Ring Construction Strategies

Note that the number of components reacting to form the ring is equal to the number of elements appearing in brackets. Hence, for a [3 -t 3] cycloaddition there are two elanents, so it is a two-component coupling reaction. The number of bond disconnections equals the number of components reacting. Note also that the sum of the elanents is 6, which correspouds to the monocycle ring size. 

For the [6 + 0] transformation, since one of the elements is zero it does not count as a component and so this transformation is a unimolecular reaction. A [2 + 2 + 2] transformation to form a six-membered ring involves three-bond disconnections and hence is a three-component coupling reaction. 

Conversely, if we have a graph that looks like 

For a [3.3.0] framework, we may have the following transformation occurring in step 8, for example. 

The abbreviated code is Bo —— [3.3.0] Ag, which means that the plan began with a five- 

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In a similar heterocyclic quinodimethane ring construction strategy, the hexacyclic adducts (64) were isolated in good yield upon condensation of appropriately functionalized indole imines with ( )-bicyclo[2.2.1]hept-5-ene-2-carboxylic acid chloride (Equation (35)) (88JA2242). In a demonstration of the utility of this new method for indole alkaloid synthesis, further transformations conducted on compound (64 R = R2 = H, R3 = Et) were shown to lead to ( —)-16-methoxy-tabersonine. 

Figure 6.30 shows the application of the algorithm to the structure of strychnine. This leads to an interesting proposed ring construction strategy as shown in Figure 6.31. 

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