Cycloaddition reactions Cycloalkanes

Accordingly, cycloaddition reactions can be carried out in solvents such as alkanes, cycloalkanes and acetonitrile. The insertion reaction into the O-H-bond of alcohols is, in every case, faster than the cycloaddition reaction. Furthermore, 100% production of isocyanate was observed upon irradiation of benzoyl azide in dichloromethane solution, however, nitrenes that can be trapped by compounds with double bonds are also formed in this solvent [21]. 

Tethering the alkene to the carbon atom of the nitrone allows the preparation of cw-l,2-disubstituted cycloalkanes such as 212. Examples in which the alkene is tethered to the nitrogen atom of the nitrone are also common. Thus, addition of formaldehyde to the hydroxylamine 213 promoted formation of the intermediate nitrone and hence the cycloadduct 214 (3.140). " Subsequent transformations led to the alkaloid luciduline. This synthesis illustrates a useful feature of the 1,3-dipolar cycloaddition reaction of nitrones, in that it provides an alternative to the Mannich reaction as a route to (3-amino-ketones, via reductive cleavage of the N-0 bond in the isoxazolidine and oxidation of the 1,3-amino-alcohol product. In another example of such an intramolecular cycloaddition reaction, the bridged bicyclic product 217, used in a synthesis of indolizidine 209B, was formed by addition of an aldehyde to the hydroxylamine 215, followed by heating the intermediate nitrone 216 (3.141).142... 

This book is considered to be a handbook about the application of chiral auxiliaries in selected areas of cycloaddition reactions. We hope it will serve as a useful tool for those working in the field of organic synthesis, e.g. the stereoselective synthesis of cycloalkanes and heterocycles,... 

As an extension of this reaction the intramolecular cycloaddition of 5-propynyloxycycloalkanepyrimidines was studied. It was found that bi-and tricyclic annelated pyridine derivatives are formed by expulsion of either X-CH2-CN and/or HCN, respectively. A marked selectivity in the product formation was observed, depending on the size of the cycloalkane ring. With cyclohexapyrimidines a mixture of A and B is formed, while with the cycloheptapyrimidine derivative exclusive formation of the tricyclic compound B takes place (92T1643, 92T1657) (Scheme 39a). 

Another interesting example is provided by the phenylethynylcarbene complex 173 and its reactions with five-, six-, and seven-membered cyclic enamines 174 to form bridgehead-substituted five-, six-, and seven-membered cycloalkane-annelated ethoxycyclopentadienes with high regioselectivity under mild reaction conditions (Scheme 38) [119,120]. In these transformations the phenylethynylcarbene complex 173 acts as a C3 building block in a formal [3+2] cycloaddition. Like in the Michael additions (reaction route F in Scheme 4), the cyclic electron-rich enamines 174 as nucleophiles attack the... 

In the Diels-Alder reaction with inverse electron demand, the overlap of the LUMO of the 1-oxa-l,3-butadiene with the HOMO of the dienophile is dominant. Since the electron-withdrawing group at the oxabutadiene at the 3-position lowers its LUMO dramatically, the cycloaddition as well as the condensation usually take place at room or slightly elevated temperature. There is actually no restriction for the aldehydes. Thus, aromatic, heteroaromatic, saturated aliphatic and unsaturated aliphatic aldehydes may be used. For example, a-oxocarbocylic esters or 1,2-dike-tones for instance have been employed as ketones. Furthermore, 1,3-dicarbonyl compounds cyclic and acyclic substances such as Meldmm s acid, barbituric acid and derivates, coumarins, any type of cycloalkane-1,3-dione, (1-ketoesters, and 1,3-diones as well as their phosphorus, nitrogen and sulfur analogues, can also be ap-... 

In addition to the alkylations discussed above, some special reactions have been reported that enable the solid-phase synthesis of cycloalkanes. These include the intramolecular ene reaction and the cyclopropanation of alkenes (Figure 5.5 see also [44]). Cyclobutanes have been prepared by the reaction of polystyrene-bound carbanions with epichlorohydrin, and by [2 + 2] cycloadditions of ketenes to resin-bound alkenes. 

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