Cycloaddition reactions 4 + 3 -oxyallyl

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). 

However, most asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides with alkenes are carried out without Lewis acids as catalysts using either chiral alkenes or chiral auxiliary compounds (with achiral alkenes). Diverse chiral alkenes are in use, such as camphor-derived chiral N-acryloylhydrazide (195), C2-symmetric l,3-diacryloyl-2,2-dimethyl-4,5-diphenylimidazolidine, chiral 3-acryloyl-2,2-dimethyl-4-phenyloxazolidine (196, 197), sugar-based ethenyl ethers (198), acrylic esters (199, 200), C-bonded vinyl-substituted sugar (201), chirally modified vinylboronic ester derived from D-( + )-mannitol (202), (l/ )-menthyl vinyl ether (203), chiral derivatives of vinylacetic acid (204), ( )-l-ethoxy-3-fluoroalkyl-3-hydroxy-4-(4-methylphenylsulfinyl)but-1 -enes (205), enantiopure Y-oxygenated-a,P-unsaturated phenyl sulfones (206), chiral (a-oxyallyl)silanes (207), and (S )-but-3-ene-1,2-diol derivatives (208). As a chiral auxiliary, diisopropyl (i ,i )-tartrate (209, 210) has been very popular. 

An extensive review of 4 - - 3-cycloaddition reactions has been presented. The 1,3-difluorooxyallyl intermediate obtained from l-bromo-l,3-difluoropropan-2-one undergoes 4 + 3-cycloaddition with cyclopentadiene and ftiran to give difluorobicyclo[3.2.1]octenones. The use of 4 + 3-cycloaddition reactions of cyclic oxyallyls in the synthesis of natural products has been extensively studied. The intramolecular 4 + 3-cycloaddition of allylic sulfones (111) possessing a diene in the side-chain in the presence of Lewis acids yield cycloadducts (112) in good to excellent yields (Scheme 43). ... 

Only a limited number of examples are known of applications of thietanes in organic synthesis. Prominent among these examples would be electrophilic ring opening reactions leading to polyfunctional sulfur compounds (33)-(37), utilization of 3-thietanones (55) and metal complexes (87) derived therefrom as oxyallyl zwitterion equivalents in cycloaddition reactions, synthesis of dipeptide (63) with a /3-thiolactone, Raney nickel desulfurization of thietanes (e.g. 120 cf. Table 7) as a route to gem-dimethyl compounds, and desulfurization of thietanes (e.g. 17) in the synthesis of cyclopropanes (also see Table 7). 

The bicyclic ketone (44), obtained from the Fe2(CO)9-promoted [3 + 4] cycloaddition reaction of a,a,a, a -tetrabromoacetone and 2-isopropylfuran followed by Zn-Cu couple reduction, has been converted to the naturally occurring troponoid, /3-thujaplicin (46) (75JOC806). Hydrogenation of (44), ether cleavage, bromination and dehydrobromination gave the tropone (45), an intermediate easily converted into the tropolone (46) by a standard procedure (Scheme 10). A related [3 + 4] cycloaddition reaction of oxyallyl metallic with furan has been used to assemble the antibiotic C-nucleosides (78JA2561). 

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... 

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. " ... 

Cyclopropanones act as three-carbon sources in [4 + 3] cycloadditions. However, these small-ring compounds are impractical for preparative scale experiments, because they are generally inaccessible and also must be handled with special precautions, in contrast to other oxyallyl species. One of the early descriptions of the synthesis of cyclopropanones appeared in 1932. NeverAeless, the development of the chemistry of cyclopropanones proceeded at quite a slow pace until the late 1960s when Turro reported their utilization in [4 -i- 3] cycloaddition reactions. 2,2-Dimethylcyclopropanone (14) adds to furan, cyclopentadiene, 6,6-dimethylfulvene and the relatively nucleophilic N-methylpyrrole to give the corresponding cycloproducts, but fails to react with anthracene and 1,3-butadiene. Parent cyclopropan-one cannot be used. 

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). 

The [4-1-3] cycloaddition between 2-aminofuran and oxyallyl cations, followed by base-induced elimination of the resulting adducts, was used in the synthesis of 3-aminotropones <05TL8475>. A chiral Lewis acid-catalyzed [4t-3] cycloaddition between furans and nitrogen-stabilized oxyallyl cations derived from allenamides was developed. As depicted below, the Cj-symmetric salen-based ligand is the most effective in promoting this asymmetric cycloaddition reaction <05JA50>. 

The furan nucleus undergoes cycloadditions with oxyallyl cations to produce compounds with the oxabicyclo[3.2.1]octene skeleton. Various research groups have found new ways of generating the oxyallyl cation and have also defined the types of substituted furans which undergo reaction. Reviews on this reaction have covered the literature up to 1987 [24-26]. The mechanism of the cycloaddition has been discussed in detail by Hoffmann [26]. 

The cycloaddition reaction of oxyallylic cations was First reported by Fort during an apparent study of the Favorskii rearrangement." Since then, the research in this area has received increasing attention and several reviews have appeared in literature. One of the most exciting modifications is the use of cyclic oxyallyls in the 4+3 cycloaddition reaction, which allows an opportunity for creating more complicated and... 

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