And ring expansion

Oxidative rearrangement takes place in the oxidation of the 1-vinyl-l-cyclo-butanol 31, yielding the cyclopentenone derivative 32[84], Ring contraction to cyclopropyl methyl ketone (34) is observed by the oxidation of 1-methylcyclo-butene (33)[85], and ring expansion to cyclopentanone takes place by the reaction of the methylenecyclobutane 35. [86,87]... 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

The addition of diphenylcyclopropenone to pyrrolidinocyclohexene and ring expansion of the adduct gave an aminocyclononadicnone (330,331), A hereas the corresponding cyclopentanone derived enamine yielded a bieyclic enone. 

In a study of the nitrosation of camphor-3-glyoxylic acid (89), Chorley and Lapworth isolated a compound whose structure (90) has recently been clarified by Hatfield and Huntsman. Decarboxylation and ring expansion occur and the reaction is rationalized in the sequence 89 90. The buttressing effect of a methyl group on... 

Very few reactions of carbenes with heterocyclic systems containing more than one hetero atom have been studied. They are confined to variants of the Reimer-Tiemann formylation of thiazoles, pyra-zoles, iminazoles, and indolizines/ and ring expansion does not appear to have been observed. 

Likewise, the isomeric fused aziridines 22 and 24, obtained from 1,4,5,6,7,8-hexahydronaph-thalene, undergo bromination. dehydrobromination and ring expansion to the 6,7,8,9-tetra-hydro-3//-3-benzazepine 23 and the 2,7-bridged azepine 25, respectively.61... 

Since aromatic substitutions, aliphatic substitutions, additions and conjugate additions to carbonyl compounds, cycloadditions, and ring expansion reactions catalyzed by Fe salts have recently been summarized [17], this section will focus on reactions in which iron salts produce a critical activation on unsaturated functional groups provided by the Lewis-acid character of these salts. 

Longifolene has also been synthesized from ( ) Wieland-Miescher ketone by a series of reactions that feature an intramolecular enolate alkylation and ring expansion, as shown in Scheme 13.26. The starting material was converted to a dibromo ketone via the Mr-silyl enol ether in the first sequence of reactions. This intermediate underwent an intramolecular enolate alkylation to form the C(7)—C(10) bond. The ring expansion was then done by conversion of the ketone to a silyl enol ether, cyclopropanation, and treatment of the siloxycyclopropane with FeCl3. 

Figure 4.21 BASED can react with molecules after photoactivation to form crosslinks with nucleophilic groups, primarily amines. Exposure of its phenyl azide groups to UV light causes nitrene formation and ring expansion to the dehydroazepine intermediate. This group is highly reactive with amines. The cross-bridge of BASED is cleavable using a disulfide reducing agent.

Skeletal Isomerization, -Elimination, and Ring Expansion Reactions... 

Scheme 16.19 A sequential carbopalladation and ring expansion of an allenylcyclobutanol.

A sequential process of carbopalladation and ring expansion of allenylcyclobuta-nols provides five-membered rings (Scheme 16.19) [24], The intramolecular version of this approach offers a facile synthetic method for 5,7- and 5,8-fused ring frameworks (Scheme 16.20) [24]. 

However, methylation of hexamethylbenzenium ion can also occur, leading to the formation of heptamethylbenzenium ion. The latter can split off, this time propylene, by a series of ring contraction and ring expansion reactions [116, 117]. 

The common motif shared by non-heme iron oxygenases contains an active site, where two histidines and one carboxylate occupy one face of the Fe(ll) coordination sphere. These enzymes catalyze a variety of oxidative modification of natural products. For example, in the biosynthesis of clavulanic acid, clavaminic acid synthase demonstrates remarkable versatility by catalyzing hydroxylation, oxidative ring formation and desaturation in the presence of a-ketoglutarate (eq. 1 in Scheme 7.22) [80]. The same theme was seen in the biosynthesis of isopenicillin, the key precursor to penicillin G and cephalosporin, from a linear tripeptide proceeded from a NRPS, where non-heme iron oxygenases catalyze radical cyclization and ring expansion (eq. 2 in Scheme 7.22) [81, 82]. 

J. O. Hoberg and J. J. Bozell, Cyclopropanation and ring-expansion of unsaturated sugars, Tetrahedron Lett., 36 (1995) 6831-6834. 

F. Trimethylsilyl enol ethers are treated with Simmons-Smith reagent to form cyclopropyl silyl ethers. These rings undergo oxidative cleavage and ring expansion when treated with... 

The bismuth ylides, Ph3Bi=CHCOR, do not react with simple ketones and electron-rich olefins probably because of their relatively low electrophilic character. However, Ph3Bi=CHCOR reacts with a-keto esters [46, 67, 68], benzils [46, 67-69], orf/to-quinones [46, 67, 68], and acenaphthenequinone [70] to give epoxides, (9-arovl enolates, 3-hydroxytropones, and 3-hydroxyphenalenones, respectively, accompanied by the formation of Ph3Bi (Scheme 11). In particular, transposition and ring expansion reactions are of interest from a mechanistic point of view, since these reaction modes are unprecedented in ylide chemistry. 

Chloromethanesulfonate is an efficient leaving group for the synthesis of 2 -trans-disubstituted tetrahydropyrans by the reanangement and ring expansion of tetrahydrofurans (Scheme 5) <99TL2145>. 

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