text: The transition state model for a six-membered oxocarbenium ring was proposed earlier in 1992 by Woods et al. The general strategy for determining the stereochemistry of a nucleophilic addition to a six-membered ring follows a similar procedure to the case of the five-membered ring. The assumption that one makes for this analysis is that the ring is in the same conformation as cyclohexene, with three carbons and the oxygen in a plane with the two other carbon atome puckered out of the plane, with one above and one below (see the figure to the right). Based on the substituients present on the ring, the lowest energy conformation is determined, keeping in mind steric and stereoelectronic effects (see the section below for a discussion of stereoelectronic effects in oxocarbenium rings). Once this conformation is established, one can consider the nucleophilic addition. The addition will proceed through the low energy chair transition state, rather than the relatively high energy twist-boat. An example of this type of reaction can be seen below. The example also highlights how the stereoelectronic effect exerted by an electronegative substituent flips the lowest energy conformation and leads to opposite selectivity.