Ring-forming reactions rings

Ring forming reaction is designated as Favored or Disfavored... 

In addition to ring-forming reactions, the transformation of preformed heterocycles serves as an efficient alternative route for the synthesis of... 

Carbon-centered radicals generated by Barton s thiohydroxamate method can also participate in ring-forming reactions (see Scheme 26).52b,s3 For example, irradiation of 129 results in the formation of compound 130 (82% yield). The outcome of this transformation is reminiscent of Stork s elegant radical cyclization/trapping processes (see Schemes 7 and 8), in that/botn alkene carbon atoms have become functionalized. / I... 

The intramolecular Heck reaction presented in Scheme 8 is also interesting and worthy of comment. Rawal s potentially general strategy for the stereocontrolled synthesis of the Strychnos alkaloids is predicated on the palladium-mediated intramolecular Heck reaction. In a concise synthesis of ( )-dehydrotubifoline [( )-40],22 Rawal et al. accomplished the conversion of compound 36 to the natural product under the conditions of Jeffery.23 In this ring-forming reaction, the a-alkenylpalladium(n) complex formed in the initial oxidative addition step engages the proximate cyclohexene double bond in a Heck cyclization, affording enamine 39 after syn /2-hydride elimination. The latter substance is a participant in a tautomeric equilibrium with imine ( )-40, which happens to be shifted substantially in favor of ( )-40. 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

The strong Bronstedt acid nature of some hexacoordinated phosphorus derivatives, [7",H ] (Et20)4 in particular, was recently used within the context of an industrial application [36]. The conjugated acid of tris(oxalato)phosphate anion 7 was found to effectively catalyze the ring-forming reaction of trimethyl-hydroquinone 63 with isophytol 64 to give (all rac)-a-tocopherol 65 (ethylene-carbonate/heptane 1 1,100 °C, 90%, Scheme 19). This process is particularly... 

Sequential pyrrolidine and hydantoin ring-forming reactions via intramolecular [2+3] cycloaddition have been applied to the stereoselective solid-phase synthesis of conformationally constrained tricyclic triazacyclopenta [C]pentalene scaffold 43 < 1999JOC8342>. These novel compounds 43 share the structural complexity characteristic of certain alkaloid natural products, angular triquinanes. The retrosynthetic analysis is shown in Scheme 87. 

TiIV-based Lewis acids are effective in ring-forming reactions such as Diels-Alder reactions (Scheme 25).94 Besides the usual TiX4 compounds (X = halide or alkoxide), Cp2Ti(OTf)2 is also a reactive catalyst for the Diels-Alder reaction,95 and it has been reported that [( PrO)2Ti(bpy)(OTf) ( PrOLQXOTf) is even more effective than Cp2Ti(OTf)2.96 In asymmetric synthesis, reactions with chiral dienophiles have been widely investigated. 

The oxidation of thioacyl amidines and related compounds is a versatile ring-forming reaction which can furnish a range of 3- and 5-substituted 1,2,4-thiadiazoles (see Section 5.08.9.4). Other ring-forming reactions that can give specific types of derivatives are discussed in the following sections. 

Hydrozirconation Ring-Forming and Ring-Opening Reactions... 

Arcadi has described a series of pyrrole ring-forming reactions leading to iV-aminopyrroIe derivatives 182-184 from common precursor 181, depending on conditions [128], Since N-aminopyrroles are widely used as pharmaceutical precursors, this chemistry could find wide appeal for the synthesis of medicinal agents. 

Larock has described the reaction of diphenylacetylene with iodosulfonamides 189 to give alkylidene dihydropyrroles 190 [130], This ring-forming reaction is similar to the large number... 

The preparation of C-1 glycals has been largely addressed by synthetic modifications on cyclic carbohydrate derivatives, although strategies that rely on ring forming reactions from acyclic derivatives have recently emerged (Fig. 1). 

Most of the cyclopropane ring forming reactions can be accommodated in the mechanism that involves a carbanion in 8 2 displacement of the second halogen... 

The most significant progress that has been described to date in the area of rhodium-catalyzed asymmetric hydroacylation of olefms/alkynes with aldehydes has involved intramolecular processes that generate either cyclopentanones or cyclopentenones. Fig. 4.2 illustrates two of the more likely mechanisms for these ring-forming reactions [12, 13]. 

Free radical reactions are proving to be synthetically useful altern-atives for producing carbon-carbon bonds. Recently, Stork has shown that vinyl radicals are valuable in ring forming reactions since they place a double bond in a predictable position. Their compatibility with many unprotected functional groups and their ability to form quaternary centers are additional features which make vinyl radical cyclization an attractive synthetic method. 

The sequence that follows illustrates how a conjugate aldol addition (Michael addition) followed by a simple aldol condensation may be used to build one ring onto another. This procedure is known as the Robinson annulation (ring forming) reaction (after the English chemist Sir Robert Robinson, who won the Nobel Prize in Chemistry in 1947 for his research on naturally occurring compounds). 

The intramolecular variant of ester enolate alkylations is a very useful ring-forming reaction. It can often be carried out under milder conditions than the corresponding intermolecular alkylation. Yields and diastereoselectivities are usually high. 

A special case is the ring-forming reaction in the lactone acetal 568,69. When 5 is treated with trimethylsilyl trifluoromethanesulfonate (trimethylsilyl triflate TMSOTf) in the presence of triethylamine at 0°C the cisjtrans mixture 6a and 6b is formed in ca. 50% yield. In this intramolecular aldol reaction the probable intermediate is the oxonium ion 7. 

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