Oxyallyl cations 4 + 3 cycloaddition reactions

Reactions on the three-carbon bridge of oxabicyclo [3.2.1] compounds have been reported but less systematically studied. Because the majority of these compounds are derived from oxyallyl cation cycloadditions, most experiments on the three-carbon bridge involve addition to the bicyclic ketone. The parent oxabicyclo[3.2.1] ketone 90 undergoes reduction with bulky hydride sources such as L-selectride to generate the endo alcohol, Eq. 68 [112]. Presumably, the selectivity is due to equatorial attack of the hydride at the ketone of the pyran-one in a pseudo chair conformation. The exo alcohol is prepared from the endo alcohol by a Mitsunobu inversion-hydrolysis sequence [113]. 

In our initial studies, begun at the University of Florida, we relied on the classic oxyallyl cation cycloaddition for the preparation of these intermediates, but quickly noticed some of the key limitations with this reaction, especially when highly substituted or aimulated furans were employed as the diene... 

Epoxidations of chiral allenamides lead to chiral nitrogen-stabilized oxyallyl catioins that undergo highly stereoselective (4 + 3) cycloaddition reactions with electron-rich dienes.6 These are the first examples of epoxidations of allenes, and the first examples of chiral nitrogen-stabilized oxyallyl cations. Further elaboration of the cycloadducts leads to interesting chiral amino alcohols that can be useful as ligands in asymmetric catalysis (Scheme 2). 

Ring opening reaction of alkylidenecyclopropanone acetals readily proceeds in the presence of Lewis or Bransted acids to produce l-alkylidene-2-oxyallyl cation, which is provided for the reaction with nucleophiles such as chloride, alcohols, siloxyalkenes, and furans. The reaction of this cation with the carbon nucleophiles gives products of [4 + 3] and [3 + 2] cycloaddition as well as those of nucleophilic addition. The modes of addition reactions are controlled by the oxy group of the cation and by the reaction conditions including solvent. 

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... 

Cycloadditions. Silver perchlorate converts the l,l-dimcthyl-2-(tri-methylsiloxy)allyl chloride (1) and related allylic chlorides into an oxyallyl cation (a), which reacts with 1,3-dienes to form seven-membered ketones, often in high yield, particularly in reactions conducted in nitromethane. 

The competing pathways in the reactions of intermediate l-alkylidene-2-oxyallyl cations (47) with furan have been investigated.54 Cycloaddition pathways compete with the electrophilic substitution pathway which initially forms the cation (48) from which a number of products may form. 

The oxyallyl cation with its two electrons delocalized over the allylic system would add to furan in a [2 + 4] cycloaddition to give a new cation stabilized by the oxyanion or, in more familiar guise, a ketone. The reaction was supposed to go like this. 

A noteworthy work on complexes of this type has been on the oxyallyl complexes (171) and their related dimers (172). Despite the fact that only electrophilic substitution is observed in the reactions of these complexes and their derived (oxyallyl)Fe(CO)4 cations, these compounds, or structurally very similar ones, have been proposed as intermediates in iron carbonyl mediated [4 + 3] cycloaddition reactions of a ,a -dibromoketones and dienes. " ... 

Allenamides have been used as precursors for oxidation reactions <2002JOC1339>. The allene oxide products that are produced are useful oxyallyl cation equivalents for cycloaddition chemistry (Scheme 30) <2001JA7174, 2003JA12694>. 

Much attention has been directed to the chemistry of p-heteroatom-substituted allylic cations (5), especially oxyallyl cations (7), from both synthetic and mechanistic viewpoints. One of the characteristic reactions of oxyallyls (7) is their [4 -i- 3] 7 cycloadditions with conjugated dienes, which provide a... 

Irradiation of tetramethylcyclobutanedione (16) in furan gives an 8-oxabicyclo[3.2.1]oct-6-en-3-one derivative via oxyallyl cation (17). This reaction is the first example of the cycloaddition of cyclopropanones. Although the syntheses of a few seven-membered ring compounds have been subsequently carried out by means of this photoreaction, interest in cyclopropanone chemistry has been directed to structure and reactivity relationships, but not to organic synthesis. ... 

Harmata, M., Shao, L., Kurti, L., Abeywardane, A. 4+3 Cycloaddition reactions of halogen-substituted cyclohexenyl oxyallylic cations. Tetrahedron Lett. 1999, 40, 1075-1078. 

The direct construction of seven-membered rings via [4+3] cycloadditions is the most attractive strategy for preparing this frequently observed natural product substructure. A great amount of effort has been focused on methods to synthetize the less accessible three-atom component of these reactions. Oxyallyl cations are the most employed intermediates to generate this moiety (see Scheme 13). Alternatively, the use of 2-(silyloxy)acroleins and related compounds in the presence of a LA catalyst as the three-atom component in [4 + 3] cycloadditions has received much interest in the last years (see Scheme 14). 

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