Rates of electrocyclic reactions. Conversion of .alpha.-pyrans to cis-dienones

Marvell, Elliot N.; Chadwick, Thomas Carl; Caple, G.; Gosink, Thomas A.; Zimmer, G.

doi:10.1021/jo00984a018

Cited by 44 publications

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“…Thus, acceleration of cyclobutene electrocyclizations by 3,4-substitution (e.g., Stephenson and Brauman, 1974) can be considered to result from increased σ-π* and σ*-7τ mixing, or by stabilization of the partial radical centers resulting in the transition state of σ bond breaking. Furthermore, such interactions can also be invoked to explain the much more facile electrocyclization of dienones to 2/f-pyrans as compared to the corresponding triene to cyclohexadiene interconversions (Marvell et al, 1972). For example, oxetenes under go electrocyclization to a,0-unsaturated ketones 107 faster than the analogous cyclobutenes (Friedrich and Schuster, 1971).…”

Section: Reactivitymentioning

confidence: 99%

Applications of Frontier Molecular Orbital Theory to Pericyclic Reactions

Houk¹

1977

Organic Chemistry: A Series of Monographs

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Section: Reactivitymentioning

confidence: 99%

Applications of Frontier Molecular Orbital Theory to Pericyclic Reactions

Houk¹

1977

Organic Chemistry: A Series of Monographs

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“…It has been shown that 1,2-pyrans exist mainly in the oxodiene form and that substituents capable of enhancing the resonance in this isomer also displace the equilibrium in its favor (2)(3)(4)(5). The formation of a-pyrones was thought at first to proceed through the pyrilium salt because of the ease with which 5,5-dimethoxy-2-n-decylpenta-2,4-dienal (1) was converted to such a compound and since the intermediate 1,2-pyrans (orthoesters) were expected to be stable in basic media.…”

Section: With 0-methoxy Enonesmentioning

confidence: 99%

Reactions of Ketene Acetals. IV. A New Synthesis of α-Pyrones. III

Bélanger¹,

Brassard²

1975

Can. J. Chem.

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ALAIN B E L A N G E R~~~ PAUL BRASSARD. Can. J. Chem. 53,201 (1975). A simple one-step synthesis of a-pyrones and 3-chloro-a-pyrones from p-functionalized a,p-enones and ketene acetals has been devised. The method has also been adapted to the preparation of some 4-methoxy-a-pyrones. Finally the mechanism of the reaction has been investigated. ALAIN BELANGER et PAUL BRASSARD. Can. J. Chem. 53,201 (1975).Une synthkse simple d'a-pyrones et de chloro-3 a-pyrones en une ttape B partir d'a,p-&nones p-fonctionnalistes a e t t rtaliste. La mtthode s'adapte aussi B la prtparation de quelques mtthoxy-4 a-pyrones. Enfin le mCcanisme de la rtaction a Cte CtudiC.In a companion paper (1) we described a new synthesis of a-pyrones. This method involved the cycloaddition of chloroketene acetals to a,p-unsaturated carbonyl compounds followed by a dehydrohalogenation which yields a-pyrones. It was then assumed that placing the leaving-group on the enone should facilitate the elimination step, prevent the formation of by-products, and simplify the overall process. This modification uses the more readily accessible ketene dialkyl acetals and produces the a-pyrones directly in moderate to good yields (30-77%) (Scheme 1; R , = OAc, OBz, or C1; R, and R, = H, alkyl, or aryl). P-Functionalized enals and P-alkylenones are either unreactive or give only exchange reactions.The great stability of a 2,2,4-trimethoxy-3,4-dihydro-1,Zpyran suggested that the cycloaddition of chloroketene acetals to (3-methoxy enones and a subsequent dehydrohalogenation would provide a new synthesis of 4-methoxy-apyrones. The 3-chloro-4-methoxy-3,4-dihydro-1,2-pyrans are formed by heating the substrates at about 150" (35-80%) and converted to the expected a-pyrones with yields of 32-90%. (Scheme 2; R, = OMe; R, = H, R, = aryl or styryl; and R, = alkyl, R, = aryl). Tile Reaction of 0-Functionalized Enones withKetene Acetals (Scheme 1) With P-Acetoxy and P-Benzoyloxy Enones The P-acetoxy enones 3a, 4a, 5a, and 7a react readily with ketene dimethyl acetal (I.) when heated at about 95" for 3-8 h. Examination of the 'Revision received August 2, 1974. n.m.r. spectra of the crude product usually reveals the presence of a mixture of the expected 4-acetoxy-2,2-dimethoxy-3,4-dihydro-1,2-pyran and elimination product, the 55-dimethoxypentadienone. The magnitude of the coupling constant between the protons in the 2 and 3 positions of the latter (-15 Hz) indicates that the tautomer has the trans-configuration (P-acetoxyacrylophenone reacts in 48 h at room temperature and gives only the relatively unstable dihydropyran).Since all attempts to separate the dihydropyran from the dienone were unsuccessful, the mixture was then heated to 120-130" when a vigorous reaction ensued. The a-pyrone could then be isolated directly by chromatography o n Florisil. Most variations of the experimental procedure such as the use of an inert solvent (xylene) or the addition of acetic acid to the reaction mixture produced no appreciable effect; no a-pyrone is obtained when the heating is carri...

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“…The strong influence of the kind of 2,3-anellation in 2H-pyrans on the tendency of an electrocyclic ring opening reaction is shown by a comparison with the known 2,5,5,8a-tetramethyl-6,7,8,8a-tetrahydro-5H-chromene [9]. An equilibrium between the two isomers can be observed, since in this case the resulting acyclic valence isomer contains the Financial support by the Fonds der Chemischen Industrie is gratefully appreciated.…”

mentioning

confidence: 97%

The enantiomers of 7aH-cyclopenta[b]pyran-7-ones and their thermal racemization

1999

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3952H-Pyrans are known to undergo ring opening reactions to their valence isomers, the pentadienones, which are capable to recyclize forming the starting pyrans [1,2]. These electrocyclic interconversions can be induced photochemically or by heat. In some cases an equilibrium between both isomers is observed. The favoured type of compound at given conditions strongly depends on the substitution pattern. Electron donating groups at position 5 of the pentadienone usually stabilize it by conjugation with the carbonyl group at C-1 whereas substituents at C-3 on the 2H-pyrans lead to a decrease of the tendency of electrocyclic ring opening [1,2].A possibility for the determination of kinetic data such as rate constants k or free enthalpies of activation ∆G ≠ for the electrocyclic ring opening/ring closure consists in the determination of the concentration of 2H-pyrans or pentadienones as a function of time. However, this method fails if one of the valence isomers is too stable that no formation of the other one by electrocyclic conversion can be detected. Investigations on spiro[indoline-chromenes], spiro[indoline-benzoxazines] [3] and spiro[cyclohexadiene-indolines][4] have shown that kinetic data for ring opening reactions can also be determined by liquid chromatographic separation of the enantiomers on non-racemic stationary phases followed by thermal enantiomerization, i.e. measuring the time-dependent loss of optical activity with a polarimeter.Racemic 7aH-cyclopenta[b]pyran-7-ones 1, the products of ring transformation of 2,4,6-triarylpyrylium salts with acyclic 1,2-diketones (butan-2,3-dione, 1-phenyl-propan-1,2-dione) [5] represent a novel type of 2H-pyran derivatives in which the pyran system is 2,3-anellated with a carbocyclic five-membered ring. At room temperature the isomeric cyclopentadienones 2 cannot be observed. Nevertheless, kinetic data for this interconversion should be obtained as in the case of spiro In this paper, we report such investigations and compare the data obtained with those of structurally related systems.According to the substitution pattern of the chiral center at C-7a, the 7aH-cyclopenta[b]pyran-7-ones studied can be divided into the 7a-methyl derivatives 1a-g and the compounds 1h -o bearing a phenyl group at C-7a. The separation of the enantiomers (R)-1 and (S)-1 of both types of pyrans by liquid chromatography on triacetylcellulose using methanol as eluent failed with the exception of the derivatives 1a, 1l, 1n and 1o (cf. Table 1). When the sorbent was changed to tribenzoylcellulose, an improved separation of the 7a-methyl substituted pyrans 1a -g could be observed whereas this system was inactive for the 7a-phenyl compounds 1h -o. The latter ones were successfully separated on (+)-poly(trityl methacrBarriers to ring opening were determined for two 7a-methyl and two 7a-phenyl substituted 2,4,5-triaryl derivatives by thermal racemization of the enantiomers. Substituted cyclopentadienones 2 are discussed as intermediates in the thermal electrocyclic ring opening/ring closure reacti...

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Rates of electrocyclic reactions. Conversion of .alpha.-pyrans to cis-dienones

Abstract: The equilibrium constant, Kw = 0.094, for the interconversion of l-oxa-2,5,5,8a-tetramethyl-5,6,7,8-tetrahydronaphthalene (3) and c¿s-/3-ionone (2) was measured at several temperatures, AH°= 5.5 keal/mol and 5°= 14 eu.

Cited by 44 publications

References 0 publications

Applications of Frontier Molecular Orbital Theory to Pericyclic Reactions

Applications of Frontier Molecular Orbital Theory to Pericyclic Reactions

Reactions of Ketene Acetals. IV. A New Synthesis of α-Pyrones. III

The enantiomers of 7aH-cyclopenta[b]pyran-7-ones and their thermal racemization

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