Cycloadditions reactions with, alkenes

Some ketones undergo a cycloaddition reaction with alkenes to form oxetanes ... 

In addition to undergoing cycloaddition reactions with alkenes and al-kynes, silenes readily undergo cycloaddition reactions with heteroatom multiple bonds such as C=0 and C=N, most commonly when the trapping reagent for the silene is either an aldehyde, ketone, or imine. In many... 

Cycloaddition Reactions with Alkenes Olefins can react with electrogenerated radicals, cationic species or dienophiles. 

An electrochemical oxidation of hydro-quinones can be used to initiate [3 + 2] cycloaddition reactions with alkenes. The... 

Other examples of the iodonium ylide-based syntheses of furan derivatives involve cycloaddition reactions with alkenes or alkynes. Although the majority of these syntheses involve stable iodonium ylides (86JOC3453 94T11541) (e.g., Eqs. 16 and 17), in some cases the ylides are unstable and are generated in situ (92JOC2135) (e.g., Eq. 18). In the case of alkenes, dihydrofuran derivatives are obtained (Eqs. 16-18). This synthetic route is especially useful for the synthesis of dihydrobenzofuran derivatives that are related to the neolignan family of natural products (Eq. 18). 

The meso-ionic l,3>2-oxathiazol-5-ones (169) show an interesting range of reactions with nucleophiles including ammonia, primary amines, and aqueous alkali. They also react with l,3-dipolarophiles, including dimethyl acetylenedicarboxylate and methyl propiolate, yielding isothiazoles (171) and carbon dioxide. 1,3-Dipolar cycloaddition reactions with alkenes such as styrene, dimethyl maleate, and methyl cinnamate also lead to isothiazoles (171) directly. BicycUc intermediates (cf. 136) were not isolable these cycloaddition reactions with alkenes giving isothiazoles involve an additional dehydrogenation step. 

As part of an extensive study of the 1,3-dipolar cycloadditions of cyclic nitrones, Ali et al. (392-397) found that the reaction of the 1,4-oxazine 349 with various dipolarophiles afforded the expected isoxazolidinyloxazine adducts (Scheme 1.78) (398). In line with earlier results (399,400), oxidation of styrene-derived adduct 350 with m-CPBA facilitated N—O cleavage and further oxidation as above to afford a mixture of three compounds, an inseparable mixture of ketonitrone 351 and bicyclic hydroxylamine 352, along with aldonitrone 353 with a solvent-dependent ratio (401). These workers have prepared the analogous nitrones based on the 1,3-oxazine ring by oxidative cleavage of isoxazolidines to afford the hydroxylamine followed by a second oxidation with benzoquinone or Hg(ll) oxide (402-404). These dipoles, along with a more recently reported pyrazine nitrone (405), were aU used in successful cycloaddition reactions with alkenes. Elsewhere, the synthesis and cycloaddition reactions of related pyrazine-3-one nitrone 354 (406,407) or a benzoxazine-3-one dipolarophile 355 (408) have been reported. These workers have also reported the use of isoxazoles with an exocychc alkene in the preparation of spiro[isoxazolidine-5,4 -isoxazolines] (409). 

This chapter deals mainly with the 1,3-dipolar cycloaddition reactions of three 1,3-dipoles azomethine ylides, nitrile oxides, and nitrones. These three have been relatively well investigated, and examples of external reagent-mediated stereocontrolled cycloadditions of other 1,3-dipoles are quite limited. Both nitrile oxides and nitrones are 1,3-dipoles whose cycloaddition reactions with alkene dipolarophiles produce 2-isoxazolines and isoxazolidines, their dihydro derivatives. These two heterocycles have long been used as intermediates in a variety of synthetic applications because their rich functionality. When subjected to reductive cleavage of the N—O bonds of these heterocycles, for example, important building blocks such as p-hydroxy ketones (aldols), a,p-unsaturated ketones, y-amino alcohols, and so on are produced (7-12). Stereocontrolled and/or enantiocontrolled cycloadditions of nitrones are the most widely developed (6,13). Examples of enantioselective Lewis acid catalyzed 1,3-dipolar cycloadditions are summarized by J0rgensen in Chapter 12 of this book, and will not be discussed further here. 

Woodward s synthesis, 4, 416-419 Chlorophyll b, 4, 382 Chlorophyll c, 4, 382 Chlorophyll d, 4, 382 Chlorophylls, 4, 378 biosynthesis reviews, 1, 99 structure, 4, 370 substituents reactions, 4, 402 Chloroporphyrin e6, 4, 404 Chloroprothixene pharmacology, 3, 942 Chloropyramine as antihistamine, 1, 177 Chloropyrifos synthesis, 2, 201 Chloropyrifos-ethyl as insecticide, 2, 516 Chloropyrifos-methyl as insecticide, 2, 516 Chloroquine, 1, 145 adsorption on nucleic acids, 1, 179 as antimalarial, 1, 173, 2, 517 Chloroquine, hydroxy-as antimalarial, 2, 517 Chlorosulfonyl isocyanate cycloaddition reactions with alkenes, azetidin-2-ones from, 7, 261 reactions... 

In trimethylenemethane complexes, the metal stabilizes an unusual and highly reactive ligand which cannot be obtained in free form. Trimethylenemethanetricar-bonyliron (R=H) was the first complex of this kind described in 1966 by Emerson and coworkers (Figure 1.2) [38]. It can be obtained by reaction of bromomethallyl alcohol with Fe(CO)5. Trimethylenemethaneiron complexes have been applied for [3+2]-cycloaddition reactions with alkenes [39]. 

Conjugated dienes have alternating single and double bonds. They may undergo 1,2- or 1,4-addition. Allylic carbocations, which are stabilized by resonance, are intermediates in both the 1,2- and 1,4-additions (Sec. 3.15a). Conjugated dienes also undergo cycloaddition reactions with alkenes (Diels-Alder reaction), a useful synthesis of six-membered rings (Sec. 3.15b). 

The dithionitronium cation SNS+ (as the AsFg salt) underwent quantitative concerted symmetry-allowed cycloaddition reactions with alkenes (ethylene, methylethylene, trans- and +r-l,2-dimethylethylene, 1,1-dimethyl-ethylene, tetramethylethylene, and norbornene) to give 1,3,2-dithiazolidine cations 76 (Equation 20) <1996JCD1997>. 

Thiobenzoyl isocyanate is widely used as a starting material for the syntheses of TAs 23-28 both in condensation with ethyl sodiocyanoacetate (86KGS3) and in [4 + 2]-cycloaddition reactions with alkenes and alkynes containing electron-donor groups (81CB2713 85ZC324) (Scheme 4). It is established that the rate of cycloaddition increases from alkenes to alkynes and with the electron-donor properties of substituents. 

Gilbert, A. (2004) 1,4-Quinone Cycloaddition Reactions with Alkenes, Alkynes, and Related Compounds, in CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds F. Lenci and W.M. Horspool), CRC Press, New York, pp. 87-1-87-12. 

Application of azomethine ylides in dipolar cycloaddition reactions with alkenes provides a route to pyrrolidine derivatives, as illustrated by the generation of the intermediate 12, and its subsequent conversion to the target system 13 (Scheme 16) <1995TL9409, CHEC-III(3.03.9)327>. 

Fluorinated cyclobutenes synthesized from the cycloaddition of fluoroalkenes with non-fluorinated alkynes (vide supra) undergo pyrolysis to give fluorinated butadienes, e.g. the pyrolysis of 3,3.4,4-tetrafluorocyclobut-l-ene gives l,1,4,4-tctrafluorobuta-l,3-diene (15) almost quantitatively. Tetrafluorodienes of this type undergo [2-F 2]-cycloaddition reactions with alkenes to give fluorinated cyclobutancs. ... 

Despite the expected enhanced reactivity of trifluoromethyl-substituted carbenes as electrophilic species, yields in their [2+ l]-cycloaddition reactions with alkenes are highly dependent on the nature of the second substituent of the carbene. Furthermore, the method of carbene generation has a large influence on carbene reactivity. 

Benzyne undergoes [2+2] -cycloaddition reaction with alkenes to give four-membered rings. 

Since the initial demonstrations of the participation of substituted l,2,4,S-tetrazines ° and oxa-zoles ° ° in [4 + 2] cycloaddition reactions with alkene and alkyne dienophiles, the investigation and application of the Diels-Alder reactions of heteroaromatic systems possessing reactive azadienes have been pursued extensively. A number of general reviewshave treated the spectrum of heteroaromatic azadienes that participate in [4 + 2] cycloaddition reactions and many of the individual heteroaromatic systems have been reviewed separatcly. ° ° An extensive account was published recently and should be consulted for descriptions of the [4 + 2] cycloaddition reactions of the common heteroaromatic azadienes that have been observed to date. 

In an unanticipated result the cation 111 did not react directly with alkenes but instead first underwent the ring expansion reaction shown in equation 12 whereas analogous cations Ar = CMc2 gave cycloaddition reactions with alkenes. 

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