Formylation,Vilsmeier reaction

Alkyl substituents. The steric effect of 1-alkyl substituents in the pyrrole series has been demonstrated in, for example, Vilsmeier formylation reactions. Thus as the bulk of the alkyl substituent on nitrogen is increased e.g. from Me to Bu ) so does the proportion of /3 substitution (70JCS(C)2573). A similar trend has been observed in a series of experiments on the trifiuoroacetylation of A-alkylpyrroles with trifluoroacetic anhydride (80JCR(S)42). 

With respect to aromatic substrates, the Vilsmeier formylation reaction works well with electron-rich derivatives like phenols, aromatic amines and aromatic heterocycles like furans, pyrroles and indoles. However various alkenes are also formylated under Vilsmeier conditions. For example the substituted hexatriene 6 is converted to the terminal hexatrienyl aldehyde 7 in 70% yield ... 

Two examples of the conversion of fused-ring triazolines to open-chain diazo isomers are reported. Whereas spontaneous thermal isomerization occurs in a pyrrolotriazoline (Scheme 143),302 isomerization of the tetracyclic triazoline in Scheme 73 is achieved by the action of phosphorus oxychloride in DMF solution through a Vilsmeier formylation reaction (87).290... 

While 5-formyluracils are easily accessible by a Vilsmeier formylation reaction, the first synthesis of 5-benzoyluracil could be achieved only by a multistep procedure involving lithiated pyrimidines (56JA2136 63JMC550). 

Formylation.1 A new variation of the Vilsmeier formylation reaction involves the reaction of triphenylphosphine dibromide with DMF in the presence of a suitably substituted substrate. The actual formylating reagent is considered to be (1). 

Formylation. Vilsmeier formylation reactions have been reviewed. Oxalyl chloride-dimethylformamide has found occasional use one application has been preparation of a piperidine substituted dialdehyde (eq 10). ... 

A very similar type of benzochlorin 50 was also synthesized by Smith s group as a result of the cyclization of the product obtained after Vilsmeier formylation reaction of Ni or Cu(OEP) using dimethylaminoacrolein and phosphorous oxychloride.The utility of this class of compounds as photosensitizers for PDT was first shown by Morgan et Jn a mouse tumor model. With a few... 

The pyridine-like nitrogen of the 2H-pyrrol-2-yiidene unit tends to withdraw electrons from the conjugated system and deactivates it in reactions with electrophiles. The add-catalyzed condensations described above for pyrroles and dipyrromethanes therefore do not occur with dipyrromethenes. Vilsmeier formylation, for example, is only successful with pyrroles and dipyrromethanes but not with dipyrromethenes. 

The benzo[6] heterocycles are generally less reactive than their monocyclic counterparts. Thus benzo[6]thiophene unlike thiophene does not undergo Vilsmeier formylation or the Mannich reaction. 

Although in general azoles do not undergo Friedel-Crafts type alkylation or acylation, several isolated reactions of this general type are known. 3-Phenylsydnone (120) undergoes Friedel-Crafts acetylation and Vilsmeier formylation at the 4-position, and the 5-alkylation of thiazoles by carbonium ions is known. 

The classical Vilsmeier-Haack reaction is one of the most useful general synthetic methods employed for the formylation of various electron rich aromatic, aliphatic and heteroaromatic substrates. However, the scope of the reaction is not restricted to aromatic formylation and the use of the Vilsmeier-Haack reagent provides a facile entry into a large number of heterocyclic systems. In 1978, the group of Meth-Cohn demonstrated a practically simple procedure in which acetanilide 3 (R = H) was efficiently converted into 2-chloro-3-quinolinecarboxaldehyde 4 (R = H) in 68% yield. This type of quinoline synthesis was termed the Vilsmeier Approach by Meth-Cohn. ... 

The reaction of electron-rich aromatic compounds with yV,A -dimethylformamide 2 and phosphorus oxychloride to yield an aromatic aldehyde—e.g. 3 from the substituted benzene 1—is called the Vilsmeier reaction or sometimes the Vilsmeier-Haack reaction. It belongs to a class of formylation reactions that are each of limited scope (see also Gattermann reaction). 

Taking into account the close relationship to pyridines one would expect 2-pyridones to express similar type of reactivities, but in fact they are quite different. 2-Pyridones are much less basic than pyridines (pKa 0.8 and 5.2, respectively) and have more in common with electron-rich aromatics. They undergo halogenations (a. Scheme 10) [67] and other electrophilic reactions like Vilsmeier formylation (b. Scheme 10) [68,69] and Mannich reactions quite easily [70,71], with the 3 and 5 positions being favored. N-unsubstituted 2-pyridones are acidic and can be deprotonated (pJCa 11) and alkylated at nitrogen as well as oxygen, depending on the electrophile and the reaction conditions [24-26], and they have also been shown to react in Mitsonobu reactions (c. Scheme 10) [27]. 

The reaction with disubstituted formamides and phosphorus oxychloride, called the Vilsmeier or the Vilsmeier-Haack reaction,is the most common method for the formylation of aromatic rings. However, it is applicable only to active substrates, such as amines and phenols. An intramolecular version is also known.Aromatic hydrocarbons and heterocycles can also be formylated, but only if they are much more active than benzene (e.g., azulenes, ferrocenes). Though A-phenyl-A-methyl-formamide is a common reagent, other arylalkyl amides and dialkyl amides are also used. Phosgene (COCI2) has been used in place of POCI3. The reaction has also been carried out with other amides to give ketones (actually an example of 11-14),... 

Formylation of alkenes can be accomplished with N-disubstituted formamides and POCl3. ° This is an aliphatic Vilsmeier reaction (see 11-15). Vilsmeier formylation can also be performed on the ot position of acetals and ketals, so that hydrolysis of the products gives keto aldehydes or dialdehydes ... 

Another useful method for introducing formyl and acyl groups is the Vilsmeier-Haack reaction.67 /V.A-dialkylamidcs react with phosphorus oxychloride or oxalyl chloride68 to give a chloroiminium ion, which is the reactive electrophile. 

Scheme 11.5 gives some examples of these acylation reactions. Entry 1 is an example of a chloromethylation reaction. Entry 2 is a formylation using carbon monoxide. Entry 3 is an example of formylation via to-chloromethyl ether. A cautionary note on this procedure is the potent carcinogenicity of this reagent. Entries 4 and 5 are examples of formylation and acetylation, using HCN and acetonitrile, respectively. Entries 6 to 8 are examples of Vilsmeier-Haack reactions, all of which are conducted on strongly activated aromatics. 

In miscellaneous oxidative processes of indoles, two methods for the preparation of 3-hydroxyindoles have been reported. The first approach involves initial Vilsmeier-Haack reaction of indole-2-carboxylates 176 to afford the corresponding 3-formyl analogs 177. Activation of the aldehyde with p-toluenesulfonic acid (PTSA) and Baeyer-Villiger oxidation with m-chloroperoxybenzoic acid (wi-CPBA) then affords high yields of the 3-hydroxy compounds 178 <00TL8217>... 

Recently, Moody et al. reported a biomimetic synthesis of calothrixin B (378) by oxidation of Hibino s 6-formylindole[2,3-fl]carbazole 1555 (870). The key intermediate 6-formyl-indole[2,3-fl]carbazole was readily obtained in six steps from indigo (1458). Using Somei s procedure, indigo (1458) was transformed to the cis-chlorohydrin 1461 in three steps and 50% overall yield (see Scheme 5.247). The reduction of the chlorohydiin 1461 gave 5-hydroxy-indolo[2,3-fl]carbazole 1564, and subsequent Vilsmeier formylation delivered the desired 6-formyl-indole[2,3-fl]carba-zole 1565 in 45% yield. Reaction of hydroxy-indolocarbazole 1565 with an excess of chloromethyl methyl ether (MOMCI) afforded the tiis-MOM-protected compound 1555. Following Hibino s approach, the tris-MOM-protected indolocarbazole 1555... 

While the Friedel-Crafts acylation is a general method for the preparation of aryl ketones, and of wide scope, there is no equivalently versatile reaction for the preparation of aryl aldehydes. There are various formylation procedures known, each of limited scope. In addition to the reactions outlined above, there is the Vilsmeier reaction, the Reimer-Tiemann reaction, and the Rieche formylation reaction The latter is the reaction of aromatic compounds with 1,1-dichloromethyl ether as formylating agent in the presence of a Lewis acid catalyst. This procedure has recently gained much importance. 

Substituted 3-hydroxythiophenes are convenient starting-points for the synthesis of the thieno[3,2-Mthiophene system. Shvedov et obtained 50% of 3-chloro-3-ethoxycarbonyl-5-methylthiophene-2-aldehyde (110) from 3-ethoxycarbonyl-4-hydroxy-2-methylthiophene (109) by Vilsmeier formylation at 100°. Reaction with thioglycolic ter formed 3-ethoxycarbonyl-5-methoxycarbonyl-2-methylthieno[3,2-6]-thiophene (111) [. (36)]. 

Brassilexin 48 and sinalexin 49 are among the most potent phytoalexins produced by economically important cruciferous plants. The most efficient preparation of brassilexin 48, sinalexin 49, and analogues 52 reported uses a Vilsmeier formylation-amination of readily available indoline-2-thiones 50 followed by an aqueous ammonia work-up procedure with subsequent oxidation of the 3-(amino)methyleneindoline-2-thione intermediates 51 using iodine in pyridine (Scheme 10) <20010L1213, 2005JOC1828>. The reaction yields are dictated by the efficiency of the... 

Benzodipyrrole 1 is selectively formylated at the 3- and 6-positions by a Vilsmeier-Haack reaction to produce 3,6-diformylbenzodipyrrole which is reduced by lithium aluminium hydride to give 3,6-dimethylbenzodipyrrole. This can also undergo subsequent Vilsmeier-Haack formylation to afford the 2,5-dialdehyde (Scheme 2) <2005AGE4053>. 

---