2//-Pyrroles, 3,4-dihydro-2-hydroxy

The importance of ring size holds also for tautomerism of -pyrrol-5-ones and. d -dihydro-6-pyridones. While the former compounds behave as cyclic 1-methyl-2-alkyl-2-hydroxy-5-pyrrolidones 179) (76) [or, on distillation, as the dehydrated l-methyl-2-alkyl-J -pyrrolones (77)], the latter compounds exist as acyclic N-methylamides of 8-oxo-acids (78) [as shown by infrared spectroscopy (/80)j. The dehydration of 78 during distillation to form l-methyl-2-alkyl-. -dihydro-6-pyridones (79) is achieved only with difficulty. 

Nicht nur Pyrrole, 3-Amino-a-p5n one und 3-Hydroxyp Tidone, sondem auch a-P5Tidone selbst sind aus den Addukten von Butadien-carbon-saure-Derivaten zuganglich die Reduktion dieser Addukte mit Zink in Eisessig liefert bei 50—60°C durch Cyclisierung der intermediar auftre-tenden l-Aminobuten-(2)-ole-(4) 3-Hydroxy-3.6-dihydro-p5ridone-(2). 

Simple pyrroles frequently give complex breakdown products. Oxidation with oxygen (UV-irradiation, a photosensitizer, or an radical initiator) produces hydroxypyrrolones <1991JOC6942, 1999TL4519, 2002TA601>. For example, pyrrole 152 (30 C) leads to 5-hydroxy-l,5-dihydro-27/-pyrrol-2-one 153 <2003T8499>. 

Before turning to the dihydro and tetrahydro derivatives of the fundamental ring systems, we deal with two special classes. The 2,2-disubstituted 2//-pyrroles and the thiophene sulfones both contain two double bonds in the heterocyclic ring, but in each case the conjugation does not include all the ring atoms. Finally, we consider hydroxy derivatives, most of which exist predominantly as the nonaromatic carbonyl tautomers. 

Dihydro-17/-pyrrolo[l,2-< ][l,3,5]oxadiazocin-6(5/7)-one ring system 78 can be obtained from 2,3-dihydro-3-hydroxy-2-(pyrrol-l-ylmethyl)-17/-isoindol-l-one 76. No intermediate acyl chloride was observed during the cyclization of 77a, and product 78 (X = O) was isolated in low 15% yield. Yields were not improved after addition of Lewis acids or use of polyphosphoric acid as a condensation reagent (Scheme 16 <1998JHC9>). Similarly, 2,3-dihydro-l/7-pyrrolo[l,2-< ][l,3,5]thiadiazocin-6(577)-one ring system 19 (X = S) was obtained in low yield. Key intermediate 77b was prepared from 76 in one step under acidic catalysis. 

With l//-pyrrole-2-carbaldehyde, [2,3]-sigmatropic and other rearrangements occur except in the case of diphe-nyl(vinyl)sulfonium trifluoromethanesulfonate where the annulation reaction does take place to give a low yield of l-(2-hydroxy-2,3-dihydro-l//-pyrrolizin-l-yl)-l//-pyrrole-2-carbaldehyde 173 (Scheme 37) <1999T10659>. 

The A -(pyrrol-2-yl)carbamates 860 are oxidized smoothly by air to a mixture of the A -(5-hydroxy- 862 and 5-hydroperoxy- 863 -l,5-dihydro-2/7-pyrrol-2-yliden)carbamates (in 63% and 8% yield, respectively, when R = H, = Bn) (Scheme 166) <1997TL1293>. Oxidation with pure oxygen gives the same products in a yield of 47% and... 

Acidic hydrolysis or methanolysis of 5-(l-ethoxyethoxy)-l//-pyrroles 1403, assembled from lithiated l-(l-ethoxy-ethoxy)-2-heptyne 1402 and isothiocyanates in one-pot (Scheme 269), leads to 2-hydroxypyrroles 1404 that, unlike the 3-hydroxy-2-(alkylsulfanyl)-l//-pyrroles 1400, exist exclusively in the form of the tautomeric l,5-dihydro-2//-pyrrol-2-ones 1405 (IR and NMR data). An unprecedented inertness of W-(2-vinyloxy)ethyl group under conditions of acidic hydrolysis (dioxane/H20, HCl, 0°C, 5min) was revealed <2002RJ0907>. 

H-Pyrrol 2-Amino-5-methyl-4,5-dihydro- -1-oxid X/4, 349 Pyrrolidin l-Hydroxy-5-imino-2-methyl- X/l, 440... 

Pyridazioe 6-Hydroxy-3-(2-thienyl)-4,5-dihydro- E9a, 570 (4-Oxo-4-het-butanoic Acid + N2H4) Pyrrol 2-Cyan-4-(ethylthiocarbo-nyl)- E5, 851 f. (H -CO-SR)... 

Phenol 6-Hydroxymethyl-4-methyl-2-thiocyanatomethyl- IX, 858 3H-Pyrrol 3-Phenylsulfonyl-4,5-dihydro-Ell, 1254 (C,C-Aufbau) Thiophen 4-Cyan-2-ethoxycarbonyl-5-ethyl-3-hydroxy- E6a, 297 [HS-CH2-COOR + RO-C(C2H 5) = C(CN) - COOR]... 

Pyrazin 2-(bzw. 4)-(4-Methyl-anilinocarbonyl)- E5, 1017 (N3 -v CO-NH-Ar) Pyridazine 4-Cyan-6-hydroxy-5-methyl-3-phenyl-4,5-dihydro-E9a, 570 (4-Oxo -4-Ar-3-CN-2-CH3 —butanoic Acid + N2H4) Pyridin 2-Methoxy-5-phenylazo-E16d, 57 (R-NH2 + R-NO) Pyrrol 2-Amino-3-cyan-1 -(4-methyl-phenyl)- 5-oxo-4,5-dihydro-E15/2, 1946 (Cl-CH2-CO-NR2 + NC-CH2-CN) Quinolin... 

Pyrrol 2-(a-Brom-benzyl)-2-hydroxy-3-methoxy-5-oxo-2,5-dihydro- E14a/2, 421 (En + NBS)... 

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