Selectivity rearrangement

There are many other examples of selective rearrangement/ although such strict steric requirements are not always observed/ ... 

The presentation of the rearranging intermediates as mesoionic ground-state species (196) and (199) has proved an adequate tool for the prediction of structural changes in this field. The steric factors which direct the partly selective rearrangement paths of the bicyclohexenone and dienone photoisomers of (174) and their corresponding alkylated homologs have been extensively discussed in a recent review by this author. ... 

Carbocation intermediates are involved and the structure and stereochemistry of the product are determined by the factors that govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. Boron trifluoride is frequently used... 

Spirocyclopropane isoxazolidines 75, obtained from alkylidenecyclopropane nitrones, underwent thermally induced selective rearrangement to pyrrolo[3,4-A]pyridinones 76 <00TA897>. The same adducts 75 in the presence of a protic acid afforded exclusively p-lactams 77 (57-60% yield) accompanied by ethylene extrusion <00JA8075>. 

Alkylidenecyclopentenediones.1 The adduct (2) of 1-lithiohexyne to the cy-clobutenedione 1 rearranges in the presence of Pd(OCOCF3)2 to alkylidenecyclopentenediones 3. A similar selective rearrangement obtains in rearrangements to 2-alkylideneindone-l,3-dione monoketals (equation I). In both cases the nonvinylic C-C bond (a) rearranges more rapidly. 

Irradiation of iV-phenyl lactams affords cyclic products selectively rearranged to the ortho position. Studies on the influence of the chain legth on the regioselectivity of the PFR of several lactams have found that, even with 11 methylenes between the nitrogen atom and the carbonyl group, the coupling product was always the ortho ketone (Scheme 36) [95,96]. 

The bisphosphane BPA was selected as a ligand for the Pd atom because of the high enantioselectivities which had been recorded in the allylation of sulfinate ions with racemic allylic carbonates (vide supra). Besides the variations of the carbon skeleton, the substituent on the S atom of the racemic allylic sulfinates was varied in order to see whether both aryl and alkyl sulfinates are amenable to a highly selective rearrangement. 

The vapor-phase pyrolysis of pcrfluorohept-1-ene at 400 C in the presence of carbon results in complete conversion into the isomeric hept-2- and hept-3-enes in a ratio of 1 3.4.34 As noted above, mixtures are also obtained with cesium fluoride and letraelhylammonium fluoride. Only the antimony(V) fluoride reaction allows selective rearrangement to the alk-2-ene. 

It is considered that the methyl-group migration leading to phenol (24) is partially suppressed as a result of steric interaction with the C 2)-methyl group. The same effect is illustrated in the selective rearrangement of 2,4,4-trimethylcyclohexa-2,5-dien-i one (26) under acidic conditions to give 2,4,5-trimethylphenol (27) rather than the 2,3,4-trimethyl isomer. 

Fragmentation of 2,3-diazabicyclo[2.2.1]hept-2-enes via photoinduced electron transfer offers a convenient route to cyclopentane-1,3-diyl radical cations, in turn undergoing interesting and selective rearrangements [42]. 

Ethyl diazoacetate has been used extensively in the homologation of ketones to 3-keto esters. Lewis acid is required for the reaction (equation 23). ° There is a tendency for the least-substituted group to migrate, particularly if one group is fully substituted (equation 24). In a total synthesis of ( )-aplysin (20) this selective rearrangement was applied (Scheme 9), and a similar approach was used in a synthesis of (+)-hirsutic acid. Ethyl diazoacetate has also been used in the homologation of acyclic... 

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