Norrish type 1 ring expansions

Ring-expansion of the 1,3-dione (27a) at —70 °C gives the acetal (28) in low yield, probably via an oxacarbene intermediate.38 The main product, the keto-ester (29) (55%), is formed by the Norrish Type I reaction and is photolabile, forming the ester (30) as well as methyl cyclopentanecarboxylate. The ring-expansion is dependent upon the ring size of the substituent groups and does not occur when the dione (27b) is used. 

The Norrish Type I photocleavage of the 2-(2-cyclopropylcyclopropyl)-alkanones (36) has been described.44 The many products from the reaction were reduced in number by catalytic hydrogenation to afford (37a 61%) and (38a 20%) from (36a), and (37b 51%) and (38b 24%) from (36b). Studies at low conversion of (36a) indicated that the six-atom ring-expansion was a primary photoprocess involving one photon. 

The photochemistry of carbonyl compounds still continues to be a major general area of interest, and physical methods, especially e.s.r., C1DNP, and C1DEP (electron polarization), continue to be widely applied for detection of radical-like transient intermediates. Quinkert and Jacobs have provided further evidence that ring expansion of cyclobutanones to oxacarbenes, and thence to tetrahydrofurans, occurs in a concerted fashion without the intermediacy of biradicals formed by Norrish Type I fission of a C—CO bond. Medary et al. report that Norrish Type I cleavage of 2-ethylcyclopentanone is non-stereospecific, and gives the cis- and /ra/z.y-hept-4-enals previous reports (Srinivasan and Cremer, 1965) that reactions of this type are stereospecific appear to require revaluation. 

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