Seven-Membered and Larger Ring Systems

Seven-Membered and Larger Ring Systems 

The dibenzodihydrothiepin 231 reacts in the same way as 229 with benzyne to give phenanthrene in high yield.115 The same pattern of reaction has been used to extrude sulfur atoms, usually two at a time, from larger rings (e.g., 232) for the synthesis of cyclophanes.116 However, in such cases elimination of thiophenol does not follow spontaneously after the Stevens rearrangement, and indirect methods for subsequent removal of the SPh groups (e.g., from 233) are necessary. 

Surprisingly, no reactions of benzyne with azepines or other seven-membered heterocycles have yet been reported. 

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VIII. Seven-Membered and Larger Ring Systems.224... 

The chemistry of the E-Hlg (E=As, Sb, Bi) bonds has been featured prominently in the reported chemistry of seven-membered and larger rings containing arsenic, antimony, and bismuth. In work directed toward the synthesis of heterocyclic systems involving transannular Sb N interaction, 64 and 65 have been obtained from chloro-substituted stibocanes 63a by reaction with LiPh in hexane or NaOSiPh3 in toluene. To obtain the NCS-substituted 66, the iodo-substituted stibocanes 63b were reacted with AgSCN in dichloromethane (Equation 2) <1998POL2655>. 

Due to the plethora of bicyclo[5.3.0]ring systems in natural products and the limited number of efficient methods that facilitate the construction of seven-membered rings the synthesis of bicyclo[5.3.0]compounds based on a PK reaction has recently been a subject of intense interest. Generally, the preparation of larger ring systems [n.3.0] (n>4) has proven challenging with traditional PK reaction catalysts. One of the few examples reported is the Co2(CO)g catalyzed formation of the azabicyclo[5.3.0j-decenonone derivative 15 (X = OTBS) from the conformationally restricted substrate 14... 

The transition metal catalysed nucleophilic attack of heteroatoms onto triple bonds has also been exploited in the preparation of larger rings. The intramolecular reaction of the phenol derivative shown in 5.12. and the pendant triple bond could result in the formation of a seven or an eight membered ring. Although the former system would be favoured by geometric considerations, the only product formed in the process is the benzoxazocin derivative, whose formation is probably driven by electronic factors.12... 

In the case of McycKc systems (133—140) there are not yet enough data available for a detailed discussion to be given. The barriers obtained for the bicyclo [2.2.2]octane (133, 134) and the bicyclo[3.3.1]nonane (139, 140) compounds are in the range expected for the corresponding mono-cyclic systems. In the case of the bicyclo[3.2.2]nonane derivatives 135 and 136 the inversion barriers are larger than in the seven-membered ring compounds 127 and 128. 

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