Cycloalkylation 6 + 4 Cycloaddition reactions

Among the reactions applied in the synthesis of fullerene derivatives cycloaddition reactions such as [2 + 1]-, [2 + 2]-, [3 + 2] and [4 + 2] cycloadditions play a dominant role. In these reactions ring-fused fullerene derivatives are obtained, at least with incorporation of heteroatoms such as oxygen, nitrogen, or silicon. In this section photochemical reactions leading to cycloalkyl ring-fused fullerene adducts will be presented. Photocycloaddition reactions leading to C6o-fused heterocycles will be discussed later. 

The heterocyclic double bond in simple indoles will take part in cycloaddition reactions with dipolar 4n components,and with electron-deficient dienes (i.e. inverse electron demand), in most reported cases, held close nsing a tetherf° a comparable effect is seen in the intermolecular cycloaddition of 2,3-cycloalkyl-indoles to ortfto-quinone generating a l,4-dioxane. ° The introduction of electron-withdrawing substituents enhances the tendency for cycloaddition to electron-rich dienes 3-acetyl-1-phenylsulfonylin-dole, for example, undergoes aluminium-chloride-catalysed cycloaddition with isoprene, " and 3-nitro-l-phenylsulfonylindole reacts with l-acylamino-buta-l,3-dienes without the need for a catalyst. Both 3- and 2-nitro-l-phenylsulfonyl-indoles undergo dipolar cycloadditions with azomethine ylides. ... 

The present preparation illustrates a general and convenient irethod for ring contraction of cyclic ketones. The first step is the usual procedure for the preparation of enamines. The second step involves 1,3-dipolar cycloaddition of diphenyl phosphorazidate to an enamine followed by ring contraction with evolution of nitrogen. Ethyl acetate and tetrahydrofuran can be used as a solvent in place of toluene. Pyrrolidine enamines from various cyclic ketones smoothly undergo the reaction under similar reaction conditions. Diphenyl (cycloalkyl-1-pyrrolidinylmethylene)phosphoramidates with 5,6,7, and 15 members in the ring have been prepared in yields of 68-76%. 

A direct and efficient route to imidazoline and pyrrolidine derivatives using copper(ll) triflate-mediated [3+2] cycloaddition of various aryl, alkyl, and cycloalkyl iV-tosylaziridines with nitriles and olefins as dipolarophiles has been reported <2006TL5399>. Formation of bicyclic imidazoline 334 with a /ra j-ring junction as a single product from aziridine 333 suggested that the reaction proceeded through an SN2-type pathway (Scheme 86). 

Competing [2+8]- and [2+4] modes of cycloaddition between pentafulvenes 2 and 2//-cyclohepta[fe]furan-2-one 72 were reported by Nair and coworkers (Scheme 7.19) [20]. The pericyclic reaction of diethylfulvene 56 and 72 in toluene in a Schlenk tube at 150-170 °C predominantly gave tricycle 73 as the major product, with trace amounts of the [4+2] adduct. The formation of 73 resulted from a [2+8]cycloaddition of 56 and 72 followed by decarboxylation. The reaction between 72 with 6,6-cycloalkyl pentafulvene 48, however, gave about 1 1-mixture of [2+8]- and [2+4]adducts, 74 and 75, respectively. The outcomes of these inverse electron-demand cycloaddltlons are probably governed by steric factors. 

---