Ring expansion two-carbon

A two-carbon ring expansion of cyclic ketones was achieved by the addition of acetylenic esters and diesters to the enamine derivatives of the ketones, and reported almost simultaneously from several laboratories (337-343). The intermediate bicyclic adduct could be isolated in some cases. 

A two-carbon ring expansion reaction of 5-membered cyclic enamines gave 6,7-dihydro-1 // azepines on reaction with dimethyl acetylenedicarboxylate <06ZN(B)385>. 

This route cannot be used for a two-carbon ring expansion because of facile direct reduction rather than insertion, but it is useful for three- and four-carbon ring expansion.14... 

A novel entry to decahydrocyclopentacyclooctene derivatives via the intramolecular photocycloaddition of fused a,/3-unsaturated y-lactones has been developed (80CC1011). Irradiation of the butenolide (153) in acetone solution gave both the fused and bridged photoadducts (154) and (155) (2-3 1). The major adduct was hydrolyzed, oxidized and esterified to afford (156). Reductive cleavage of the unsaturated keto ester (156) with lithium in ammonia afforded a five-component mixture of a,/3- and /3,y-unsaturated esters. Equilibration with 0.1M sodium methoxide in methanol converted the mixture into a single a,j8-unsaturated ester (157 Scheme 34). This annelative two-carbon ring expansion method may find application in the synthesis of ophiobolin and ceroplastol sesterterpenes. 

The reaction between electrophilic alkynes and enamines gives cyclobutene intermediates270 which, on heating, give cyclic compounds resulting from a two-carbon ring expansion (see Chapter 18). 

The two-carbon ring expansion which involves the [2 + 2] cycloaddition of enamines of cyclic ketones with electron-deficient acetylenes followed by thermal rearrangement of the resulting fused cyclobutenes (see Section II.E) has been successfully used in the synthesis of medium-size heterocycles. Examples include the preparation of compounds 407246, 408247 and 409248. 

Two-carbon ring expansion. Some year. ago Criegee- reported that the benzoate (1) of 9-decalyl hydroperoxide rearranges on standing by way of l-benzoyloxy-1,6-epox-ycyclodecane (2) to the hydroxycyclodecanone L... 

This type of rearrangement has now been used to effect two-carbon ring expansion of cycloalkenylethyl brosylates by reaction with I HF/90% HiO, (caution). For example, 4 is converted in this way into 5, presumably via a cyclopropyl hydroperoxide (a). 

Riiedi et al. discovered a novel and efficient two-carbon ring expansion of vinylcycloalkanones 148 (Scheme 32). Precursors with n = 9 to 15 were ther-molysed at 600-630 °C to generate diradical intermediates, which took part in intramolecular recombinations to yield ring-expanded y,8-imsaturated cy-cloalkanones 149 [120]. In each case a mixture of E and Z isomers was obtained. Thermolysis of the 12-membered-ring ethynyl-substituted cycloalka-none 149 yielded a novel 14-membered-ring allenyl-cycloalkanone 150 via an analogous mechanism. 

Two-carbon Ring Expansion via Cyclobutene Ring Openings... 

Reaction of DMAD with the enolates of keto esters results in a two-carbon ring expansion, an example being the preparation of (103a) from (103). These latter results suggest a general sequence of ring expansions involving cyclic p-diketones, -keto esters or -keto sulfones and alkynes. 

Thies ° has shown that the 1,3-oxy-Cope rearrangement can be used to advantage as a two-carbon ring expansion method (equation 9). ° Addition of vinyllithium to a cyclic p. -unsaturated ketone (6), followed by pyrolysis of the TMS ether (7) of the resulting alcohol, gives predominandy the 1,3-rearrangement product (8). 

In summary then, the oxyanion-accelerated rearrangement of 2-vinylcyclobutanol derivatives is now established as an attractive method for achieving two-carbon ring expansion under relatively mild conditions. In conjunction with the efficient synthetic routes to vinylcyclobutanones outlined in Scheme 21, this version of the VCB rearrangement provides several strategically novel annulation methods for the construction of six-membered carbocycles. ... 

By Two-Carbon Ring Expansion with Hydrogen Transfer... 

Addition of quadricyclane to 1,2,3,3-tetrafluorocyclopropene also resulted in two-carbon ring expansion to give 6. ... 

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