Cyclization reactions thermal

Other photoiaduced cyclization reactions can occur by conrotatory bond formation to give the 9 P,10 P-antiisomers, isopyrocalciferol2 [474-70-4] (23) or isopyrocalciferol [10346-44-8] (24) (Fig. 5), whereas thermal cychzation at >100°C leads to the two 9,10-syn isomers, (9a,10 a)-pyrocalciferol (27)... 

Unsaturated hydrazones, unsaturated diazonium salts or hydrazones of 2,3,5-triketones can be used as suitable precursors for the formation of pyridazines in this type of cyclization reaction. As shown in Scheme 61, pyridazines are obtainable in a single step by thermal cyclization of the tricyanohydrazone (139), prepared from cyanoacetone phenylhydrazone and tetracyanoethylene (76CB1787). Similarly, in an attempted Fischer indole synthesis the hydrazone of the cyano compound (140) was transformed into a pyridazine (Scheme 61)... 

A -acetyl groups attached to the aniline have been shown to withstand the Conrad-Limpach reaction. Phenols and alcohols also survived unless in proximity to a reactive center. Jaroszewski reported the formation of 64 by reaction of aniline 63 with ethyl acetoacetate (5). Cyclization under thermal conditions in paraffin gave a mixture of quinolone 65 and quinoline 66. 

Polymeranalogous reactions considered above may be referred to as intramolecular condensation transformations since they are accompanied by elimination of low-molecular products. On the other hand, PCSs can be obtained via polymeranalogous transformations, principally intramolecular polymerization reactions . Thermal and chemical cyclization of poly(acrilonitrile) (PAN) is an example of processes of this type. It was demonstrated by a number of researchers216-225 that thermal transformations of PAN follow the scheme ... 

The thermal solid-to-solid cyclization reaction of diallene derivatives also proceeds stereospecifically. Reaction of 1,6-diphenyl-1,6-di(p-tolyl)hexa-2,4-diyne-l,6-diol (113) with HBr gave meso- (114) and rac-3,4-dibromo-l,6-di-... 

The methyl hydrogens in 1,3,5- and 1,4,5-trimethyl tetrazoliums, as well as the proton on C-5 in 2,3-disubstituted tetrazoliums are acidic, and can be abstracted with butyl lithium.289,290 Tetrazolium methylides, e.g., the dicyano derivative (143), and the phenacyl compound (171, R = Ph, R = Me) are known.291 The latter undergoes an unexpected thermal cyclization reaction to yield imidazolones (173) (Scheme 27).292... 

Furthermore, as described by Mori and coworkers, the domino aldol/cyclization reaction of the 3-keto sulfoxide 2-422 with succindialdehyde (2-423) in the presence of piperidine at r.t. afforded the chromone 2-424 which, on heating to 140 °C, underwent a thermal syn-elimination of methanesulfenic acid to provide 2-426 in 22 % overall yield (Scheme 2.100) [227]. This approach was then used for the synthesis of the natural products coniochaetones A (2-425) and B (2-427) [228]. 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

The rhodium-catalyzed hydroboration has opened the way to cyclization reactions starting from dienes [92], For instance, rhodium-catalyzed hydroboration of the terminal alkenyl group of an os/Tunsaturated lactone followed by reaction with the PTOC-OMe chain transfer reagent afforded the bicyclic a-S-pyridyl lactone in 63% yield (Scheme 39). After oxidation of the sulfide with m-CPBA, thermal elimination of the sulfoxide afforded the corresponding a-methylene lactone in 65% yield. Interestingly, such bicyclic a-methylenelactones are substructures that can be found in many natural products such as mirabolide [93]. 

Many hydrocarbon derivatives of 22 have since been prepared [81, 82] they are mostly of interest because of their thermal cyclization reactions (see below) [8, 83, 84], although their behavior in cycloaddition reactions could also lead to new preparative insights [79]. 

The acetylene 27 became of interest since it undergoes a novel type of thermal cyclization reaction, later to be called the Myers or Myers-Saito cyclization (see Chapter 20 for a discussion of its relevance). The Z-diastereomer of 1,2,4-heptatrien-6-yne (27) was prepared by the sequence summarized in Scheme 5.33 [85]. 

That the thermal isomerizations of these substrates lie completely on the side of the cyclic structures 324 comes as no surprise after what has been said about the energetics of the allene cyclization reactions above [125] the cydoisomerization may also be induced by metal salts, such as cuprous chloride, incidentally [136]. 

The ability of (Z)-l,2,4-heptatrien-6-ynes (enyne-allenes) and the benzannulated derivatives to undergo cyclization reactions under mild thermal conditions to produce biradicals has been the main focus of their chemical reactivities [1-5]. With the development of many synthetic methods for these highly conjugated allenes, a variety of biradicals are readily accessible for subsequent chemical transformations. Cyclization of the enyne-allene 1 could occur either via the C2-C7 pathway (Myers-Saito cyclization) leading to the a,3-didehydrotoluene/naphthalene biradical 2 [6-10] or via the C2-C6 pathway (Schmittel cyclization) producing the fulvene/benzofulvene biradical 3 [11] (Scheme 20.1). 

Step 10 is a thermal cyclization reaction not requiring a catalyst. The participation of a radical intermediate in the rearrangement reaction was verified by radiotracer studies (94a). 

The 5,6-double bond in activated pyrimidines can participate in thermal [4-1-2] cyclization reactions as demonstrated by the 1,3-dipolar cycloaddition reactions of O-protected thymidine derivatives 483 with the nonstabilized azo-methine ylide 484, which is generated from trimethylamine AT-oxide by reaction with EDA <2002SC1977>. 

The thermal cyclization reaction of 2,4,6-triaminopyrimidine 466 with ethyl 2-cyclohexanonecarboxylate 588 in PhgO at 190 °C gave the tetrahydropyrimido[4,5-r-]isoquinolin-6(5//)-one 589 as the sole product (Equation 48) <1999JOC634>. 

Cis-stilbene (Zl) also undergoes a conrotatory cyclization reaction into trans-4a,4b-dihydrophenanthrene (DHP, 2), a short living, not isolated product with an absorption in the visible spectrum at 450 nm. In the absence of an oxidizing agent DHP will return to the starting material, by both a thermal and a photochemical ring opening reaction. 

The classical synthetic pathway to prepare polyimides consists of a two-step scheme in which the first step involves polymerization of a soluble and thus processable poly(amic acid) intermediate, followed by a second dehydration step of this prepolymer to yield the final polyimide. This preparative pathway is representative of most of the early aromatic polyimide work and remains the most practical and widely utilized method of polyimide preparation to date. As illustrated in Scheme 4, this approach is based on the reaction of a suitable diamine with a dianhydride in a polar, aprotic solvent such as dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), dimethylformamide (DMF), or AT-methylpyrrolidone (NMP), generally at ambient temperature, to yield a poly(amic acid). The poly(amic acid) is then cyclized either thermally or chemically in a subsequent step to produce the desired polyimide. This second step will be discussed in more detail in the imidization characteristics section. More specifically, step 1 in the classical two-step synthesis of polyimides... 

Diastereoselective ene reactions. Thermal cyclization via an intramolecular ene-tvpc reaction of the (7.)-1,6-diene 1 results in a mixture of trans-1 and cis-2 in the ratio 75 25. This reaction is markedly accelerated by addition of (C2H5)2AIC1, with only trans-1 being formed. The (E)-isomer of I is also cyclized thermally to give... 

There is less evidence for the participation of azomethine ylides 88 in the early examples of the thermal cyclization reactions of Meldrum s acid derivatives 86. This reaction, conducted under flash vacuum pyrolysis conditions, may proceed via the methyleneketene 87. Hydrogen transfer from this highly unstable species may lead to the dipolar intermediate 88, which could cyclize either in a 6zr, to give 89, or 8tt, to give 90, manner (Scheme 27) [83JCS(CC)988 85TL833 87JCS(CC)140]. 

A comparison of the behavior of the R = H, 5-Me-, and 6-Me-substituted succinates (77) in the competitive cyclization reactions (Table I) revealed that in ethanolic sodium ethoxide at 25°C the 6-methyl group fully inhibited the ring closure onto the ring nitrogen. It also revealed that in phosphoryl chloride-polyphosphoric acid at 120 C the formation of 4-oxo-4//-pyrido-[1,2-a]pyrimidine (78 R = 6-Me) was only hindered, whereas at 250 C under the conditions of thermal ring closure, only a slight, if any, steric hindrance occurred. This suggests that the formation of the 6-substituted pyrido[l,2-a]-... 

Pyrolysis of poly(methylacetylene) shows rather similar behaviour 528>, with mesi-tylene as the major product but substantial yields of methyl and proton-enriched products. Thermal decomposition of this polymer sets in at around 150 °C and the mechanism is postulated to involve chain scission followed by cyclization reactions and both electron-proton and electron-methyl exchanges. Pyrolysis of poly(phenyl-acetylene) has been reported to start at 270 °C in nitrogen 529). 

Alko xy ally 1)stannane aldehydes (57) can cyclize either thermally or with Lewis or protic acid catalysis to give cyclic ethers (58).85 The interrelationship of the reactant and product stereochemistries has been investigated, as have the methods used to promote the reaction. For both thermal and proton-promoted reactions, [(Z)-57 gave (cis-58), and [(E)-57] gave trans-58), whereas trans-58) was the predominant or exclusive product of Lewis acid mediation, regardless of the double bond geometry of... 

Disubstituted adamantanes are most frequently obtained from internal cyclization reactions of a bridgehead substituent. Thus, photolysis of the azido-formate 79 gives 80 by nitrene insertion 279). 80 may be readily hydrolyzed to 2-amino-1-adamantanol (81) 75 Subsequent oxidation gives l-hydroxy-2-ada-mantanone (82) from which other derivatives may be prepared 280>2S1). An analogous carbene insertion is initiated by the thermal decomposition of the... 

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