In cycloaddition reactions

Acylisocyanates or isothiocyanates undergo cycloaddition with 5-hydroxy-THISs under so mild conditions that isolation of the initial adducts becomes possible (23). In cycloaddition reactions the 5-hydroxy-THISs can be replaced by their precursors (23). 

More recently, Cheeseman and coworkers have investigated cycloaddition reactions of 2,6-dioxypyrazines (80jCS(Pl)1603). 2,6-Dihydroxy-3,5-diphenylpyrazine (77) reacts with electron deficient dienophiles such as iV-phenylmaleimide, diethyl maleate and diethyl fumarate (Scheme 26) to yield adducts of the 3,8-diazabicyclo[3.2.1]octane class such as (78). This reaction is believed to proceed by way of the betaine (79) and has precedent (69AG(E)604) in that photolysis of the bicyclic aziridine (80) generates analogous betaines which have been trapped in cycloaddition reactions. 

Furan has the greater reactivity in cycloaddition reactions compared with pyrrole and thiophene the latter is the least reactive diene. However, A -substituted pyrroles show enhanced dienic character compared with the parent heterocycle. 

The different possibilities for the creation of the pyrazole ring according to the bonds formed are shown in Scheme 46. It should be noted that this customary classification lacks mechanistic significance actually, only two procedures have mechanistic implications the formation of one bond, and the simultaneous formation of two bonds in cycloaddition reactions (disregarding the problem of the synchronous vs. non-synchronous mechanism). 

Extrapolation from the known reactivity of cyclobutadiene would suggest that azetes should be highly reactive towards dimerization and as dienes and dienophiles in cycloaddition reactions and the presence of a polar C=N should impart additional reactivity towards attack by nucleophiles. Isolation of formal dimers of azetes has been claimed as evidence for the intermediacy of such species, but no clear reports of their interception in inter-molecular cycloaddition reactions or by nucleophiles have yet appeared. 

The chalcogene heterocycles have been used as stable precursors for sulfur-said selenium-cantaining hetero-l,3-dienes in cycloaddition reactions 3//-l,2,4-Thiaselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-thia-3-aza-buta-1,3-dienes, and 3//-diselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-selena-3-azabuta-l,3-dienes as well as bis(tnfluoro-methyl)-substrtuted nitrile ylides [137]... 

Formation of 4- to 6-membered heterocycles in cycloaddition reactions 98JCS(P1)3873. 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

In numerous synthetic studies,9" 6 100 it has been demonstrated that porphyrins react at the chromophore periphery in cycloaddition reactions, rearrangements, conjugative additions and substitution reactions to yield interesting porphyrin derivatives. Thus, metal-free protoporphyrin IX dimethyl ester reacts in Diels-Alder reactions108a b with dienophilcs like ethenetetra-carbonitrile and acetylenedicarboxylates at the diene structural parts to yield, according to the reaction conditions, the corresponding monoadducts 2 and 3 (see also Section 1.2.) and bisadducts 1 (see also Section 1.4.), respectively. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

Synthesis of alkyl perfluoroalkanedithlocarboxylates and some aspects of their reactivity in cycloaddition reactions [161]... 

Whereas tropones usually act as dienes in cycloaddition reactions (Section 5.4), tricarbonyl (tropone) iron 59 displays a reactivity that is almost identical to that of a normal enone. High pressure cycloadditions of 59 with 1-oxygen substituted dienes 60 gave the desired cycloadducts 61 in good to excellent yields (Equation 5.9). The subsequent decomplexation of the cycloadducts has been accomplished by treatment with CAN [20]. 

Although 1-vinylnaphthalene thermally reacts with 4-acetoxy-2-cyclopenten-1-one (98) to regioselectively afford 99, the isomer 2-vinylnaphthalene gives the same thermal cycloaddition with low yield (30 %) and reacts satisfactorily only with 98 at 10 kbar (Scheme 5.10). Both products 99 and 101 were converted into the cyclopenta[a]phenanthren-15-one (100) and cyclopenta[c]phenanthren-l-one (102) isomers. Acetoxyketone 98 acts as a synthetic equivalent of cyclo-pentadienone (114 in Scheme 5.14) in cycloaddition reactions [33]. 

Sauer J. The Structure-Reactivity Problem in Cycloaddition Reactions to Form Heterocyclic Compounds Khim. Geterotsikl. Soedin. 1995 1307-1322 Keywords structure-reactivity, heterocyciic compounds... 

It has been shown that cross-coupling reactions constitute a very mild method to introduce different alkyl and aryl groups to the most active C-3 position of the pyrazinone ring [26]. The broadly functionahzed 2-azadiene system of the title compounds was studied in cycloaddition reactions with various electron-reach and electron-poor dienophiles to provide highly substituted heterocycles [24]. 

Both ( )-l-phenylsulfonyl and (5)-(+)-3-p-tolylsulfmyl -alk-3-en-2-ones can exhibit high diastereoselectivity in their reactions with vinyl ethers and styrenes, with the dienophile having a dominant influence on the stereochemical outcome <96T1205,96TL3687>. Indol-2-ylideneacetic acid esters can act as both dienophile and heterodiene in cycloaddition reactions in the latter case pyrano[3,2-h]indoles are formed <96SYN519>. 

The l,3-dithian-2-ylidene substituted carbene (54), accessible from the tosylhydrazone (53) by a Bamford - Stevens reaction, not only participates in cycloaddition reactions but is also a source of 4,8-dithiaspiro[2.5]oct-l-ene 6JCS(P1)2773>. 

As complex 40 proved to be active in cycloaddition reactions and is isoelectronic to Rh(+1), which is a potent catalyst for [2 + 2 + 2] cycloadditions [20, 21], it was expected that 40 might also be active in those reactions, which is indeed the case. Triyne 54 could be converted to the [2 + 2 + 2]-cycloaddition product 55 in good yield (eq. 3 in Scheme 11). Mechanistically, this reaction is also assumed to proceed via a metallacyclic intermediate. 

The complex PPN[Au(CgF5)C1] was used as a catalyst in cycloaddition reactions [76] and, although the ratio endo/exo was practically unchanged, the reaction time was reduced from 96h to 72 h (using 10mol% of catalyst). 

Thus, nitroheterocycles are important synthons of five-membered heteroarynes in cycloaddition reactions, which are generally difficult to be generated.38... 

The 3-oxo-2-pyrazolidinium ylides 315, easily available by reaction of the corresponding pyrazolidin-3-one with aromatic aldehydes, function as 1,3-dipoles in cycloaddition reactions with suitable alkenes and alkynes to provide the corresponding products. When unsymmetrical alkynes are used, mixtures of both possible products 316 and 317 are usually obtained (Equation 45). The regioselectivity of cycloadditions of the reaction with methyl propiolate is influenced by the substituents on the aryl residue using several 2,6-di- and 2,4,6-trisubstituted phenyl derivatives only compound 316 is formed <2001HCA146>. Analogous reactions of 3-thioxo-l,2-pyrazolidinium ylides have also been described <1994H(38)2171>. 

The most important aspect of ADC compounds, as far as heterocyclic synthesis is concerned, is the great reactivity of the N=N bond in cycloaddition reactions with dienes, monoenes and 1,3-dipoles, and as an electrophile. Other aspects of ADC reactivity are discussed briefly in Section V. 

PORPHYRINS IN CYCLOADDITION REACTIONS 2.2.1 Porphyrins as dienophiles in Diels-Alder reactions... 

Azomethine ylide cycloadditions have been utilized to prepare a number of novel fused pyrroles including pyrrolo[2,1 -a isoquinolincs <06CHJC279, 06TL1469> and pyrrolo[l,2-Zdpyridazines <06SL804>. Fused hydroxypyrroles were obtained in cycloaddition reactions with trimethylsilylketenes (TMS ketene) <06TL1469>. 

In this paper we review the techniques and applications of microwave irradiation in cycloaddition reactions. This mode ofheating requires not only appropriate micro-wave equipment and glassware, but also a new experimental outlook. A special section will focus on modifications of chemo-, regio- and stereoselectivity which result from use of microwaves, and possible explanations of this behavior will be given. 

In cycloaddition reactions few examples have been described in which changes in selectivity have been observed on use of microwave irradiation. In concerted pro-... 

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

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