Cycloheptanone 2- cyclobutanone

The chemical shift dependence of the carbonyl resonances on ring size in cycloalkanones is particularly remarkable In the series of cycloalkanones, cyclopentanone is found to have the largest carbonyl shift (219.6 ppm). The CO signals of cyclobutanone and cyclohexanone are both observed at higher field (x 209 ppm). The carbonyl carbons of cy-clooctanone and cyclononanone are much more deshielded than those of cyclohexanone, cycloheptanone, cyclodecanone and cycloundecanone. The carbonyl resonances of the twelve to seventeen membered ring ketones occur at S values similar to those of acyclic ones [282, 288]. 

Regarding the substrate, the asymmetric deprotonations are relatively tolerant as demonstrated by the synthetic applications listed in O Brien s review. Cyclohexanones and (bridged) cycloheptanones are commonplace while cyclopentanones and cyclobutanones have been more rarely investigated. 

In hydrogenolyses with HAICU, the dimethyl acetals of cyclobutanone and cyclohexanone are cleaved more slowly than that of 3-pentanone, while those of cyclopentanone and cycloheptanone are cleaved more rapidly (Table 1), as would be expected for a carbonium ion process. The differences in rate are small, suggesting that carbonium ion character is not strongly developed in the transition state. With the dimethyl acetal of 4-t-butylcyclohexanone, the hydride addition step occurs with strongly predominating axial addition when HAlCh is used Zn(BH4)2 with TMS-Cl, and TMS-H with TMSO-Tf are less selective (Table 2). Equatorial attack predominates, however, in the reduction of the ketone itself with TBDMS-H and TBDMS-OTf. ... 

Reaction I (in a peculiar form), as well as reactions II, III and IV all occur in the photolysis of cyclobutanone, though the quantum yield of step IV is very low. The occurrence of reaction III in the photolysis of cyclopentanone was not reported however, in that of cyclohexanone and cycloheptanone again all four reactions seemed to take place. 

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