Enolene Rearrangements. Relationship to the “Abnormal Claisen Rearrangement” and Other 1,5-Hydrogen Shift Processes

Roberts, Royston M.; Landolt, Robert G.; Greene, R. N.; Heyer, Ernest W.

doi:10.1021/ja00982a023

Cited by 56 publications

(18 citation statements)

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“…The three compounds formed in the pyrolytic study had already been described from the treatment of CBC and other cannabinoids with acids and/or by heating or irradiation . The formal [2π + 2π] thermal cyclization of CBC ( 3 ) to CBL ( 8 ) was, nevertheless, mechanistically intriguing, and a similar transformation is documented also for other homoisoprenylchromenes. − The reaction has been considered a Gassman-type cationic [2π + 2π] cyclization, but the anti-Markovnikow regiochemistry is at odds with the considerations of carbocation stability that governs this type of cycloaddition. , An alternative, and more plausible mechanism, is presented in Figure . The reaction might involve a cyclopropane intermediate ( 9 ), the result of a concerted process triggered by electrophilic (acidic) activation of the carbonyl tautomer of the resorcinyl moiety, and terminated by electrophilic Markovnikov addition to the electron-rich homoisoprenyl terminal double bond (Figure ).…”

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confidence: 97%

“…15−17 The reaction has been considered a Gassman-type cationic [2π + 2π] cyclization, 18 but the anti-Markovnikow regiochemistry is at odds with the considerations of carbocation stability that governs this type of cycloaddition. 18,19 An alternative, and more plausible mechanism, is presented in Figure 3. The reaction might involve a cyclopropane intermediate (9), the result of a concerted process triggered by electrophilic (acidic) activation of the carbonyl tautomer of the resorcinyl moiety, and terminated by electrophilic Markovnikov addition to the electron-rich homoisoprenyl terminal double bond (Figure 3).…”

mentioning

confidence: 99%

“…Attempts to expand the application of the reaction to heteroaromatic analogues of alkylresorcinols also met with limited success. Thus, the pyranocoumarin ferprenin (18) 32 gave the corresponding dibenzochromene (19) in a mixture with two "benzylic" cyclization products (20a,b) that became the exclusive reaction products with the pyranopyrone 21 (Figure 5).…”

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confidence: 99%

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One-Pot Total Synthesis of Cannabinol via Iodine-Mediated Deconstructive Annulation

et al. 2019

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The thermal degradation of cannabichromene (CBC, 3) is dominated by cationic reactions and not by the pericyclic rearrangements observed in model compounds. The rationalization of these differences inspired the development of a process that coupled, in an aromatization-driven single operational step, the condensation of citral and alkylresorciniols to homoprenylchromenes and their in situ deconstructive annulation to benzo[c]chromenes. This process was applied to a total synthesis of cannabinol (CBN, 5) and to its molecular editing.

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One-Pot Total Synthesis of Cannabinol via Iodine-Mediated Deconstructive Annulation

et al. 2019

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“…Since it was important to establish the absence of 17 in the sensitized irradiation of 11, an authentic sample was prepared from mesityl oxide and dimethyloxosulfonium methylide. 36 No trace of 17 could be detected in irradiated solutions of 11 under VPC conditions where the known product was easily observed. Ketone 22 was prepared independently by reaction of methyllithium with 4-methyl-3-pentenoic acid while 24 was made from Grignard reagent 35 and the appropriate acid chloride.…”

Section: Resultsmentioning

confidence: 99%

Systematic structural modifications in the photochemistry of .beta.,.gamma.-unsaturated ketones. II. Acyclic olefins and acetylenes

Schexnayder¹,

Engel²

1975

J. Am. Chem. Soc.

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“…The final ratios of 11 to 3, as assessed by VPC on column G, were 0.097:l and 0.072:1, respectively, as compared to 0.23:l for a simultaneously irradiated control solution. trans-2-Methylcyclododec-4-enone (15). In a dry flask under nitrogen, THF (2 ml) and HMPA (2 ml) were added to sodium hydride (9 mg, 0.375 mmol).…”

Section: Methodsmentioning

confidence: 99%

Photochemical formation of 12-methylene-cis-bicyclo[8.2.0]dodecan-1-ol from 2-methylenecyclododecanone. Restricted rotation in a biradical intermediate

Hemperly¹,

Wolff²,

Agosta³

1975

J. Org. Chem.

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Irradiation of 2-methylenecyclododecanone (3) leads to 12-methylene-cis-bicyclo[8.2.0]dodecan-l-01 (11) in 87% yield, unaccompanied by the cyclobutyl or cyclopropyl ketones usually formed photochemically from CYmethylene ketones. Pyrolysis of 11 leads to enone 15 and bicyclic ketone 14, which is the cyclobutyl ketone anomalously absent from irradiation of 3. The structures of 11,14, and 15 are supported by spectroscopic and chemical data. Restricted rotation about the C(a)-C(P) bond in biradical intermediate 13 is advanced as an explanation for the exceptional behavior of 3. These results suggest the importance of conformational mobility of a short-lived biradical in determining the products formed on photolysis of a-methylene ketones.Photochemical isomerization of a variety of a-methylene ketones leads to cyclobutyl ketones, accompanied in some cases by related cyclopropyl ketones and methylenecyclobutano1s.l Equation 1 gives a typical example in which all three types of products are found. There is good evidencethat the cyclopropyl and cyclobutyl ketones arise by way of biradical intermediates in which 0 hydrogen (1, for cyclopropyl ketones) or y hydrogen (2, for cyclobutyl ketones) has been abstracted by the carbonyl oxygen.2 It is quite likely that the methylenecyclobutanols also arise by way of biradical intermediates (as 2),' although there is no direct evidence for this, and the possibility remains open that these alcohols represent a concerted [ , , 2 + ,2] cycloaddition3 of the y carbon-hydrogen bond with the carbonyl group. The major photoproduct is cyclobutyl ketone in most cases, and frequently no methylenecyclobutanol is found a t all. An exception to this generalization is the photolysis of 2-methylenecyclododecanone (3), which furnishes solely a single isomer of the related methylenecyclobutanol 4 in high yield (eq 2).l Ketone 3 is the only medium-or large-ring compound investigated, and its exceptional photochemical behavior presumably is related to this structural feature. Our purpose in the present study was to investigate this isomerization of 3 in more detail; in this connection we have determined the stereochemistry of 4 and carried out sensitization and quenching experiments on its formation from 3. In addition we have studied the thermolysis of 4 and succeeded thereby in preparing the cyclobutyl ketone which is anomalously absent in photolysis of 3. The results are discussed in detail below; they permit us to offer a reasonable explanation for the behavior of 3 in terms of restricted rotation in a biradical intermediate, thus providing an instructive example of conformational control over the fate of a short-lived intermediate.The convenient availability of both isomers of bicyclo-[8.2.0]dodecan-l-o1, 5 and 6, from photolysis of cyclododecanone4 suggested that a simple proof of the stereochemistry of 4 would involve its correlation with one of these known alcohols. This approach was successful in demonstrating a cis ring fusion in 4, and the structural formulas showing the degradation ...

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Enolene Rearrangements. Relationship to the “Abnormal Claisen Rearrangement” and Other 1,5-Hydrogen Shift Processes

Cited by 56 publications

References 1 publication

One-Pot Total Synthesis of Cannabinol via Iodine-Mediated Deconstructive Annulation

One-Pot Total Synthesis of Cannabinol via Iodine-Mediated Deconstructive Annulation

Systematic structural modifications in the photochemistry of .beta.,.gamma.-unsaturated ketones. II. Acyclic olefins and acetylenes

Photochemical formation of 12-methylene-cis-bicyclo[8.2.0]dodecan-1-ol from 2-methylenecyclododecanone. Restricted rotation in a biradical intermediate

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