In 4+2 cycloaddition

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). 

The importance of both electronic and steric effects is clear in cycloadditions as in cross-oxidations. One example is a heterocycHc modification leading to the thermodynamically less stable natural form of juglone derivatives such as ventiloquinones JT [124917-64-2] (84) and I [124917-65-3] (85) (83). The yields are 97% (84) from 6-chloro-2,3-dimethoxy-l,4-ben2oquinone [30839-34-0] and 100% (85) upon hydrolysis. 

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). 

The double bond in A7-methoxycarbonyl-2-azetine (237 Z = COaMe) undergoes acid or photocatalyzed hydration and subsequent ring opening to give the aldehyde (238). In cycloadditions it is inert to TCNE and diphenylisobenzofuran but it does react with dipyridyltetrazine (77CC806). 

The chemistry of benzazetidin-2-ones (251) can also be explained in terms of facile ring opening to the iminoketenes (252) which dimerize, rearrange or can be intercepted by nucleophiles or in cycloadditions depending on the conditions. Indeed, this ring opening precludes their isolation in all but exceptional cases (Section 5.09.4.3.5) (76AHC(19)215). 

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 same conclusions are drawn by analysis of the frontier orbitals involved in cycloadditions. For the most common case of the Diels-Alder reaction, which involves dienophiles with electron-attracting substituents, the frontier orbitals are l/2 of the diene (which is the HOMO) and n of the dienophile (which is the LUMO). Reaction occurs by interaction of the HOMO and LUMO, which can be seen from the illustration below to be allowed. 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

It can be assumed that in cycloadditions only one reactant is electronically excited, in view of the short lifetimes of excited species in solution and the consequently low probability of a collision between two excited molecules. Also, the cycloadditions are conducted with light of wavelengths above 2800 A... 

Fluorinated allenes are especially reactive in cycloadditions because of their highly strained double bonds [118, 119] 1,1-Difluoro- and 1-fluoroallene readily undergo both [2+2] and [4+2] cycloadditions [118 124] (equations 50-52) Exten sive studies of stereochemistry and regioselectivity show that cyclobutane forma-... 

It IS likely that the syn selectivity exhibited in cycloadditions of fluoroallene IS due to electrostatic interactions [23 25] As in the case of difluoroallene the reactions of fluoroallene with diazoalkanes and nitrile oxides are facile, but such reactions, other than that shown in equation 18, are neither regio nor stereospeutic [23, 25] Indeed, the addition of phenylnitrile oxide to fluoroallene occurs with preferential anti addition for both regioisomenc products (equation 20)... 

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]. 

For a system of classification of cycloaddition reactions, see Huisgen, R. Angew. Chem. Int. Ed. Engl., 1968, 7, 321. For a review of certain types of cycloadditions leading to three- to six-membered rings involving 2, 3, or 4 components, see Posner, G.H. Chem. Rev, 1986, 86, 831. See also the series Advances in Cycloaddition. 

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>. 

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