Ring-expansion of strained

Oxidative rearrangement and ring expansion of strained molecules of tert-cyclo-butanols are known [206]. Recently it has been claimed that the reaction can be explained more reasonably by elimination of jS-carbon (see Chapter 3.8.2) [207]. Conversion of the vinylcyclobutanol 490 to the ring-opened product 493 by relief of the ring strain under oxidative conditions is explained by j6-carbon elimination as shown by 491 to generate 492, followed by j6-H elimination. In this ease. 

Ring expansion of small rings is once again favored by ring strain, and many 3 5 conversions are known. Four-membered rings can expand to five- or six-membered ones. Examples are given in Scheme 13. 

The driving force for ring expansion of 17-hydroxy-20-keto steroids probably comes from relief of strain accompanying conversion of trans-fused hydrindanes to tra/75-fused decalins. The greater susceptibility to rearrangement of 17/ -hydroxy-20-ketones as compared to the 17a-hydroxy-20-... 

Why then does excited cyclobutylchlorodiazirine (21, Scheme 4) so readily undergo 1,2-C RIES 28 Presumably, the difference is in the energetics of the 1,2 shifts. Ring expansion of 21-C1 (or 17-CI) to chlorocyclopentene relieves about 20 kcal/mol of ring strain, but the analogous conversion of 33 or 34 to chlorocyclobutene actually increases ring strain by 2 kcal/mol.21... 

At the opposite end of the philicity spectrum, nucleophilic carbenes have proven useful in synthesis. Warkentin" pioneered the thermolysis of oxadiazolines as precursors for (CH30)2C and related dioxacarbenes (Scheme 7.3). Dimethoxycarbene generated from an oxadiazoline undergoes a variety of intermolecular reactions." One example is the ring enlargement of strained cyclic ketones, for example, cyclo-butanone. In this reaction, the nucleophilic carbene initiates the ring expansion by... 

Scheme 31 illustrates, as an example, the Co2(CO)8-catalyzed carbonylative ring expansion of bicyclic aziridine 220, forming the corresponding highly strained /ra r-bicyclic /3-lactam 221, and a proposed reaction pathway. In this reaction, it is believed that tetracarbonylcobaltate, Co(CO)4, generated from Co2(CO)8 is the active catalyst... 

There appears to be at least one factor, in addition to angular strain and resonance stabilization, which determines the relative importance of photoinduced decarbonylation, 6-cleavage, and ring expansion of cyclobutanones. It has already been demonstrated that increasing a-alkylation of cyclo-butanone results in an increase in the extent of cyclic acetal formation (i.e., ring expansion). Comparison of [64a] and [64b] reveals a similar trend. The 3-phenyl-cyclobutanones [141] and [144], however, provide the most spectacular effect of a-substitution (25). In both cases, a-cleavage occurs exclusively at the most highly sub-... 

This specificity can be inverted in the case of the ring expansion of 9-chloro-9-fluorobicyclo [6.1.0]nonane thus, due to the relative stability of the strained transition state, the ring expansion of the exo-chloro isomer (87) occurs faster than that of the endo-chloro isomer (88) (equation 55) . ... 

Reduction of the ketone to the alcohol is trivial and then acid treatment allows the loss of w= and a ring expansion of the second four-membered ring. You may have drawn this as a concerted stepwise process. The elimination gives the most highly substituted alkene. Both rearrangemE.-happen very easily because of the relief of strain in going from four- to five-membered rings. 

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