestic microwave oven. 102EGs exhibit a crown ether role by complexing Na + and with microwave irradiation high yields are obtained in just 2-3 minutes.The mechanism proposed was similar to that described by Berkoff.Using this methodology 4,4-diphenylbutylamines 46, reported to be antilipidperoxidative, neuroleptic and antihypertensive agents, were prepared.Nitriles 44 are obtained from diphenylacetonitrile by alkylation under phase transfer catalysis conditions (PTC).The next step is the substitution of the remaining halogen atom by a secondary amine leading to 45 (Scheme 19).Milart et al. prepared a series of di-or triamino-m-terphenyls as described in Scheme 20.Gewald's method was applied for the preparation of 47. 103 The cyano groups were removed from 47 under harsh basic conditions.The authors consider that treatment with ethanolic sodium hydroxyde solution at high pressure (2.5 MPa) and temperature (220°C) may cause the conversion of the nitrile groups to anions of the corresponding carboxylic acids (48)

Scheme 20
Under severe acidic conditions, the reductive decyanation is equally observed with good yields. 104The nitrile group is probably hydrolyzed and then decarboxylated.Such conditions have been employed in the course of the synthesis of benzo[b]thiophenes.The method is described in Scheme 21.Benzoannelation is developed via conjugate addition-displacement on β,β-(dimethylthio)ketone 50 by carbanion derived from 2-cyanomethylthiophene and is followed by acid induced cyclisation of 51.The nitrile group is efficiently removed using a mixture of sulfuric acid, acetic acid and water at 180 °C (Scheme 21).

Bu 3 SnH or SmI 2 promoted decyanation reactions
Curran and Seong discovered accidentally that mono-and dialkylated malonitriles (geminal dinitriles) are reductively decyanated to mononitriles on treatment with tributyltin hyride and a small amount of AIBN in refluxing benzene. 108They proposed a chain mechanism to account for this reduction, the first step being the addition of the tin radical to the carbon-nitrogen triple bond to generate a carbon radical species (Scheme 25).So far, the reaction is limited to geminal dinitriles.Attempts to reduce 2-cyanoacetate under the same conditions gave no reaction.The first radical adduct is apparently generated, but its fragmentation is not fast enough with respect to the reverse addition to propagate the chain.Thus, the fragmentation of the carbon-carbon bond is facilitated by the ability of the nitrile group to labilize adjacent bonds.

Scheme 25
A variety of mono-and dialkylated malonitriles were reduced with good yields (75-94%) in this way.The α-nitrile radical was trapped by cyclization using the 5-hexenyl rearrangement.The rate constant of cyclization of the unsubstituted 5-hexenyl radical is k = 2 10 5 s -1 at 20 °C.Substitution of the radical with the cyano group should decrease this constant of approximately two orders of magnitude. 109

Scheme 26
Gerlach developed the synthesis of tricyclic cyano substituted tetrahydroquinolines 70 in good yields from the geminal dinitriles 69 110 by radical decyanation using Bu 3 SnH (Scheme 27).

Scheme 27
Hattori and Grossman used this method to prepare the fused bicyclic α-amino acid 74 that lacked a quaternary center at the ring junction. 111Tethered diacid 71 undergoes a double Michael addition with the alkynone and affords 72.This cyclic geminal dinitrile was treated with tributyltin hydride to afford mononitrile 73 in 59% yield (5:1 mixture of diastereomers, Scheme 28).The reaction of the intermediate decyanated radical with tributyltin hydride apparently occurred predominantly from the less hindered face to give the higher energy cis isomer of the product.

Scheme 28
It is worth noting that the reductive decyanation on mononitrile 75 promoted by tributyltin hydride proposed by Rychnovsky and Swenson is, in fact, a nitrile transfer reaction. 112The reaction proceeds by a mechanism of cyclization followed by β-scission affording the 1,4-or 1,5-cyano transfer, when n = 1, 2, respectively (formation of 77).When the cyclization is not allowed (n = 0, 3), the nitrile transfer reaction is not observed and the simple dehalogenation products 76 are formed (Scheme 29).

Scheme 29
Kang et al. proposed an alternative samarium (II) iodide (2-3 eq., in THF/HMPA, 5-10% v/v versus THF, 0°C) promoted reductive decyanation of malonitriles. 113,114Whithout HMPA no decyanation reaction was observed.The reaction was successfully applied to α-ethoxycarbonyl substituted nitriles at room temperature and extended to a range of substrates with an hydroxyl group or a carbon-carbon double bond located on the side chain (Scheme 30, entries 1-7).The mechanism could be similar to the tin hydride method, however, no cyclization products were observed when the side chain has a C=C bond.

Scheme 30
With this method, Molander and Wolfe reduced also a serie of α-cyano esters and show that nitriles, amides and chlorides are well-tolerated (Scheme 30, entries 8-11).The reaction conditions were similar to these described by Kang (two equivalents of ethanol were added). 115n a review, Kamochi and Kudo described the rapid reduction of a variety of organic functionalities including decyanation with SmI 2 . 116

Organometallic promoted decyanation reactions
There are literature reports on the decyanation of tertiary nitriles by organometallic reagents. 117,118In 1953, Blicke and Tsao studied the action of several Grignard reagents on nitriles. 119Diphenylacetonitrile reacts with ethylmagnesium bromide in refluxing toluene giving the evolution of about one equivalent of ethane.After acid hydrolysis, 1,1-diphenyl-2-butanone (20%) and diphenylmethane (45%) were found.When the reaction mixture was carbonated, diphenylmalonic acid (55%) was found together with 1,1-diphenyl-2-butanone (5%).The nitriles were difficult to isolate but propionic acid was isolated after hydrolysis.Similar results were obtained with other Grignard reagents but the yield of diphenylmethane was much lower.The reaction scheme 31 may explain the formed products.The first step seems to be a nucleophilic addition of the Grignard reagent on the nitrile group, followed by the elimination of propionitrile and formation of diphenylmethylmagnesium bromide.The same process can occur after an initial deprotonation of diphenylacetonitrile.

Scheme 31
King et al., studying the decyanation of 9-cyanofluorenes promoted by ethylmagnesium bromide in refluxing xylene, proposed a simple general S N 2 mechanism to this reaction. 120ulp and Romanelli extended this method to alkyldiphenylacetonitriles and formed decyanation products using either methyllithium or phenyllithium as reagents. 121They also observed the formation of the decyanation product when treating butyldiphenylacetonitrile with phenylmagnesium bromide at a reaction temperature rising to 180°C.
During the synthesis of imidazoles next to a quaternary carbon center, Gregory et al. treated tertiary nitriles 80 by methylmagnesium bromide and obtained the desired ketone.But when alkyllithiums in ether solvents were tried as an alternative to Grignard reagents, both ketone and decyanation product 82 were obtained.With methyllithium or butyllithium, in THF, yields of decyanation are quantitative. 122The authors explained the formation of decyanation products through a 4-membered transition state 81, the collapse of which is accompanied by a hydride ion abstraction from the alkyl substituent R by the incipient benzylic carbonium ion at the 1-position of the tetrahydronaphthalene nucleus (Scheme 32).The formation of a monodeuterated compound as the sole decyanation product in an experiment with CD 3 Li, but not when the reaction was quenched in THF with DCl supports this mechanism.Formation and fragmentation of a 4-centered transition state in THF probably occurs because THF is a stronger Lewis base than diethyl ether or benzene and is better to solvate the lithium cation.Marr et al. proposed a similar mechanism for decyanation of ferrocenyl-α,α-dibenzylmethylcyanide induced by n-BuLi. 117An electron transfer pathway was also proposed for this kind of decyanation.

Raney nickel or hydrogenation promoted decyanation reactions
A Raney nickel mediated decyanation was used to modify piperidine scaffolds containing the chiral, non racemic N-(cyanomethyl)oxazolidine system.Alkylation of the commercialy available 86 is followed by treatment with Raney nickel. 126,127Compounds 88 are obtained as a mixture of two epimers at the C-6 position without any racemisation at the C-2 methyl group.The authors proposed the formation of a prochiral iminium ion which could be explained in terms of a "push pull" mechanism.The delivery of the hydrogen present in Raney nickel is the result of a stereoelectronically controlled process. 86The oxazolidine epimerization probably occurs during workup of the reaction mixture.

Scheme 34
Albaneze-Walker et al. have developed a mild and robust protocol for the deoxygenation/ decyanation of pyrazine carbonitrile-N-oxides such as 89.These transformations occur by hydrogenation with platinium on carbon in the presence of activated carbon under acidic conditions. 125The generality of this decyanation was explored and good yields were obtained (Scheme 35).

Hydrolytic decyanation
The hydrolytic decyanation of α-cyano-γ-lactones 91 with wet alumina in a variety of solvents affords the corresponding γ-lactones 92.The transformation occurs under neutral conditions and without the need for an aqueous workup.We report here yields obtained in THF with 1.9 g of alumina per mmol of substrate.The proposed mechanism involves the initial formation of ketenimines which react with water to form successively ketenes and carboxylic acids.The later subsequently decarboxylate to yield γ-lactones (Scheme 36).

Electrochemical cleavage of nitriles
The use of electrochemically generated electrons was first reported in 1968 by Arapakos and Scott. 129Decyanation of dehydroabietonitrile, cycloheptyl nitrile and octanenitrile, in anhydrous amine media, produces dehydroabietene, cycloheptane and n-heptane, respectively, in 60-80% yields.They showed that the electrolytic generation of solvated electrons provides a reductive selectivity with sensitive nitriles and avoids the side reactions encountered with the chemical methods of generating solvated electrons, for example, the competitive reduction of some primary and secondary nitriles to their corresponding amines.This method was used to perform reductive decyanation during the total synthesis of (±)-Hirsutene 95 (Scheme 37).

Scheme 37
Shono et al. reported the electroreductive decyanation of a great range of nitriles with a Zn cathode in DMF containing Et 4 NOTs as a supporting electrolyte. 130The yield of the reaction is highly influenced by electrolytes, electrode material and solvents but the method is applicable to epoxy, α-alkoxy or α-amino nitriles.As a mechanism, they proposed the dissociation of the radical anion formed by a first electron transfer, the radical formed being reduced by a second electron and then protonated (see Scheme 2, M = zinc electrode) Nevertheless a two-electron reduction yielding the corresponding dianion was proposed in the electrochemical reduction of benzenedicarbonitriles. 131

Transition metals promoted decyanation reactions
In representative cases, the reductive decyanation of alkyl nitriles was carried out in 58-100% yields by the action of Fe(acac) 3 -Na in benzene at room temperature (acac = C 5 H 7 O 2 , acetylacetonato). 132More recently, the carbocyclization reaction of o-iodophenyl ketones 96 with acrylonitrile in the presence of cobalt catalyst and Zn power in acetonitrile at 80 °C afforded indene derivatives 97 in good yields.Interestlingly, no cyano group was present in the formed products (Scheme 38).The reason for this reductive decyanation remains unclear. 133The reductive decyanation of a pyrazinecarbonitrile or pyridinecarbonitriles by titanium (III) chloride was also reported. 125,134

Photoinduced decyanations
][137][138] Homma and Yamada reported that irradiation of phenylacetonitrile and its derivatives 98 in the presence of triethylamine give α-benzylated triethylamine 99, bibenzyl 100 and toluene 101 (decyanation product, 16-67% yields based on the substrate consumed) derivatives (Scheme 39); the formation of these products is explained in terms of a benzylic intermediate radical formed by electron transfer between the triethylamine and the substrate, followed by elimination of a cyanide anion from the radical anion of the starting compound. 139

Conclusions
This review describes a number of methods to perform reductive decyanation.A large variety of procedures are available and much of them proceeds with good yields and stereoselectivities allowing their exploitation in organic synthesis.The most popular one is probably the reductive decyanation using metal dissolving conditions.This methodology was first restricted to tertiary nitriles, poor yields were obtained with primary and secondary nitriles.Improvements using HMPA with a protic cosolvent or the K/dicyclohexano-18-crown-6/toluene system allow decyanation of all classes of nitriles with good yields.The hydride reduction of α-aminonitriles is also a well-explored reaction.The success of the decyanation process depends on the substitution pattern of the α-aminonitrile, the stereoelectronic effects and the strain of the molecule.Factors promoting the formation of the iminium ion also favored the decyanation over the reduction to diamine.Acid or basic promoted decyanation usually requires harsh conditions, nevertheless, several applications are now described.The reduction under radical conditions was first restricted to geminal dinitriles.Extension to α-cyanoesters using SmI 2 could open new perspectives.
Different mechanisms were proposed depending on the reducing agent used : electron transfer (alkali metals), radical chain (Bu 3 SnH) reaction or S N 1 reaction (metal hydrides with αaminonitriles).Sometimes the mechanism involved remains a puzzle.Thus, the mechanism of decyanation of substrates other than α-aminonitriles induced by LiAlH 4 is not yet totally clarified. 94,97Several pathways were proposed with organometallic reagents, a 4-centered transition state, an addition-elimination process or an electron transfer mechanism.Similarly, the decyanation induced in basic medium could proceed through an addition-elimination process or a sequence involving hydrolysis and decarboxylation.

They are then reduced with NaBH 4 in ethanol to give 22. ISSN 1424-6376 Page 97
58,79,80itriles can also be decyanated with other hydride donors such as AgBF 4 /Zn(BH 4 ) 2 ,58,79,80 90r instance the reduction of substituted α-aminonitrile 26 led exclusively to cyclopenta[c]quinoline derivative 27 (Scheme 12).90 bicyclo[2.2.1]hept-5-ene-2-carbonitrile 38, a mixture of the expected amine 39 and the decyanated product 40 was obtained in an approximate 1-1 ratio (Scheme 17).The mass balance was 88%.The stereochemical outcome of this reaction was uncertain since inconsistent results were obtained from this point of view. ARKAT(isopropylsulfonyl) 107verdov et al. managed to reduce selectively one cyano group in position 2 of tetracyanocyclohexanones 54.This reaction is explained by intramolecular interaction of the cyano group with the carbonyl group.On heating with 5% H 2 SO 4 , hydroxy compounds 55 were isolated.More severe conditions (30% H 2 SO 4 ) led to the formation of tricyanocyclohexanones 58.Intermediates 56 are formed by retro-cyclisation of 55.Hydrolysis of amides 56 and subsequent decarboxylation of the corresponding carboxylic acids would lead to the final products (Scheme 22).105They investigated the reaction of cyanomethyllithium with optically active sulfoxide 59 and obtained enaminonitrile 60.The latter is treated under reflux optimally with phosphoric acid in acetic acid, containing a small amount of water, to afford the decyanated compound 62 with 93% yield and over 99% ee (Scheme 23).In the last step, they heat the precursor 64 in H 2 SO 4 with a focused microwave reactor.The transformation involves deprotection of the nitrogen group and decyanation of the thiazole moiety (Scheme 24).107 ARKATAlexandre et al. prepared thiazoloquinazolinones derivatives such as 65.
When 66 is reduced, the cyclized product 68 is formed together with 67 (Scheme 26).

Sodium dithionite promoted decyanation reactions
125Uridine 84 was prepared in high yield from 5-cyanouridine 83 by the action of sodium dithionite in aqueous sodium hydrogen carbonate.The reaction presumably involves initial reduction of 83 into 85 followed by a base catalysed elimination of hydrogen cyanide (Scheme 33).124Similar conditions were used for the decyanation of a pyrazinecarbonitrile but the reaction was not entirely reliable.125