Aldehyde heteroaromatic

Silylated cyanohydrins have also been prepared via silylation of cyanohydrins themselves and by the addition of hydrogen cyanide to silyl enol ethers. Silylated cyanohydrins have proved to be quite useful in a variety of synthetic transformations, including the regiospecific protection of p-quinones, as intermediates in an efficient synthesis of a-aminomethyl alcohols, and for the preparation of ketone cyanohydrins themselves.The silylated cyanohydrins of heteroaromatic aldehydes have found extensive use as... 

Van Leusen and co-workers also demonstrated the condensation of heteroaromatic aldehydes with TosMIC. Table 6.7.1 shows the 5-heteroaryloxazoles 6 prepared in 47-88% yield in the presence of equimolar amounts of potassium carbonate in refluxing methanol. 

Hexamethylphosphorous triamide general reaction with aromatic and heteroaromatic aldehydes to give diaryl ethylene oxides, 46,... 

A large range of aromatic and heteroaromatic aldehydes, for example benzalde-hyde 462a or pyridine-2-aldehyde 462b, condense with two equivalents of N-silyl-ated dimethylamine 463, piperidine, or morpholine 294 in the presence of... 

Pentacarbonyl(l-ethoxyethyliden)chromium 1651 condenses readily with aromatic or heteroaromatic aldehydes such as benzaldehyde [35, 37] in the presence of TCS 14 and triethylamine to give chromabutadiene 1652 in 30-82% yield and HMDSO 7 (Scheme 10.16). Condensation of pentacarbonyl(l-methoxyethyli-... 

Tu found that when aniline was used instead of the secondary amine under otherwise identical conditions 2,4-diphenyl-substituted quinoline was formed in 56% yield. Phenylacetylene and aniline were initially used as model substrates for exploring the aldehyde scope. With aromatic aldehydes the reactions proceeded smoothly to give the corresponding quinolines in moderate to good yields. A heteroaromatic aldehyde is also compatible with this transformation and the expected product was afforded in 83% yield. However, when ahphatic aldehydes were subjected to the reaction, the desired product was obtained in low yield (Scheme 19) [34]. 

Until 1987, the (R)-PaHNL from almonds was the only HNL used as catalyst in the enantioselective preparation of cyanohydrins. Therefore, it was of great interest to get access to HNLs which catalyze the formation of (5 )-cyanohydrins. (5 )-SbHNL [EC 4.1.2.11], isolated from Sorghum bicolor, was the first HNL used for the preparation of (5 )-cyanohydrins. Since the substrate range of SbHNL is limited to aromatic and heteroaromatic aldehydes as substrates, other enzymes with (5 )-cyanoglycosides have been investigated as catalysts for the synthesis of (5 )-cyanohydrins. The (5 )-HNLs from cassava (Manihot esculenta, MeHNL) and from Hevea brasiliensis (HbHNL) proved to be highly promising candidates for the preparation of (5 )-cyanohydrins. Both MeHNL and HbHNL have been overexpressed successfully in Escherichia coli, Saccharomyces cerevisiae and Pichia pastoris. 

Rovis and co-workers have also extended the intermolecular Stetter reaction to inclnde nitroaUcenes as the electrophilic component. Fluorinated triazolinm precatalyst 155 was effective in catalysing the reaction of a variety of heteroaromatic aldehydes 153 with nitroalkenes 154 to generate P-nitroketones in excellent yields and enantioselectivities. The authors propose that stereoelectronically induced conformational effects on the catalyst skeleton are key to the high selectivities observed with flnorinated catalyst 155 (Scheme 12.33) [69],... 

Heteroaromatic aldehydes undergo similar enantioselective hydrophosphonylation reactions (Scheme 5-34). 

Scheme 5-34 LLB-catalyzed asymmetric hydrophosphonylation of heteroaromatic aldehydes LLB = La/Li/BINOL...

A-Acylimidazoles are easily reduced to the corresponding aldehydes with LiAlH4 in THF or ether as solvent.[1] Thus, aliphatic, conjugated aliphatic, aromatic, conjugated aromatic, and heteroaromatic aldehydes can all be obtained in this way in moderate to high yields. 

In general, a broad range of substituted aromatic and heteroaromatic aldehydes are obtained at unprecedented low catalyst concentrations in excellent yield (up to 99%). The simplicity of the reaction conditions and the practicability and usefulness of this novel method allows for the first time to obtain such reactions on industrial scale. 

A one-pot reaction of aromatic or heteroaromatic aldehydes with equimolar... 

Allylic chromium species can also add to aldehydes. In this regard, an efficient catalytic enantioselective variant using allylic halides as substrates and manganese as co-oxidant has been described recently (Eq. 117). This method provides high enantiomeric excesses in the simple allylation of a wide range of aliphatic, aromatic, and heteroaromatic aldehydes. Crotylation examples are also very enantioselective, albeit with modest anti/syn diastereoselectivity. 

In addition, heteroaromatic aldehyde such as furfural can also serve as a substrate in this reaction, giving the corresponding aldol in a moderate yield (Table 20, entry 9). Conjugated aldehydes were also good substrates (Table 20, entry 10). Aliphatic aldehydes lead to a poor yield of the aldol due to incomplete conversion (Table 20, entries 11 and 12). 

A variety of methods have been developed for the preparation of substituted benzimidazoles. Of these, one of the most traditional methods involves the condensation of an o-phenylenediamine with carboxylic acid or its derivatives. Subsequently, several improved protocols have been developed for the synthesis of benzimidazoles via the condensation of o-phenylenediamines with aldehydes in the presence of acid catalysts under various reaction conditions. However, many of these methods suffer from certain drawbacks, including longer reaction times, unsatisfactory yields, harsh reaction conditions, expensive reagents, tedious work-up procedures, co-occurrence of several side reactions, and poor selectivity. Bismuth triflate provides a handy alternative to the conventional methods. It catalyzes the reaction of mono- and disubstituted aryl 1,2-diamines with aromatic aldehydes bearing either electron-rich or electron-deficient substituents on the aromatic ring in the presence of Bi(OTf)3 (10 mol%) in water, resulting in the formation of benzimidazole [119] (Fig. 29). Furthermore, the reaction also works well with heteroaromatic aldehydes. 

A microwave-assisted Se02 oxidation of heteroaromatic methyl groups to give the corresponding heteroaromatic aldehydes has been reported <2003SC475>. 

In contrast to the large variety of aromatic, olefinic, and aliphatic aldehydes which can be used as donor substrates, wild-type BFD does not tolerate a modification of the methyl group of acetaldehyde in the case of aliphatic acceptor aldehydes. Apart from acetaldehyde, BFD shows activity with aromatic and heteroaromatic aldehydes as the acceptor substrate, forming enantiomerically pure (R)-benzoin and derivatives (Table 2.2.7.3, entries 6-8) [55]. 

From mechanistic considerations and assuming that cleavage and formation of (R)-benzoin are in equilibrium, BAL should also catalyze carboligation. Consequently, BAL-catalyzed acyloin condensation of benzaldehyde in an aqueous buffer/DMSO mixture resulted in almost quantitative formation of enantiomeri-cally pure (R)-benzoin [Scheme 2.2.7.21, Eq. (1)]. The reaction was carried out on a preparative scale with different aromatic and heteroaromatic aldehydes [62]. From the viewpoint of the organic-preparative chemist, it is important to mention that crude cell extracts of the recombinant E. coli strain overexpressing the BAL gene are sufficient for catalysis, hence, purification of the enzyme is not necessary. 

Methyl-substituted 2-phenylbenzo[b]furans and dibenzofurans can also react with Schiff s bases derived from heteroaromatic aldehydes to yield styryl compounds, provided that these heterocycles are stable at the relatively high reaction temperatures required. Thus, for example,... 

Apart from the anils derived from carbocyclic aldehydes, those from heteroaromatic aldehydes can also react with 66. Schiff s bases obtained from p-chloroaniline and 3-formylpyridine and 2-formyIthiophene thus yield products 69 and 70, respectively. Table V shows some further stilbenyl-l,3,4-oxadiazoles that have been obtained from the p-tolyl-1,3,4-oxadiazoles and Schiff s bases by treatment with potassium hydroxide. 

In Tables VI, XI, XV, and XVII are compiled a number of styryl derivatives of oxazoles and oxadiazoles that have been prepared from SchifFs bases of heteroaromatic aldehydes. 

---