Evidence for cyclopropoxide intermediates in α-methyl H/D exchange of cyclic ketones under homoenolization conditions

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. J. Chem. 62, 1385Chem. 62, (1984.The effect of strongly basic conditions (t-BuOL/t-BuOH/I85"C) on the exo and endo isomers of 7,7-dimethyltricyclo[3.2. I .02."]octan-6-one has been examined. While the exo isomer is stable, the endo isomer readily rearranges to 4,4-dimethyltricyclo[3.3.0.0'.H]octan-3-one (6) which subsequently undergoes reduction of the three-mcrnbered ring. 'The initial transformation is one of very few examples of y-enolate rearrangement. Experiments in tert-butyl alcohol-0-d, were carried out to establish the several sites of deuterium incorporation in the initial ketones and in 6 using '~m r . These results reveal the stereochemistry of y-enolate formation and cleavage as well as the stercoselectivities and relative rates of the exchange processes.A. K. CHENG, ASHIM K. GHOSH et J. B. STOTHERS. Can. J. Chem. 62, 1385 (1984.On a CtudiC I'effet de conditions fortement basiques (t-BuO-/t-BuOH/l85"C) sur les isomeres exo et endo de la dimkthyl-7,7 tricyclo

Section: Hid Exchange Experimentsmentioning

confidence: 93%

Nuclear magnetic resonance studies. XL. Tricyclooctanone rearrangement, [3.2.1.0] → [3.3.0.0], via γ-enolization and a ²Hmr study of H/D exchange in tricyclic ketones

Cheng¹,

Ghosh²,

Stothers³

1984

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“…In the course of a recent study (6), however, we found that, with our standard conditions, 6-trimethylsilyloxy-exo-tricyclo-[3.2.1.02.4]oct-6-ene (1) presented unexpected difficulties, such that with reflux times of 20 h or more, which are typical for good results with all other systems investigated to date (1)(2)(3)(4)(5), a mixture of products resulted that was too complex to warrant detailed examination. With a reflux time of 15 h, the reaction led to a product containing, by gas-liquid chromatography (glc), one major component (65%) and a minor component (-15%) together with small amounts of additional material.…”

mentioning

confidence: 71%

“…Ether l l b was prepared in 80% yield using the procedure (LDA, Et3N, t-BDMSCI in THF) analogous to that employed previously for a variety of TMS en01 ethers (1)(2)(3)(4)(5)(6). Since the completion of this study, however, two, more efficient, general methods for the preparation of t-BDMS en01 For personal use only.…”

Section: Resultsmentioning

confidence: 99%

¹³C magnetic resonance studies. 120. The Simmons–Smith reaction with some silyl enol ethers. Unusual ring expansions of some norcamphorsti

Ragauskas¹,

Stothers²

1985

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Unexpected difficulties with Simmons-Smith cyclopropanation of the tr~methylsilyl en01 ether of exo-tricyclo[3.2.1 .02.4]-octan-6-one led to a reexamination of this reaction with the parent norcamphor derivative which under "concentrated" conditions gave ring-expanded allylic ethers in low yield. With the tert-butyldimethylsilyl ethers, however, the allylic product is formed in high yield in the norcamphor and exo-trimethylenenorcamphor (tricyclo[5.2. l .0'.6]decan-8-one) systems. The tert-butyldimethylsilyl en01 ethers of bicyclo[3.2. Iloctan-6-one and nopinone do not undergo ring expansion under similar conditions but the silyl ether of 3-methylenenopinol is a major product in the latter case. ARTHUR J. RAGAUSKAS et J. B. STOTHERS. Can. J. Chem. 63, 2969.Les difficultCs inattendues qui se sont manifestees lors de la rCaction de cyclopropanation selon Simmons-Smith de 1'Cther Cnolique trimCthylsilyle de la tricyclo[3.2.1.02~4]octanone-6 exo nous ont incite a reetudier cette reaction avec le derive norcamphre apparent6 qui, lorsqu'il est soumis a des conditions "concentrCes", conduit avec un faible rendement aux Cthers allyliques ayant subi une expansion du cycle. Toutefois, avec les ethers tert-butyldimCthylsilyle du norcamphre et du trimCthyltnenorcamphre-exo (tricyclo[5.2.1 .02.6]dCcanone-8), on obtient le produit allylique avec un rendement elevt. Dans des conditions analogues, les ethers Cnoliques tert-butyldimethylsilyles de la bicyclo[3.2. Iloctanone-6 et de la nopinone ne subissent pas d'extension de cycle; toutefois, dans ce dernier cas, le produit principal est 1'Cther silylk du methylbne-3 nopinol.[Traduit par le journal] Introduction Some years ago we described an efficient synthetic method for ring expansion of several bicyclic ketones (1). Our sequence involves generation of the trimethylsilyl en01 ether followed by Simmons-Smith cyclopropanation and basecatalyzed cleavage of the cyclopropyl ether (Scheme 1). The cyclopropoxide produced by ether cleavage immediately undergoes homoketonization, which strongly favors ring expansion in [2.2.1], [2.2.2], and related systems. Subsequently this sequence was employed for ring expansion of a variety of polycyclic ketones (2-4). From these and other cases ( 3 , we have collected more than 25 examples of the Simmons-Smith reaction with polycyclic trimethylsilyl en01 ethers and, in each case, a single cyclopropanation product was obtained in good to high yield (70-95%) under a standard set of conditionsIn the course of a recent study (6), however, we found that, with our standard conditions, 6-trimethylsilyloxy-exo-tricyclo-[3.2.1.02.4]oct-6-ene (1) presented unexpected difficulties, such that with reflux times of 20 h or more, which are typical for good results with all other systems investigated to date (1-5), a mixture of products resulted that was too complex to warrant detailed examination. With a reflux time of 15 h, the reaction led to a product containing, by gas-liquid chromatography (glc), one major component (65%) and a minor component (-15%) toget...

“…Invariably, exchange has been found to occur at the a-methyl sites but no ring-expanded products have been observed corresponding to those expected through ketonization of the putative cyclopropoxide intermediate 1, i.e., cleavage b in Scheme 1. Evidence has been presented (4) in support of cyclopropoxide species as reasonable intermediates for a-methyl exchange in cyclic systems, and one example of rearrangement through an a-methyl carbon in a bicyclic diketone has been described (5). In contrast to the cyclic systems, however, some acyclic monoketones do undergo skeletal isomerization through P-enolizable methyl groups as was first observed with di-tertbutyl ketone and 5,5,7,7-tetramethylundecan-6-one (6).…”

Section: Introductionmentioning

confidence: 92%

¹³C magnetic resonance studies. 142. The fate of highly substituted acyclic ketones in strong base: β-enolization, Haller–Bauer cleavage, and an alkyl to aryl acyl shift

Dyllick‐Brenzinger¹,

Patel²,

Rampersad³

et al. 1990

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. Di-tert-butyl ketone (2a), 5,5,7,7-tetramethyl-6-undecanone (2b), and 2,4-dimethyl-2,4-diphenyl-3-pentanone (2d) have been found to undergo slow rearrangement by P-proton abstraction from the a-methyl groups upon treatment with t-BuO-/t-BuOH/> 185°C. This is in contrast to the absence of such rearrangement in cyclic systems. It is the only significant process observed for 2 a and 2b, whereas with 2 d and its monomethyl analog 22 Haller-Bauer type cleavage and an unprecedented 1,3-acyl shift from sp3 to sp2 carbon accompany the p-enolate rearrangement. The rates of 'H/'H exchange of the methyl protons in 2a, b were found to be similar to those reported for a variety of cyclic systems but this exchange does not occur in 2d, where only aryl proton exchange was observed.Key words: p-enolization, acyclic ketones, acyl shift. On a trouvC que, si on soumet la di-tert-butyl-cCtone (2a), la 5,5,7,7-tttramtthyl-undCcan-6-one (2b) ainsi que la 2,4-dimCthyl-l,4-diphCnyl-pentan-3-one ( 2 4 A un traitement par le tert-BuO-/tert-B~OH/5185~C, il se produit une transposition lente impliquant un enlkvement du proton-P des groupements mtthyles en a. Ce rtsultat est en opposition avec le fait que l'on n'observe pas de telles transpositions dans les systkmes cycliques. Cette transportation est le seul processus qui est observk partir des composCs 2 a et 2 b alors que, avec le produit 2d ainsi que son analogue monomCthylC 22, on observe un clivage de Haller-Bauer et un dCplacement acyle-1,3 sans prCcCdent du carbone sp3 vers le carbone sp2 qui accompagne la transposition de 1'Cnolate P. On a trouvC que les taux d'tchange 'H/'H des protons des groupements mCthyles des composCs 2 a et 2 b sont semblables aceux rapportts antkrieurement pourune variCtC de systkmes cycliques; toutefois, cet Cchange ne se produit pas avec le produit 2 d alors que l'on n'observe que I'Cchange des protons aromatiques.