Oxyallyl cations, intramolecular reactions

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

Photogenerated oxyallyl cations undergo intramolecular cycloadditions when tethered to a furan an example is illustrated in Eq. 16 [44], The cyclo-hexadienone precursors required substituents which provide the right electronic characteristics for the photoconversion to the oxyallyl zwitterions, therefore limiting the versatility of this reaction. 

The intramolecular 4+3 cycloaddition reactions of cyclopentenyl oxyallylic cations have been studied and developed in our laboratory for quite some time. Our group appreciates the power of cyclic oxyallylic cations such as 31 that allow us to perform a 4+3 cycloaddition reaction to create a product that is a formal 4+(3+m) cycloadduct (32) (Scheme 10). So does Fred West s group. 

The intramolecular reaction of cyclic oxyallylic cations with tethered dienes (33) can lead to structurally complex molecules (34) that are... 

The lack of diastereoselectivity in the intramolecular 4+3 cycloaddition reactions of cyclopentenyl oxyallylic cations with tethered butadienes has prohibited the application of this methodology to the synthesis of natural produets. Therefore, the development of stereocontrol in cycloadditions of this type is of great significance. Our study in this area was conducted in the context of the synthesis of (+)-dactylol. 

There have been several examples of good stereocontrol in 4+3 cycloaddition reactions. For instance, it is known that cyclopentenyl and cyclohexenyl oxyallylic zwitterions bearing a stereogenic center react preferentially with dienes from the face opposite to the substituent. One of the precedents from our laboratory is the intramolecular reaction of ketones 50 and 52 (Scheme 15). Treatment of these ketones with LDA/TfCl afforded the corresponding a-chloroketones, which reacted intramolecularly with the tethered furans to yield cycloadducts 51a,b and 53a,b upon treatment with base. The endo isomers (51a, 53a) dominated in both cases. Only those products derived from the transition states having furans approach the least hindered faces of the oxyallylic cations were produced. 

The synthesis of seven-membered rings from furans and a,a -dibromoketones via oxyallyl intermediates is well known. It was reported that this reaction occurs quite easily when water is used as solvent <97TL8031>. In a study on the influence of steric and electronic effects on a function attached at C-2 of furans in the yield and diastereoselectivity of [4 + 3] cycloaddition reactions with oxyallyl cations it was found that in almost all cases a c/x-diastereospecificity and a high endo diastereoselectivity is obtained <97T11669>. A tandem Peterson olefination-[4 + 3] cycloaddition reaction with furans has been reported <97JOC1578 97TL386l>. For an intramolecular [4 -t- 3] cycloaddition see <97JOC6051>. The first asymmetric [4 + 3]... 

A convenient method for the formation of 2-oxyallyl cations has been developed from ketones and applied to both inter- and intramolecular cycloaddition reactions. The ketone is converted to the intermediate a-chloroketone, from which the 2-oxyallyl cation is formed with lithium perchlorate. For example, intramolecular cycloaddition of the 2-oxyallyl cahon generated from the cyclic ketone 194, gave the exo adduct 195 as the major stereoisomer (3.128). 

A one-step Lewis acid-catalysed intermolecular 4- -3-cycloaddition of aromatic a,)3-unsaturated aldehyde and ketones (105) with epoxides (106) formed seven-membered oxacycles (107) under mild conditions (Scheme 34).The effect of oxygen-, sulfur-, and halogen-substituents on the reactivity of nitrogen-stabilized oxyallyl cations in 4- -3-cycloaddition reactions has been extensively investigated. Aza-oxyallyl cationic intermediates react with cyclopentadiene and furan via an aza-4 -I- 3-cycloaddition reaction to form bicyclic cycloadducts in moderate yields. The intramolecular formal 4- -4-cycloaddition of conjugated enynes with an e-deflcient cyclobutene (108) yielded a strained six-membered cyclic allene (109) that isomerized to a 1,3-cyclohexadiene (110). This intermediate underwent a thermal or acid-promoted six-electron electrocyclic ring opening to yield a 2,4,6-cyclooctatrienone (111) (Scheme 35). ... 

The intramolecular aza-[4 + 3] cycloaddition reactions of aza-oxyallylic cations with furans provided macrocyclic furans (13S1825). The [4 + 3] cycloaddition products of 2-chlorocyclopentanones with furans were prone... 

Scheme 7.25 Intramolecular reactions of azides with oxyallyl cations...

Both intermolecular and intramolecular [4+3] cycloaddition reactions are known [6], The first example of this process was published by Fort in 1962 in the course of his studies on the mechanism of the Favorskii rearrangement [7]. He was able to capture the oxyallylic cation generated upon treatment of 1 with base in the presence of furan (Scheme 19.2). While the yield for the process was low, this contribution was seminal. It should be noted that this year (2012) marks the golden anniversary of this reaction. Research in both fundamental and applied aspects of the process continues unabated and the reaction can truly now be called synthetically useful. 

The Ga(III)-catalysed three-component 4 -I- 3-cycloaddition reactions of indoles (128), ketones (129), and cyclohexadienes (130) produced cyclohepta[h]indole derivatives (131) in a single step at room temperature (Scheme 40). The 4-i-3-cycloaddition reactions of pyrroles with stabilized 2-(silyloxy)allyl cations formed tropinones in high yields (85%)4 The intramolecular 4-t-3-cycloaddition reaction of diaza-oxyallyl cationic intermediates and cyclic dienes yielded bicyclic ureas in good to excellent yields. ... 

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