F Cycloadditions

Louerat, F., Bougrin, K., Loupy, A., Ochoa de Retana, A.M., Pagalday, J. and Palacios, F., Cycloaddition reactions of azidomethyl phosphonate with acetylenes and enamines. Synthesis of triazoles, Heterocycles, 1998, 48, 161-170. 

F. Cycloaddition Reactions of Ruthenium Acetylide and Vinylidene Complexes... 

Some substituted alkenes react with singlet oxygen to form a dioxetane in a -f- cycloaddition reaction. Most dioxetanes readily decompose to carbonyl compounds in an exothermic reaction that is accompanied by a bluish luminescence. The chemiluminescence will be dealt with in more detail in Section 7.6.4. 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Malpass, 1977). Diels-Alder type [2 + 4]-cycloadditions are possible with certain hetero-"ene components (J.R. Malpass, 1977 S.F. Martin, 1980) or with highly reactive o-quinodimethanes as diene components (W. Oppoizer, I978A). 

Cycloadditions of diazaquinones with unsaturated compounds yield diazacyc-lobutanes, from which N-substituted 3-hydroxypyridazin-6(l/f)-ones are formed after addition of water, t-butanol or acetic acid (Scheme 56). The same types of compound are also obtained from enamines. 

The distinction between these two classes of reactions is semantic for the five-membered rings Diels-Alder reaction at the F/B positions in (269) (four atom fragment) is equivalent to 1,3-dipolar cycloaddition in (270) across the three-atom fragment, both providing the 47t-electron component of the cycloaddition. Oxazoles and isoxazoles and their polyaza analogues show reduced aromatic character and will undergo many cycloadditions, whereas fully nitrogenous azoles such as pyrazoles and imidazoles do not, except in certain isolated cases. 

The functionalized allene, DIMETHYL 2,3-PENTADIENEDIOATE, the first in the series, is an intriguing substrate for various addition and cycloaddition reactions. Finally, a new reagent, DI-ferf-BUTYL DICARBONATE, for he formation of A-f-BOC derivatives which eliminates the use o the hazardous fert-BUTYL AZIDOFORMATE (WARNING) is intrqduced. 

The selection rules for cycloaddition reactions can also be derived from consideration of the aromaticity of the transition state. The transition states for [2tc -f 2tc] and [4tc -1- 2tc] cycloadditions are depicted in Fig. 11.11. For the [4tc-1-2tc] suprafacial-suprafacial cycloaddition, the transition state is aromatic. For [2tc -F 2tc] cycloaddition, the suprafacial-suprafacial mode is antiaromatic, but the suprafacial-antarafacial mode is aromatic. In order to specify the topology of cycloaddition reactions, subscripts are added to the numerical classification. Thus, a Diels-Alder reaction is a [4tc -f 2 ] cycloaddition. The... 

The behavior of strained,/Zuorimiret/ methylenecyelopropanes depends upon the position and level of fluorination [34], l-(Difluoromethylene)cyclopropane is much like tetrafluoroethylene in its preference for [2+2] cycloaddition (equation 37), but Its 2,2-difluoro isomer favors [4+2] cycloadditions (equation 38). Perfluoromethylenecyclopropane is an exceptionally reactive dienophile but does not undergo [2+2] cycloadditions, possibly because of stenc reasons [34, 45] Cycloadditions involving most possible combinations of simple fluoroalkenes and alkenes or alkynes have been tried [85], but kinetic activation enthalpies (A/f j for only the dimerizations of tetrafluoroethylene (22 6-23 5 kcal/mol), chlorotri-fluoroethylene (23 6 kcal/mol), and perfluoropropene (31.6 kcal/mol) and the cycloaddition between chlorotnfluoroethylene and perfluoropropene (25.5 kcal/mol) have been determined accurately [97, 98] Some cycloadditions involving more functionalized alkenes are listed in Table 5 [99. 100, 101, 102, 103]... 

Only a few reactions of benzodithiadiazines have been investigated. In common with dithiatriazines 12.8, the anti-aromatic system 12.12 (R = H) undergoes a reversible 5,5 -cycloaddition with norbornadiene. The reaction of 12.12 (R = F) with triphenylphosphine results in a ring contraction to give the imino 2 -phosphane 12.13. ... 

Intramolecular cycloadditions of 4/f-pyrido[l,2-n]pyrimidin-4-ones 235 (R = H, Me Ph) and MeNHOH HCl gave tetracyclic isoxazolo derivatives 237. In the case of 235 (R = Me) a minor epimer 238 was also isolated (00JCR(S)414). Similar reaction of 235 (R = H, Me, Ph) and sarcosine ethyl ester HCl afforded an isomeric mixture of epimeric tetracyclic pyrrolo derivatives 239 and 240. In the reaction of 235 (R = H) and PhCHjNHCHjCOOEt only one product 241 was obtained. 

The above described reaction has been extended to the application of the AlMe-BINOL catalyst to reactions of acyclic nitrones. A series chiral AlMe-3,3 -diaryl-BINOL complexes llb-f was investigated as catalysts for the 1,3-dipolar cycloaddition reaction between the cyclic nitrone 14a and ethyl vinyl ether 8a [34], Surprisingly, these catalysts were not sufficiently selective for the reactions of cyclic nitrones with ethyl vinyl ether. Use of the tetramethoxy-substituted derivative llg as the catalyst for the reaction significantly improved the results (Scheme 6.14). In the presence of 10 mol% llg the reaction proceeded in a mixture of CH2CI2 and petroleum ether to give the product 15a in 79% isolated yield. The diastereoselectiv-ity was the same as in the acyclic case giving an excellent ratio of exo-15a and endo-15a of >95 <5, and exo-15a was obtained with up to 82% ee. 

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