Cyclobutanol, formation mechanism from

Scheme 9.—Proposed Mechanism for Formation of Cyclobutanol from the Photolysis of 1,3,4,5,6-Penta-O-acetyl-fceto-L-sorbose (18).

Observation of Scheme 54 shows a close analogy between these results and the ones obtained in arynic condensations. Thus we observe the formation of normal ketones 138, transposed ketones 139 or 140, and methylene cyclobutanols 141. However, we met large difficulties with the mechanism. Indeed, starting from the hypothesis of cyclohexyne being the intermediary (what is consistent with our works on amine condensations) and by analogy with arynic condensations, the reaction should be written as in Scheme 55. 

Larock and Reddy obtained the 2-alkylidenecyclopentanone 72 by the reaction of l-(l-alkynyl)cyclobutanol 71 with iodobenzene. The bicyclononanone 74 was obtained from 73. Selective formation of 74 demonstrates that the more substituted bond a in the eyelobutanol 73 undergoes exclusive cleavage (or migration) [8,13]. Larock proposed the mechanism of the reaction of 75 involving ring expansion of 76 to form palladacycle 77 and reductive elimination to give 78. 

Upon irradiation in solution aliphatic ketones containing 7-hydrogens from cyclobutanols as well as undergo photoelimination Cc g-) Eq. (46) Two mechanisms have been considered to account for the photochemical formation of cyclobutanols. The first is a stepwise mechanism, Eq. (59), and the second is a concerted process, Eq. (CO). 

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