Symmetry properties for 1,4-sigmatropic shifts with inversion and retention

The two mechanisms require the formation of stereochemically different products when the aryl groups at C(4) are different. When the experiment was carried out on 5, only the product corresponding to that formed by inversion of configuration at C(4) was observed  

The rearrangement step is a ground-state thermal process, and may be classified as a [l,4]-sigmatropic shift of carbon across the face of a 2-oxybutenyl cation. The Woodward-Hoffmann rules require a sigmatropic shift in this system to proceed with inversion of configuration. The symmetry properties for 1,4-shifts with inversion and retention are shown in Fig. 11.6. 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is very dependent on the structure of the reactant. We have been able to discuss only some of the best-studied reactions. Many other examples can be found in the literature. This variation in reaction path with reactant structure may make photochemical reactions seem somewhat more complex and capricious than ground-state reactions. The real problem is that structure-reactivity relationships in excited states are not so well established as in ground-state chemistry. Continued study of photochemical reactions will no doubt lead to a general understanding of structure-reactivity effects in excited states. 

Despite the variety of overall processes that can be carried out photochemically, the number of individual types of steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the jS -carbon atom of a,/8 -unsaturated 

Many ketones undergo cycloaddition reactions with alkenes to form oxetanes  

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Fig. 13.7. Symmetry properties for [1,4] sigmatropic shifts with inversion and retention.

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