Intramolecular Reactions of Acyclic Diketones

Acyclic diketones 120 (and sufficiently flexible cyclic compounds) possessing a hydrogen atom at C-4 of the 1,2-diketo system undergo intramolecular reactions via their lowest triplet states to form cyclobutanolones 122 in high chemical and quantum yields 1). The proposal that excited states of diones assume, whenever possible, a pla- 

The quantum-yields for reaction of compounds 120a, 120b, and 120 c in benzene solution were 0.054, 0.50, and 0.57 (0.62 in t-butyl alcohol) respectively 100). Rates of hydrogen atom transfer were obtained from Stem-Vollmer treatment of quenching 

As part of the synthesis of 1,6-dimethyldodecahedrane 108), an interesting case has been reported 109) of cyclization of a polycyclic diketone in which two new cyclo-pentanol rings were formed, as illustrated by the partial structures shown below. X-ray crystallographic analysis showed that conformational factors were particularly favorable for this unique result. 

The key role played by conformations in these reactions prompted investigation of results in liquid crystals n0). However, results obtained with octane-4,5-dione and l,6-diphenylhexane-3,4-dione in several types of liquid crystals did not differ appreciably from those obtained in hexane solution. 

The absence of intramolecular H-abstraction in cyclopropyl diketones reflects a combination of conformational factors, difficulty in formation of cyclopropyl radicals, and strain in the cyclization product. 

---