Lactone ring cleavage

Of interest is the fact that a Lewis acid with bulky substituents, such as methylaluminium di(2,6-di-t-butyl-4-methylphenoxide), also has an accelerating effect on the polymerisation of /1-lactone, the extent of acceleration being dependent on the mode of lactone ring cleavage. The polymerisation of /i-butyrolactone in the presence of (tpp)AlOMe [scheme (9)] was slower than that in the presence of (tpp)AlCl [scheme (10)], but the accelerating effect of the bulky Lewis acid was more significant for the (tpp)A10Me catalyst [125]. Thus, the acceleration effect is considered [125] to be due to the coordination of... 

Bringman et al. have investigated biaryl lactones and biaryl thionolactones as precursors to enantiomerically enriched axially chiral biaryls. Both, the lactones and the thionolactones are configurationally labile. In this method, biaryl products are obtained by coordination of a Lewis acid followed by reductive lactone ring cleavage. Asymmetric induction requires either the Lewis acid or the reducing agent to be chiral. Both approaches have been realized for biaryl thionolactones with mild Ru Lewis acids (Scheme 10.18) [29]. 

Typical reactions are exemplified by the transformation of leucomy-sine (10a) to artemazulene (63ZOB2734, 68MI1). Reductive lactone ring cleavage leads to 1,4-glycol 10b that when dehydrogenated suffers recyclization. 

Fragmentation of Lactols to Unsaturated Medium-ring Lactones. Ring cleavage to form a medium-sized ring lactone with a transannular double bond has been observed (eq 21). ... 

Unsubstituted 3-alkyl- or 3-aryl-isoxazoles undergo ring cleavage reactions under more vigorous conditions. In these substrates the deprotonation of the H-5 proton is concurrent with fission of the N—O and C(3)—-C(4) bonds, giving a nitrile and an ethynolate anion. The latter is usually hydrolyzed on work-up to a carboxylic acid, but can be trapped at low temperature. As shown by Scheme 33, such reactions could provide useful syntheses of ketenes and /3-lactones (79LA219). 

Results of fluorination of lactones with sulfur tetrafluoride depend on the ring size. y-Butyrolactone undergoes ring cleavage to give y-fluorobutyryl fluoride, which IS further fluormated to 1,1,1,4-tetrafluorobutane. The six-membered 1,4-di-... 

The synthesis in Scheme 13.41 is also built on the desymmetrization concept but uses a very different intermediate. cA-5,7-Dimethylcycloheptadiene was acetoxylated with Pd(OAc)2 and the resulting all-cA-diacetate intermediate was enantioselectively hydrolyzed with a lipase to give a monoacetate that was protected as the TBDMS ether. An anti Sw2 displacement by dimethyl cuprate established the correct configuration of the C(2) methyl substituent. Oxidative ring cleavage and lactonization gave the final product. 

Oxidative ring cleavage of cyclohexanones is achieved in ethanol at a platinum anode. Cleavage of the carbon-carbonyl bond occurs and the carbonyl centee is trapped as the carboxylic acid. The radical centre formed after bond cleavage is oxidised to the carbocation. This rearranges to the most stable centre and is then trapped by the carboxyl group to form a lactone [4, 5], An identical process is... 

When heated with trimethyloxonium fluoroborate in 1,1-dichloroethane, dilactone 842 experiences cleavage of both lactone rings to give a mixture of diene diesters, catalytic reduction of which produced 853 Reductive methylation of 853 proceeded with installation of the methyl groups on the exterior face for obvious steric reasons. Acyloin condensation followed by ferric cloride oxidation furnished a-diketone 854 which proved to be highly responsive to photoexcitation. However, irradiation of 854 did not provide 855 as expected. Rather, a most unusual reaction pathway was followed to deliver diol 856. 

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