Ring Expansion Reactions Leading to Carbocycles

The incorporation of the side chain, placed in 2-position of a cycloalkanone, is influenced by strong electron withdrawing groups as Y. There are a number of experiments published, in which the nitro, the phenylsulfonyl, the ketone carbonyl, and the cyano group were used as Y. 

In addition to the two alternative active methylene groups in the side chain ring, strain effects have to be considered when cyclic analogues of VII/7 and VII/10 are treated with base. Thus, compounds with the general formula VII/13 give, apart from some side products (e.g. retro-Michael and bicyclic compounds 

2) The twelve-membered VII/18 in CHC13 solution exists in four conformers (NMR spectra). Most similar compounds also have more than one conformation. - Hydrolysis of the ester in VII/18 was not possible, and decarboxylation could only be realized by hydrogenolyses of the corresponding benzylester [2]. 

3) Pentylamine gave the best results in the enamine ring enlargement route. For comparable reactions, see ref. [13]. 

R=CH3) by the enamine route of ring enlargement (yields are given for R=H) [14]. 

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In 1968, an excellent review on Carbocyclic Ring Expansion Reactions was published [1], Most of the reaction discussed there are one carbon atom insertions. Our review will be limited to discussions of newer methods. Well known reactions are summarized only by giving the principal reaction and leading additional references. The principal reactions for one carbon insertion are summarized in Scheme II/l. 

Kuninobu and Takai developed a rhenium-catalyzed reaction of ketoester 43 with an alkyne, leading to cyclooctenone 44, in which an alkyne is formally inserted into a C-C=0 bond (Eq. (6.10)) [22, 23]. This ring-expansion reaction is applicable to the synthesis of up to 10-membered carbocycles. The C-C bond cleavage is proposed to proceed via retro-Aldol fragmentation of intermediate 45 or 46. 

Finally, alkyltins can participate in radical chemistry, especially when the /3-elimination is thermodynamicaly favored, leading, for instance, to carbocyclic ring expansions [313], In a similar way, radical reactions involving a 1,3-stannyl shift could afford 5-exo cyclizations [314]. 

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