New Syntheses of 2-Methoxycyclopentanone

Barco, A.; Giuli, G. De; Pollini, G. P.; Traverso, G.

doi:10.1055/s-1972-21960

Cited by 6 publications

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“…Hydroxy ketal 2 (0.46 g) gave 0.32 g (89%) of 8: bp 67-70 "C (15 mm), lit.23 bp 68-70 "C (15 mm). Ketone 8 had spectra identical to that previously reported for 2-methoxycyclopentanone 23. Rearrangementof3.…”

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“…Product analyses were therefore carried out to evaluate the effect of increasing inductive destabilization on neighboring-group participation as reflected by rearranged products. Solvolysis of 2,2-dimethoxycyclobutyl tosylate (23) in 70% aqueous acetone gave only methyl cyclopropanecarboxylate (32). The same product was observed in the pyrolysis of 1bromo-2,2-dimethoxycyclobutane4a and was suggested to arise via a concerted process.4» In the solvolytic reaction, the rearranged ester 32 suggests an assisted ionization leading to 33.…”

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Rearrangements of .alpha.-hydroxy ketals and derivatives of .alpha.-hydroxy ketals

Creary¹,

Rollin²

1977

J. Org. Chem.

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a-Hydroxy ketals are found to rearrange when treated with acid to give alkoxy ketones. The k H / k D value of 3.1-3.5 for endo-2,2-dimethoxybicyclo[2.2.1] heptan-3-01 (4) suggests a mechanism involving rate-limiting hydride migration to an a-alkoxy cation. exo-2,2-Dimethoxybicyclo[2.2.l]heptan-3-01(5) gives a dimeric product consistent with the low propensity for endo-hydride migration in the norbornyl system. Solvolysis of tosylate and triflate derivatives of a-hydroxy ketals gives rates largely inductively retarded by the dimethoxy grouping. Hydrolysis or acetolysis of 2,2-dimethoxycyclobutyl tosylate (23) gave methyl cyclopropanecarboxylate (32) via a ring-contraction process. 2,2-Dimethycyclopentyl triflate (26) gave 2,3-dimethoxycyclopentene (38) on acetolysis and l,l,2-trimethoxycyclopentane on methanolysis. These products arise from methoxy participation. endo-2,2-Dimethoxybicycl0[2.2.l]hept-3-y1 triflate (28) gave products arising from a k , process and from methoxy participation. Additionally, methyl cyclohex-3-enecarboxylate (41) was produced. A deuterium-labeling study showed that 41 was produced from the rarely observed c1-C~ participation in the norbornyl system. The migration is suggested to arise from the increased demand for participation as a result of the inductively destabilizing dimethoxy substituents.@-Hydroxy ketals are readily available from the corresponding acyloins, which are in turn prepared using the Ruhlmann and Schrapler modification of the acyloin condensation.1 Complimenting this route is the recently developed procedure involving epoxidation, hydrolysis of silyl enol ethers.2 The presence of both the alcohol and masked keto functionality makes the hydroxy ketal potentially interesting from a synthetic viewpoint and additionally with respect to potential rearrangement processes. We have undertaken a study of some of the reactions which these acyloin derivatives undergo with the goal of elucidating mechanistic processes. Reported here are the results of these studies. Results and DiscussionAcid-Catalyzed Rearrangements of a-Hydroxy Ketals. a-Hydroxy ketals 1-4 were prepared by ketalization of the appropriate a-hydroxy ketone produced in the acyloin condensation. The exo-hydroxy ketal 5 was produced in the unusual reaction of endo-keto triflate 6 with sodium methoxide in methan01.~ U N O H+ n = 2,3,4 4 OH U ' O C H , 1 , n = 2 2 , n = 3 3 , n = 4 6 5 Ketal alcohols 1-5 undergo rearrangement when treated with hydrochloric acid vapors. Table I gives the products and reaction conditions employed for these transformations. Conia has previously reported rearrangement of 1 under thermal w Me,SO-Et,N OH 0 1 12 12.dz 1-d, D H ----t H+ f i O c H 3 + D 0 D 6a 6bconditions a t temperatures greater than 200 "C and has suggested mechanisms ranging from entirely concerted to acid ~a t a l y z e d .~ We have found that 1 is thermally stable in basewashed glassware and that the reaction of 1 is truely acid catalyzed. The mechanism of rearrangement of 1 remains open to question and we therefore sough...

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confidence: 75%

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confidence: 99%

Rearrangements of .alpha.-hydroxy ketals and derivatives of .alpha.-hydroxy ketals

Creary¹,

Rollin²

1977

J. Org. Chem.

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Erzeugung und [4 + 3]‐Cycloaddition von Cyclopentenylium‐2‐olat aus 2‐Chlorcyclopentanon unter Alkoholyse‐Bedingungen

Föhlisch

Joachimi

1987

Chem. Ber.

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2-Chlor-und 2-Brom-3-pentanon reagieren in Methanol, Ethanol und 2-Propanol mit den entsprechenden Natriumalkoxiden hochstereoselektiv zu Halbacetalen des cis-2,3-Dimethylcyclopropanons. Das Cyclopropanon erleidet eine disrotatorische Ringoffnung zum Dimethylallylium-2-olat, einem W-konfigurierten Oxallyl, welches sich an Furan und andere 1,3-Diene cycloaddieren kann. Anstelle des Natriumalkoxids kann zur Durchfuhrung der Cycloaddition auch Triethylamin verwendet werden ".Der erste Schritt der postulierten mehrstufigen Reaktionsfolge ist ohne Zweifel die a'-Deprotonierung des 2-Halogen-3-pentanons an C-4. Fur den zweiten Schritt, den nucleofugen Abgang des Halogens aus dem gebildeten Halogenenolat, kann entweder eine intramolekulare S~2-Reaktion zum Cyclopropanon oder eine Ionisation zu einem Oxallyl (SN1) diskutiert werden, wobei dem lctzteren ProzeD aus stereoelektronischen Grunden 3, der Vorzug gegeben werden sollte').Urn zwischen diesen beiden Moglichkeiten zu differenzieren, haben wir untersucht, wie sich 2-Chlor-cyclopentanon (la) bei der baseninduzierten Alkoholyse und der Reaktion mit 1,3-Dienen verhalt; denn ein intramolekularer SN2-RingschluD des primar gebildeten') Halogenenolats 2 zu Bicyclo[2.l.O]pentan-5-on (5) ist stereoelektronisch und wegen der hohen Spannungsenergie von 5 5, auhrst unwahrscheinlich. Dasselbe gilt fur das zweite denkbare Valenzisomere, das Allenoxid 4. Aus diesen Griinden ist es auch nicht verwunderlich, daD bei 1 a keine Favorskii-Umlagerung zu Cyclobutancarbonsaure bzw. deren Ester beobachtet wurde6). Nicht-enolisierbare a-Halogencyclopentanone konnen jedoch wie 2-Halogencyclobutanone nach dem Semibenzilsaure-Mechanismus zu ,,Favorskii-Produkten" reagieren').Die Ionisierung von 2 zum cyclischen, U-konfigurierten Oxallyl 3 sollte jedoch leicht moglich sein; denn die allylische C-HalogenBindung bildet nach dem Dreiding-Modell mit der Ebene der C -C-Doppelbindung -je nach planarer nder Halbsessel-Konformation -einen Winkcl von 60-75", der nicht weit von dem optimalen 90"-Winkel entfernt ist. Es ist bekannt, daD Cyclopentenyl-Derivate unter Solvolysebedingungen leicht ionisieren; das resultierende Cyclopentenylium-Kation ist sehr stabil'l. Alkoholyse von 2-Chlorcyclopentanon (1 a)Mousseron und Mitarbeiter berichteten, dai3 bei der Reaktion von l a mit einem fjberschul3 an methanolischem Natriummethoxid bei 0°C mit ca. 25% Ausbeute Eliminierung zu 2-Cyclopenten-i-on eintritt und in betrachtlicher Chem. Ber.

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