Cyclobutoxy radical

It is possible to combine ring-opening reactions of the cyclobutoxy radical with a subsequent carbonylation/oxidation sequence (Scheme 4-48) [84]. The pattern of Cl-substitution affects the final products, cyclized or uncyclized, and this can be ascribed to the different reactivity of formyl and acyl functionalities to electrophilic attack by an acyl cation. 

The dimer of (74) is isolated as one of the products. In the flow ESR spectra, nitroxides (76) and (77) can be recognized. These are proposed to arise by H-abstraction from (74) by a cyclobutoxy radical, giving (75), which either reacts with a further molecule of (74), yielding (76), or undergoes, 5-exo ring closure to (77). The steroidal nitrite (78) (Scheme 17) yields 50% of the ketone (79) and 16% of the alcohol (80) when photolysed in the solid state (X > 300 nm), but only 5% of (79) and 52% of the Barton-type product (81) when photolysed in toluene solution. It is usual for ketones to be produced only in low yields from photoreactions of nitrites in solution, and so the promotion of this reaction pathway in solid-state photolysis is of considerable interest. Similar results were obtained for the solid-state photolyses of a number of other steroidal nitrites, but nitrites prepared from acyclic alcohols showed much less selectivity in favour of the corresponding ketones. 

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