Rings, seven-membered, from acyclic

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... 

The zirconium complex 315 reacts with diphenylacetylene to give the coupling product 326 [Eq. (55)].116 The regiochemistry of the reaction parallels that of an acyclic allene complex.133 Complex 315 also reacts with stilbene, but in this case, the seven-membered ring is displaced from the metal. The fate of the seven-membered ring is unknown. 

II,G,4), six- (Section II,G,5), and even seven-membered rings (Section II,G,6) are described and finally the syntheses of coupled ring systems (Section II,G,7). Two main methods of preparation are described one is the intermolecular cleavage of hydrogen chloride from hydrazine and dialkyl- or aryldichlorosilanes,55 another is based on the cyclization of acyclic fluorosilylhydrazines.153839,56 57... 

Besides the common oleftnic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acyclic electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cyclic polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

All the above cascade alkene metathesis reactions are based on the ROM of a cycloalkene moiety. Harrity and co-workers have described the synthesis of functionalized spiro cyclic systems by cascade selective olefin ringclosing metathesis reactions from an acyclic tetraalkene. The selectivity for five-membered ring closure over seven-membered ring closure would be the result of a kinetically favored cyclization process [42] (Scheme 20). The syn-... 

The palladium(0)-catalyzed asymmetric O-allylation of phenols has been described using five-, six- and seven-membered ring allylic carbonates and acyclic allylic carbonates (eq 9). The products from these reactions were subjected to a Claisen rearrangement to provide C-alkylated phenols. A study of various ligands for the reaction of phenol with 2-cyclohexenyl-l-methyl carbonate clearly showed that the Trost ligand is superior. ... 

II. SEVEN-MEMBERED RINGS FROM ACYCLIC PRECURSORS... 

Shibasaki and co-workers used a ring-closing metathesis approach to prepare a number of five-, six-, and seven-membered rings from electron-deficient olefins. Treatment of acyclic enol ether 18 with 7 mol % of 3 in refluxing benzene provided the corresponding cyclic enol ether 19 in 94% yield. Deprotection of the silyl ether 19 (not shown) resulted in the corresponding cyclic ketone, a valuable synthetic intermediate in natural products synthesis and a number of industrial processes. The authors reported additional examples of the synthesis of five-membered ring carbocycles as part of this study. 

Shibasaki and co-workers used a ring-closing metathesis approach to prepare seven-membered rings from electron-deficient olefins. Reaction of acyclic enol ethers 226 and 228 with 7 mol % of 3 in benzene provided cyclic enol ethers 227 and 229 in 88 and 93% yield, respectively. 

Hoffmann" and Noyori later developed and popularized the analogous higher order [4+ 3]-cycloaddition of an oxyallyl cation equivalent and 1,3-dienes as an attractive method for the formation of historically difficult-to-access seven-membered rings. The oxyallyl cations are typically generated from the reduction of a,a -dihalo ketone derivatives, often generating symmetric cations. Subsequent addition to both cyclic and acyclic dienes produces the bicyclo[3.2.1]oct-6-en-3-ones, meso compounds that have found wide use in organic syntheses. ... 

---