Pyrroles nitroso

Pyrrole, 2-methyoxycarbonyl-1 -methyl-formylation, 4, 45 Pyrrole, nitro-rearrangement, 4, 297 Pyrrole, 2-nitro-nitration, 4, 211 photolysis, 4, 203 radical methylation, 4, 260 synthesis, 4, 210, 362 Pyrrole, 3-nitro- N NMR, 4, 13 nitration, 4, 211 synthesis, 4, 49, 210, 362 Pyrrole, nitroso-rearrangement, 4, 297 Pyrrole, 2-nitroso-reactions... 

This synthesis of the pyrrole ring system, due to Knorr, consists in the condensation of an a-aminoketone with a 1,3-diketone or the ester of a p-keto-acid, a-Aminoketones are unstable in the free state, readily undergoing self-condensation consequently they must be prepared, by the reduction of an a-nitroso (or oximino) ketone, in the presence of the 1,3-diketone or p-ketoester, to ensure rapid interaction. 

The nitrosation of pyrroles and indoles is not a simple process. The 3-nitroso derivatives (84) obtained from indoles exist largely in oximino forms (85) (80IJC(B)767). Nitrosation of pyrrole or alkylpyrroles may result in ring opening or oxidation of the ring and removal of the alkyl groups. This is illustrated by the formation of the maleimide (86) from 2,3,4 -trime thylpyrrole. 

We have shown (10) that base (dr nucleophilic) catalysed hydrolysis contributes to the decomposition of N-nitroso-2-pyrrol-idone at 25°C in CI2HCCO H and CIH CCO H buffers at pH 1-3, and... 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

Upon reaction with nitrous acid, indole produces a complex mixture of products. In addition to 3-oximino-3H -indole (16), which is the stable tautomeric form of 3-nitrosoindole (17), dimeric products of the type (18) and (19) are also formed. In contrast, (16) appears to be the sole product of the nitrosation of indole with amyl nitrite and sodium ethoxide (72HC(25-2)537). Studies of the nitrosation of pyrrole are somewhat indecisive. The mononitrosopyrrole, obtained from the reaction of pyrrole with nitrous acid, has not been fully characterized, but there is some evidence that nitrosation of pyrrole with amyl nitrite and sodium ethoxide leads to the sodium salt of the 3-nitroso derivative. However, upon the addition of acid, the product rearranges to give the oxime of 3-formylisoxazole (20) (B-77MI30502). 

Predictably, nitrosation of 2-acetylpyrrole and pyrrole-2-carboxylic esters with alkyl nitrites or nitrous acid preferentially yields the relatively stable 4-nitroso derivatives, whilst 2,4-dialkyl- or -diaryl-pyrroles are nitrosated at the 5-position. Further reaction of the dialkyl and diaryl nitrosopyrroles with an excess of alkyl nitrite in the absence of a base can result in the formation of the nitropyrroles, whereas the reaction with nitrous acid converts the nitrosopyrroles into diazopyrroles (B-77MI30502). 

The C-2 or C-3 amino derivatives of pyrroles can be obtained by reduction of nitro or nitroso pyrroles or by Curtius degradation of pyrrolecarboxylhydrazides. A few ring closures give 2-aminopyrroles directly (see Section 3.06.2.3, for example), but these are rather specialized (B-77MI30608). 

An example of the Knorr pyrrole synthesis is provided by the formation of 3,5-diethoxycarbonyl-2,4-dimethylpyrrole (55). Overall ring construction in this case may be related to (46) above. A retrosynthetic analysis involving disconnection of the N—C2 bond, appropriate prototropic shifts, and finally a retro-aldol reaction to effect disconnection of the C3—C4 bond, reveals ethyl acetoacetate and ethyl a-aminoacetoacetate (ethyl 2-amino-3-oxo-butanoate) (56) as reagents. An FGI transform on this latter compound generates the corresponding nitroso (oximino) compound which may also be derived from ethyl acetoacetate. 

As mentioned above, the very stable dihydro systems of triazolopyrimidines and pyrazolopyrimidines proceed in an unusual direction upon treatment with HN02. Under the conditions of the well-known heteroaromatization method, i.e., the action of NaN02 in the presence of an acid, compounds 381 and 382 are transformed into their corresponding nitroso derivatives 383 and 384 [174, 297, 323]. Heating compound 383 in the presence of polyphosphoric acid causes cyclization involving aromatic rings at positions 2 and 4 with the formation of a pyrrole ring (compounds 385 and 386) [323] (Scheme 3.101). 

S)-l-Nitroso-2-methoxymethyl pyrrol idine Pyrrolidine, 2-(methoxymethyl) -1-nitroso-, (S-) (9) (60096-50-6)... 

The nitration was avoided by employing a two-step process. The pyrrole (29) was converted into 3-nitroso-2,5-diphenylpyrrole by treatment with pentylnitrite and sodium ethoxide. The resulting nitroso compound was converted into the diazo compound (31) by treatment with gaseous nitric oxide. Exactly the same problem was encountered in the preparation of 2-diazo-3,5-diphenylpyrrole.12... 

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