Seven-, Eight- and Nine-membered Rings

The synthesis of oxygen-containing seven- or eight-membered rings by IMSC reaction has been little studied as compared to their five- and six-membered analogues. 

This approach should, however, receive more attention since the IMSC reaction is usually highly stereoselective. 

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In the years following the discovery of bis(cyclopentadienyl), bis(arene) and cyclopentadienyl(arene) metal complexes, the structural feature of a sandwich was also established for compounds containing three-, four-, seven-, eight- and nine-membered ring systems. In these compounds the metal centre... 

Figure 4 Structures of three sets of n-electron sextet isomeric seven-, eight- and nine-membered rings with three Z-type atoms no restriction on X-X or Z-Z adjacencies mesoionic structures are indicated by an asterisk.

In the case of the diphosphites, compounds containing seven-, eight-, and nine-membered rings have been studied. The bis-equatorial coordination mode that involves an eight-membered ring provides the highest enantioselectivities. Chiral cooperativity between the central backbone and the aryl substituents as well as the influence of the terminal groups has been observed for several systems. 

Carling, R.W., Clark, J.S., and Holmes, A.B. (1992) Synthesis of medium ring ethers. Part 2. Synthesis of the firlly saturated carbon skeleton of Laurenda non-terpenoid ether metabolites containing seven-, eight- and nine-membered rings. J. Chem. Soc., Perkin Trans. I, 83-94. 

The molecular stmcture of the BTXs consists of 10 or 11 ttansfused polycyclic rings (including five-, six-, seven-, eight-, and nine-membered oxacycles) fused in a linear array adorned by 22 tetrahedral stereocenters. 

Seven-, Eight-, and Nine-Membered Chelating Ring Systems... 

There are fewer examples concerning the formation of higher ring compounds, such as seven-, eight- and nine-membered cyclic compounds, from metallacy-cles of titanocene and zirconocene. 

Figure 6.38 The process for drawing the bridged compound bicyclo[4.3.2]undecane. Notice that the resulting compound contains three rings that are seven-, eight-, and nine-membered.

In the case of the seven-membered ring, the diastereoselection dropped to 9 1 using the Simmons-Smith reagent. A reversal of relative stereocontrol was observed with eight-and nine-membered cychc aUylic alcohol derivatives. This reversal was explained using a simple conformational analysis of the ground state in which attack from the hydroxy side of the most stable conformation led to the anti isomer (Figure 4). 

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