Pyrrole derivatives, hydroxyalkylation

The precedent is strong for the involvement of oxetanes as Intermediates in carbonyl additions to pyrroles. " NMR evidence has been obtained far an oxetane adduct of acetone and N-methylpyrrole. The initial photoadduct was shown to rearrange readily on workup to the 3-(hydroxyalkyl)pyrrole derivative. 

Oxidation of the 3-(hydroxyalkyl)pyrrole derivative gives a pure 3-acylpyrrole derivative which is difficult to obtain by direct substitution in the pyrrole ring. Acylation of pyrrole yields 1- and/or 2-acetyl pyrrole, whereas acylation of 1-methyl pyrrole forms both 2- and 3-acetyl-1-methyl-... 

The condensation of 2,5-diunsubstituted pyrroles with formic acid20 is a viable method to produce porphyrins. However, the most common procedure21 22 involves the heating of the corresponding pyrroles 1 with aldehydes and aldehyde derivatives like imines or a Mannich reagent in the presence of acid. The reaction is initiated by electrophilic attack of the aldehyde (or aldehyde derivative) to the pyrrole 1. The formed (hydroxyalkyl)pyrrole 3 then undergoes electrophilic substitution with another pyrrole to form a dipyrrylmethane 4. Repeated addition of aldehyde and pyrrole finally forms a tetrameric (hydroxyalkyl)bilane 5. 

SCHEME 13.68 Aminodeoxyaldonolactams through hydroxyalkylation of a pyrrole derivative. Copyright 2006 by Taylor Francis Group LLC... 

Among the (2 + 2)-cycloadditions, the Paterno-Buchi reaction (see p. 46) with pyrroles has been investigated. The pyrrole-derived oxetanes may isomerize to give 3-(hydroxyalkyl)pyrroles under the reaction conditions, for example ... 

The Reimer-Tiemann reaction is not an effective route to formyl-pyrroles or -indoles (see Section 3.05.1.6) and the oxidation of alkyl and hydroxyalkyl derivatives of the heterocycles and the reduction of carboxylic acid derivatives are discussed in Sections 3.05.2.2 and 3.05.2.4, respectively. 

Extention of the reaction of ketoximes with acetylene to oximes of hydroxyalkyl ketones can lead to the synthesis of inaccessible, functionally substituted pyrroles and A-vinyl derivatives. However, the methyl a- and /3-hydroxyalkyl ketoximes used by Trofimov et al. (80ZOR410) appeared to react with acetylene in the KOH/DMSO system in an abnormal way. 

In addition to furan, other heterocycles have been examined." Thiophene undergoes efficient photocycloaddition with benzaldehyde to afford a single exo photoproduct (183) in 60% yield. As reported by Jones and coworkers,the photolysis of IV-methylpyrrole in the presence of aldehydes or ketones yields the corresponding 3-hydroxyalkyl derivative (184), even when the reaction mixture is free from any trace of acid. In order to use the pyrrole nucleus for stereoselective alkaloid synthesis (cf. caesalpinine, 185) in the fashion developed with the furan nucleus, pyrrole substituents that stabilize the presumed intermediate bicyclic oxetane must be discovered. 

Kobayashi, K., Matoba, T., Irisawa, S., Matsumoto, T., Morikawa, O., Konishi, H. Synthesis of pyrrolo[1,2-a]quinoxaline and its 4-(1-hydroxyalkyl) derivatives by Lewis acid-catalyzed reactions of 1-(2-isocyanophenyl)pyrrole. Chem. Lett. 1998, 551-552. 

Heterocyclic compounds such as furan, thiophene, and pyrrole are subject to oxidation. Similar to secondary allyUc alcohols, 2-(l-hydroxyalkyl) derivatives of these heterocychc compounds are also good substrates for kinetic resolution using a titanium-tartrate and TBHP system (Scheme 15). 

---