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Halogen-lithium exchange, Parham cyclization

Several chromans have been synthesized using the Parham cycloalkylation technique (76JOC1184). l-Bromo-3-(2-bromophenoxy)propane, prepared from phenol and 1,3-dibromopropane, is treated with n-butyllithium at — 100 °C. Halogen-lithium exchange yields the aryllithium (278) which cyclizes either at — 100°C or in some instances only at an acceptable rate at higher temperatures. This method offers the advantage of regio-specificity since cyclization is controlled by the location of the o-bromine atom (Scheme 72). [Pg.783]

The Parham cyclization is the generation by halogen-lithium exchange of aryllith-iums and heteroaryllithiums, and their subsequent intramolecular cyclization onto an electrophilic site. [Pg.413]

The main variation of the Parham cyclization might be considered the generation of the requisite aryllithium or hetaryllithium species by "directed-lithiation", rather than via halogen-lithium exchange. However, this protocol does not fall under the rubric of the classic Parham cyclization and is not covered herein. [Pg.751]

One alternative to the halogen-lithium exchange that has been employed in a Parham cyclization is halogen-magnesium exchange, as exemplified by the synthesis of 5-substituted 2,3-dihydrobenzofurans 05)- The use of n-butyllithium is less successful and leads to butylated by products. Lithium tributylmagnesate has also been employed, e.g., 16- 17. ... [Pg.751]

In view of three excellent recent reviews on the Parham cyclization, " this section focuses mainly on the more recent literature. While -butyllithium and tert-butyllithium are commonly used to effect halogen-lithium exchange, these reagents do present problems as delineated earlier. Mesityllithium does not pose such problems since the resulting mesityl halides are not susceptible to the side reactions inherent with -butyl and tert-butyl halides. Mesityllithium has been used to prepare the camptothecin precursor 28. This base can also be used with added electrophiles, as for the conversion of 29 to phthalides 30. A more conventional Parham synthesis of phthalides, i.e., 31—>32, has been applied to the synthesis of the aristocularine alkaloids. ... [Pg.753]

A Parham cyclization can be followed in the same pot by a second, remote halogen-lithium exchange and intermolecular reaction. Such a case is 33—>34. ° A related one-pot sequential lithiation process has... [Pg.754]

A Parham cyclization strategy has been used to synthesize a wide variety of isoquinolines. For example, iodide 66 is smoothly converted to 67, which can be reduced (sodium borohydride/trifluoroacetic acid, 99%) or treated with nucleophiles and a Lewis acid to effect a-amidoalkylation. Likewise, imidazo[4,3-a]isoquinolinones (68), ° thiazolo[4,3-a]isoquinolinones (69), ° pyrrolo[2,l-a]isoquinolines (70), pyrrolo[ 1,2-6] isoquinolines (71), pyrrolo[l,2-6]acridinones (72), ° pyirolo[l,2-g]quinolones (73), ° thieno[3,2-y]indolizinones (74), ° and furo[3,2-/lindolizinones (75). ° It should be noted that the latter ring system 63 was prepared by deprotonation (LDA) rather than halogen-lithium exchange. °... [Pg.758]

The very fast metal-halogen exchange allows intramolecular cyclization reactions, which are known as Parham cyclizations171. The potential of Parham cyclizations as a useful stereoselective cyclization procedure has proven to be extremely interesting172. Thus, it has been recently demonstrated that iodinated IV-phenethylimides tolerate iodine-lithium exchange, giving rise to the isoquinoline nucleus 304, via a Parham-... [Pg.117]

This reaction was initially developed by Parham in 1975 and extended primarily by Bradsher after Parham s death in 1976. It is the synthesis of aryl or heteroaryl ring fused carbocylces or heterocycles through the intramolecular reaction between a side chain electrophile and an aryl or heteroaryllithium generated from lithium-halogen exchange. Therefore, this reaction is generally known as the Parham cyclization. Occasionally, it is also referred to as the Parham reaction. J This reaction is very useful for the synthesis of alkaloids. ... [Pg.2114]


See other pages where Halogen-lithium exchange, Parham cyclization is mentioned: [Pg.247]    [Pg.749]    [Pg.195]   


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Parham cyclizations

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