Alkenes, cycloaddition reaction with

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

Cycloaddition reactions also have important applications for acyclic chalcogen-nitrogen species. Extensive studies have been carried out on the cycloaddition chemistry of [NSa]" which, unlike [NOa]", undergoes quantitative, cycloaddition reactions with unsaturated molecules such as alkenes, alkynes and nitriles (Section 5.3.2). ° The frontier orbital interactions involved in the cycloaddition of [NSa]" and alkynes are illustrated in Fig. 4.13. The HOMO ( Tn) and LUMO ( r ) of the sulfur-nitrogen species are of the correct symmetry to interact with the LUMO (tt ) and HOMO (tt) of a typical alkyne, respectively. Although both... 

More recently, further developments have shown that the reaction outlined in Scheme 4.33 can also proceed for other alkenes, such as silyl-enol ethers of acetophenone [48 b], which gives the endo diastereomer in up to 99% ee. It was also shown that / -ethyl-/ -methyl-substituted acyl phosphonate also can undergo a dia-stereo- and enantioselective cycloaddition reaction with ethyl vinyl ether catalyzed by the chiral Ph-BOX-copper(ll) catalyst. The preparative use of the cycloaddition reaction was demonstrated by performing reactions on the gram scale and showing that no special measures are required for the reaction and that the dihydro-pyrans can be obtained in high yield and with very high diastereo- and enantioselective excess. 

Scheeren et al. reported the first enantioselective metal-catalyzed 1,3-dipolar cycloaddition reaction of nitrones with alkenes in 1994 [26]. Their approach involved C,N-diphenylnitrone la and ketene acetals 2, in the presence of the amino acid-derived oxazaborolidinones 3 as the catalyst (Scheme 6.8). This type of boron catalyst has been used successfully for asymmetric Diels-Alder reactions [27, 28]. In this reaction the nitrone is activated, according to the inverse electron-demand, for a 1,3-dipolar cycloaddition with the electron-rich alkene. The reaction is thus controlled by the LUMO inone-HOMOaikene interaction. They found that coordination of the nitrone to the boron Lewis acid strongly accelerated the 1,3-dipolar cycloaddition reaction with ketene acetals. The reactions of la with 2a,b, catalyzed by 20 mol% of oxazaborolidinones such as 3a,b were carried out at -78 °C. In some reactions fair enantioselectivities were induced by the catalysts, thus, 4a was obtained with an optical purity of 74% ee, however, in a low yield. The reaction involving 2b gave the C-3, C-4-cis isomer 4b as the only diastereomer of the product with 62% ee. 

V-Acyliminium ions act as dienophiles in [4 + 2] cycloaddition reactions with conjugated dienes13, while A-acylimimum ions that (can) adopt an x-cis conformation are able to act as heterodienes in an inverse electron demand Diels-Alder process with alkenes or alkynes3 (see Section D. 1.6.1.1.). 

The 2-azadiene system of the pyrazinone scaffold undergoes inter- and intramolecular cycloaddition reactions with a variety of (functionalized) alkenes forming bicyclic adducts, leading to the stereoselective generation of a variety of natural product analogues as well as peptidomimetics [58]. These bicyclic compounds could serve as key intermediates in the synthesis... 

Cycloaddition reactions with the Si(lOO) surface have been investigated for the purpose of designing microelectronics, nonlinear optical materials, sensors, and biologically active surfaces. The features of the [2+2] cycloadditions characteristic of the reactions in the pseudoexcitation band [133] predicts that [2+2] cycloadditions of electron-donating alkenes with Si(100)-2 x 1 surface could proceed with retention of configurations, in agreement with the observation [134]. Such stereospecific functionalizations of surfaces are of potential use for specific applications. 

The reactivity of the prototype o-QM as heterodiene in Diels-Alder cycloaddition reactions with several substituted alkenes such as methyl vinyl ether (MVE), styrene,... 

Given their extraordinary reactivity, one might assume that o-QMs offer plentiful applications as electrophiles in synthetic chemistry. However, unlike their more stable /tora-quinone methide (p-QM) cousin, the potential of o-QMs remains largely untapped. The reason resides with the propensity of these species to participate in undesired addition of the closest available nucleophile, which can be solvent or the o-QM itself. Methods for o-QM generation have therefore required a combination of low concentrations and high temperatures to mitigate and reverse undesired pathways and enable the redistribution into thermodynamically preferred and desired products. Hence, the principal uses for o-QMs have been as electrophilic heterodienes either in intramolecular cycloaddition reactions with nucleophilic alkenes under thermodynamic control or in intermolecular reactions under thermodynamic control where a large excess of a reactive nucleophile thwarts unwanted side reactions by its sheer vast presence. 

Method G is used to introduce the alkyl fragment when less reactive alkenes are employed or for cases where functionality within the dienophilic alkene undergoes reaction with the Grignard reagent. Following this procedure, a lithium anion is first added to the aldehyde 5 at 78 °C.27 After consumption of the aldehyde has been determined by TLC, the dienophile is added and magnesium bromide is introduced. The cycloaddition occurs as the reaction warms to room temperature. In the case of... 

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... 

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>. 

Sulfonyl imides (78) are, like sulfenes, prepared by dehydrohalogenation of the corresponding sulfonyl chlorides (79) (usually called sulfamoyl chlorides). Like sulfenes, they take part in [2 + 2] and [4 + 2] cycloaddition reactions with electron-rich alkenes or with 1,3-dienes, yielding 1,2-thia-zetidine 1,1-dioxides (80)104 or dihydro-1,2-thiazines (81),105 respectively. 

Chromone-3-carbonitrile oxide obtained from 3-formylchromone oxime by bromination and subsequent dehydrobromination underwent cycloaddition reactions with terminal alkenes to give isoxazolines 34 (175). 

The Lewis acid catalyst 53 is now referred to as the Narasaka catalyst. This catalyst can be generated in situ from the reaction of dichlorodiisopropoxy-titanium and a diol chiral ligand derived from tartaric acid. This compound can also catalyze [2+2] cycloaddition reactions with high enantioselectivity. For example, as depicted in Scheme 5-20, in the reaction of alkenes bearing al-kylthio groups (ketene dithioacetals, alkenyl sulfides, and alkynyl sulfides) with electron-deficient olefins, the corresponding cyclobutane or methylenecyclobu-tene derivatives can be obtained in high enantiomeric excess.18... 

Cycloadditions. Desilylation of 1 in CH,CN with LiF results in an azomethine ylide (a), which undergoes cycloaddition reactions with dipolarophiles and activated alkenes to give pyrrolidines. 

Another example of a diene undergoing a [2 + 2] cycloaddition reaction with an alkene has been reported recently4. 2-Dimethylaluminumoxy-l,3-cyclohexadiene (7) reacted with phenyl vinyl sulfoxide (8) to afford a diastereomeric mixture of cis substituted cyclobutanols 9 (equation 3). The occurrence of a [2 + 2] cycloaddition as well as the high cis stereoselectivity observed were explained by a pre-organization of the reactants by complexation of the diene bound aluminum with the sulfoxide oxygen on the olefin. 

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 triphenyl derivative (91, R = R = R = Ph, R = H) is formed in a mechanistically interesting reaction between benzoyl formic acid anil (Ph-N=CPh-C02H), trifluoroacetic anhydride, and pyridine. Its 1,3-dipolar cycloaddition reactions with alkynes and alkenes have been reported. ... 

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. 

Elsewhere, Heaney et al. (313-315) found that alkenyloximes (e.g., 285), may react in a number of ways including formation of cyclic nitrones by the 1,3-APT reaction (Scheme 1.60). The benzodiazepinone nitrones (286) formed by the intramolecular 1,3-APT will undergo an intermolecular dipolar cycloaddition reaction with an external dipolarophile to afford five,seven,six-membered tricyclic adducts (287). Alternatively, the oximes may equilibrate to the corresponding N—H nitrones (288) and undergo intramolecular cycloaddition with the alkenyl function to afford five,six,six-membered tricyclic isoxazolidine adducts (289, R = H see also Section 1.11.2). In the presence of an electron-deficient alkene such as methyl vinyl ketone, the nitrogen of oxime 285 may be alkylated via the acyclic version of the 1,3-APT reaction and thus afford the N-alkylated nitrone 290 and the corresponding adduct 291. In more recent work, they prepared the related pyrimidodiazepine N-oxides by oxime-alkene cyclization for subsequent cycloaddition reactions (316). Related nitrones have been prepared by a number of workers by the more familiar route of condensation with alkylhydroxylamines (Scheme 1.67, Section 1.11.3). 

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