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  • Regioselectivity  (3)
  • Transition structures  (3)
  • 1,7,9-Decatrien-3-ones  (2)
  • 1
    ISSN: 1434-193X
    Keywords: Intramolecular Diels-Alder reaction ; 1,7,9-Decatrien-3-ones ; α-Eudesmol ; Lewis acids ; Chelates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Trienones 1-3 were subjected to Lewis acid-promoted intramolecular Diels-Alder reactions. It was shown that with monocoordinating Lewis acids the endo selectivity of the cycloaddition was generally high. The preference for either of the two possible endo products cis-a and cis-b, however, was shown to be highly dependent on the nature of the Lewis acid, and on the substitution pattern of the trienone substrates. Lewis acids with two coordination sites furnished predominantly the exo product trans-b via seven-membered ring chelate intermediates. The chelate-controlled intramolecular Diels-Alder reaction was then utilised as the key step in a stereocontrolled synthesis of the sesquiterpene α-eudesmol. This reaction mode, performed on a model trienone, also paved the way for the synthesis of the pharmaceutically important natural product dihydromevinolin.
    Additional Material: 6 Tab.
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  • 2
    ISSN: 1434-193X
    Keywords: Natural products ; Intramolecular Diels-Alder reactions ; 2-Siloxycyclopropanecarboxylates ; 1,7,9-Decatrien-3-ones ; Octalones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Starting from enones 7 and 8 siloxydienes 9 and 12 were synthesized. Cyclopropanation and subsequent alkylation with sorbyl bromide provided tetrasubstituted siloxycyclopropanes 15 and 16 in good overall efficiency. Their ring cleavage with fluoride reagents gave 1,7,9-decatrienones 5 and 17; the latter was converted into the protected compound 6. The thermal intramolecular Diels-Alder reactions of 5 and 6 were compared with those of related trienones 3 and 4. Octalones 1, 2, 21, and 22 were formed as mixtures of three or four diastereomers. Thus, for the stereoselective construction of natural products such as α-eudesmol or dihydromevinolin alternative reaction conditions for the cycloaddition step have to be developed.
    Additional Material: 5 Tab.
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  • 3
    ISSN: 1434-193X
    Keywords: Rearrangements ; Density functional calculations ; Substituent effects ; Transition structures ; Small-ring compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Two possible types of competing [1,3] rearrangements of divinylcyclopropane derivatives and of the hetero-analogous compounds vinylcyclopropanecarbaldehyde and vinylcyclopropanecarbthioaldehyde to five-membered ring compounds have been investigated theoretically by (U)DFT (Becke3LYP/6-31G*) methodology. Although both of the rearrangement pathways correspond to a reaction of the monosubstituted cyclopropane substructure, i.e. a reaction of vinylcyclopropane or of cyclopropanecarb(thio)aldehyde, new features in the reactivity are seen due to the combination of two substituents. Reaction parameters such as reaction and activation energies of the [1,3]-sigmatropic rearrangements have been calculated and compared. Substituent effects of hydroxy and formyl (donor and acceptor) groups in specific positions have also been considered. On the basis of these calculations, the [1,3] rearrangement of vinylcyclopropanecarbaldehydes to dihydrofurans can be expected to be kinetically favored over their conversion to cyclopentenes, while formation of the latter should be favored thermodynamically. This prediction is borne out by the experimental findings. Depending on the substitution pattern, the [1,3]-sigmatropic rearrangement of vinylcyclopropanecarbaldehydes may compete with the [3,3]-sigmatropic rearrangement. For the other investigated structures, the [1,3] rearrangement is strongly disfavored.Supporting information for this article is available on the WWW under -http://www.wileY-Vch.de/contents/jc_2046/1999/98456_s.pdf or from the author.
    Additional Material: 9 Ill.
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  • 4
    ISSN: 0947-3440
    Keywords: Electrocyclic reactions ; Hetero Diels-Alder reaction ; Thionitroso compounds ; Nitroso compounds ; N,S-Heterocycles ; N,O-Heterocycles ; Transition structures ; Calculations, ab initio, semiempirical ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structure and reactivity of the title compounds 1 were examined by ab initio methods at the post Hartree-Fock level of theory. Both compounds are expected to undergo electrocyclic ring closure to 4H-1,2-oxazete (4O) and 4H-1,2-thiazete (4S), respectively. The alternative electrocyclic reaction affording 2H-azirine 1-oxide (3O) and 2H-azirine 1-thioxide (3S) is definitely less favoured. (Thionitroso)ethylene (1S) is more reactive than the oxygen-containing congener. The same holds for the cycloaddition reaction with ethylene. [4 + 2] Hetero Diels-Alder reactions furnishing 4H-1,2-oxazine (5O) or 4H-1,2-thiazine (5S) are favoured over the [3 + 2] cycloaddition reactions providing 2H-pyrrole 1-oxide (2O) and 2H-pyrrole 1-thioxide (2S). The results of RMP2/6-31G* calculations and results obtained at lower levels of theory (RHF, PM3) are compared and discussed with respect to the experimental material available. Some additional single-point calculations at the computationally more demanding QC1SD/6-31G* level confirmed conclusions drawn from RMP2 calculations. Some test calculations also showed that the theoretical results are less affected by consideration of the temperature by statistic thermodynamics and by inclusion of solvent effects by a self-consistent reaction-field method.
    Additional Material: 5 Ill.
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  • 5
    ISSN: 0947-3440
    Keywords: Pericyclic reactions ; Hetero-Cope-type rearrangement ; Cyclopropanes ; Heterocycles ; Substituent effects ; Transition structures ; Density functional calculations ; Ab initio calculations ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The prototypical 1,2-cis-vinylcyclopropanecarbaldehyde-to-2,5-dihydrooxepin hetero-Cope-type rearrangement was studied by “exact” first-principle methods. The reaction pathway was examined. The reaction, as well activation energies, was calculated for the unimolecular transformation of vinylcyclopropanecarbaldehyde and various derivatives. The derivatives differ from vinylcyclopropanecarbaldehyde by replacement of the formyl (CH=O) by the thioformyl (CH=S) or formiminyl (CH=NH) group and, in part, by replacement of hydrogen atoms at the substituted carbon atoms of the cyclopropane ring by hydroxyl or formyl substituents. The experimental reaction parameters of vinylcyclopropanecarbaldehyde are surprisingly well reproduced by B3LYP/6-31G* density functional and MP2/6-31G* ab initio quantum theoretical calculations. Reactant and product are nearly isoenergetic, while the activation energy amounts to about 25 kcal/mol. In the case of the nitrogen and sulfur containing compounds the isomeric seven-membered ring structures are considerably favoured over the cyclopropanes. Due to a low calculated activation energy a rapid formation of the 2,5-dihydrothiepin is expected. Substitution of the hydrogen at the substituted ring carbon atoms in vinylcyclopropanecarbaldehyde by OH and CH=O also lowers the barrier and increases, in general, the exothermicity of the reaction. As shown by the reaction energies of isodesmic reactions both reactants and products are stabilized by substitution. However, the seven-memberd ring compounds are more strongly stabilized than the cyclopropanes. The OH group exerts a different effect depending on whether the linkage is geminal or vicinal to the C=X group. The difference is caused by hydrogen bond formation in geminal arrangements. The substituent effects in the cyclopropane series parallel those for the prototypical Cope-type and Claisen-type series. The particular feature of the cyclopropane series is the lower stability of the cyclopropanes relative to the corresponding open chain congeners. This is obviously due to the ring strain which over-compensates for attractive interactions between cyclopropane and the substituents. In consequence, the formation of seven-membered ring compounds proceeds more easily than the formation of the corresponding compounds in related Cope-type rearrangements.
    Additional Material: 7 Ill.
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  • 6
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines, lithiated ; Deprotonation ; Alkylation ; Deuteration ; Regioselectivity ; Calculations, MNDO ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5,6-Dihydro-5-methylene-4H-1,2-oxazine 1 is smoothly converted by n-butyllithium into 1-Li which reacts with electrophiles such as D2O, carbonyl compounds, dimethyl sulfide, or an azo diester to give the γ-adducts 4a-4f. On the other hand, alkylation of 1-Li occurs exclusively at C-4 of the heterocycle and provides the α-adducts 3g and 3h. These reactions require the activation of 1-Li by tetramethylethylenediamine. Treatment with allyl bromide and methyl acrylate affords mixtures of regioisomers 3 and 4. 1,2-Oxazine 5 with a conjugated C = C bond is less acidic than 1 but is also converted into 1-Li, whilst compound 6, lacking the 6-methoxy group, is not deprotonated under standard conditions. The dianion of 1,2-oxazine 7 is generated by employing an excess of n-butyllithium. This dianion displays a similar regiochemical behavior as 1-Li. Deuterium is exclusively incorporated into the γ-position to give product 8, while methylation occurs at C-4 to produce 9. 1,2-Oxazine 3g with an additional 4-methyl group can also be metalated and affords γ-adducts 10 and 11 upon reaction with D2O or acetone. Treatment with methyl iodide gives a 3:1 mixture of regioisomers 12 and 13. Deuteration of 1,2-oxazines 14 and 16 bearing a 3-CF3 or 3-CO2Et substituent requires more severe deprotonation conditions to provide the γ-adducts 15 and 17 in moderate yields. MNDO calculations of neutral 1,2-oxazines, the corresponding carbanions, and the lithium compounds allow an insight into the structure and charge distribution of these species, and also an estimation of the relative acidities. The regioselectivity of reactions of 1-Li is discussed on the basis of these semiempirical calculations and comparison with related ambident nucleophiles.
    Additional Material: 1 Ill.
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  • 7
    ISSN: 0009-2940
    Keywords: Chromium complexes ; Carbene complexes ; 1,3-Dienes, electron-deficient ; [2 + 1] Cycloadditions ; Vinylcyclopropanes, donor-acceptor-substituted ; Diastereoselectivity ; Regioselectivity ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermal reactions of Fischer carbene complex 1 with trisubstituted electron-deficient 1,3-dienes 7, 8, 23, 25, 32, and 35 provided highly substituted vinylcyclopropanes in good yields. The carbene transfer proceeds highly regioselectively favouring cyclopropanation of the double bond not bearing the ester function. In addition, the diastereoselectivity is generally fairly high in preference of cyclopropanes with the methoxy group cis-positioned with respect to the olefin moiety. The reaction of methylcarbene complex 2 with diene ester 8 displays inversed regioselectivity. These observations are discussed together with solvent effects, and a mechanistic rationale is presented.
    Additional Material: 5 Tab.
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  • 8
    ISSN: 0009-2940
    Keywords: Chromium complexes ; Carbene complexes ; 1,3-Dienes, electron-deficient ; [2 + 1] Cycloaddition ; Vinylcyclopropanes, donor-acceptor-substituted ; 1,1′-Bicyclopropane-2-carboxamides ; Regioselectivity ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermal reaction of Fischer carbene complex 1 with electron-deficient 1,3-dienes such as methyl (E,E)-2,4-hexadienoate (2) provided functionalized vinylcyclopropanes like 3 in good yields. Similar results were obtained by employing related unsaturated esters 4, 17, and 21 or nitrile 6. The periselectivity as well as regioselectivity of these carbene-transfer reactions are generally very high, and the diastereomer with the methoxy group cis-positioned with respect to the olefinic moiety is largely favoured (〉85:15). A mechanistic rationale of these observations is given. Double adducts were only formed as minor sideproducts in these reactions, but by employing amide 13 as electron-deficient diene they were very easily formed with 1. The [2 + 1] cycloaddition could also be extended to methylcarbene complex 24 and diene 2, but the corresponding vinylcyclopropane 25a was so far only obtained in low yield.
    Additional Material: 5 Tab.
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