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Houben–Weyl Methods of Organic Chemistry will be published starting in the year. information intended to help the user choose the most suitable method for their applica- . Reduction of Carbonic and Carboxylic Acid Derivatives.
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- EPA2 - Process for the preparation of substituted piperidines - Google Patents
- EP0181618A2 - Process for the preparation of substituted piperidines - Google Patents
The formed adduct undergoes an elimination of methanol to form the enolate. This adducts suffers the Claisen rearrangement to afford the j unsaturated ester 6.
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There is a correlation between chiral centers at C-2 and C-3 in compounds 2 and 4 Fig. Also let us remark the dependence on the temperature with regard to the stereochemistry during the Claisen rearrangement, as shown in Fig. In Fig. The Ireland's ester-enolate procedure described previously [9,11] is considered among the best methods when controlling the geometry of the double bond of the enol ether [11,13].
In this sense, the conversion of the allylic ester 13 into the silyl ketene acetal by using lithium diisopropylamine in THF with subsequent silylation with chlorodimethylterbutyl silane, usually conducts to the E-ketene acetal 14 and later to its acid 15 . Is at this stage that the stereochemistry EorZof the double bond of the enol moiety of the enol ether will be defined depending on the solvent lithium diisopropylamine in THF or lithium diisopropylamine in THF plus hexamethyl-phosphoric triamide respectively according to Ireland.
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There is after, a transferring of charge from the carbanion-lithium to the oxygen of the carbonyl to form a new oxygen-lithium pair. For example, let us survey the E-crotyl propanoate 18, which produces principally the erythro acid 19 when the enolisation is done in THF, however, the threo form 20 predominates if the solvent THF contains hexamethyl-phosphoric triamide [11,14]. When the Z-crotyl propanoate 21  is employed, the opposite stereochemistry is observed [11,14]. Non-bonding interactions favor a transition state like 22 a and consequently the formation of the Z-enolate is favored.
If HMPA is added to the solvent, co-ordination is less important and the ether oxygen is the more demanding sterically, consequently, the E-enolate is favored . The authors express their gratitude to Prof. Bravo, J. Theoretical mechanistic approach to diasteroselective synthesis of cis-1,2-dialkenylcyclopropanols and subsequent oxy-Cope rearrangement by Jin Kun Cha et al, Rev.
EPA2 - Process for the preparation of substituted piperidines - Google Patents
Vila, J. Carruthers, W.
Stork, G, Raucher, S. In the present method, the reagents were mixed in a pressure resistant tube equipped with a stirring bar and placed in a microwave oven apparatus.
The spectral data of 4d and 4e are in agreement with published data. After the time indicated in Table 1 , the tube was opened and the solid was dissolved in chloroform and precipitated with hexane to give analytically pure samples with the yields shown in Table 1.
The extraction, drying and silica gel filtration steps were eliminated. The recrystallization process of the products obtained by this method was much easier as compared with the recrystallization of the products obtained when benzene was used as the reaction solvent. In all but one case a single product was formed. The structure assignment of the obtained products was made by comparison of their spectral data with those published previously, when a detailed study of the regio- and stereochemistry of the addition products of p -methoxyphenyltellurium trichloride to alkynes was made.
These results are in agreement with a previous work. This transformation was important to determine the isomer ratio in the reaction with propargyl alcohol Table 1 , entry 3d , since the chromatographic separation of the tellurium IV compounds failed Scheme 4. In conclusion, an easy and ecofriendly method to prepare tellurium IV compounds was developed using microwave irradiation of the reaction mixture in the absence of solvents.
Under the conditions employed in this communication, the reactions occur in shorter reaction times and at lower temperatures than in the preceding published methods, avoiding the use of harmful solvents and harsh reaction conditions. Analytical thin layer chromatography TLC for monitoring reactions was performed using Merck 0. All reagents are commercial grade and were pretreated before use, when needed all reagents were purchased from Sigma-Aldrich Co. The mixture was refluxed for 72 h until all the tellurium powder was consumed.
After this time, the excess of SO 2 Cl 2 was removed by distillation under vacuum.
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A white solid was obtained, which was submitted to high vacuum under a water bath heating to remove any trace of SO 2 Cl 2 and then used without further purification. Yield: After this time all the tellurium powder was consumed and the excess of SO 2 Cl 2 was removed by distillation under vacuum, leaving behind a white solid which was submitted to high vacuum and heating as described above and then used for further reactions without purification.
Yield: 7. Preparation of p -methoxyphenyltellurium trichloride 2. The procedure described by Cunha et al.
EP0181618A2 - Process for the preparation of substituted piperidines - Google Patents
The yellow solid obtained was recrystallized from acetic acid. In a glass pressure resistant tube 35 mL equipped with a magnetic stirring bar were placed tellurium tetrachloride prepared as described for 1 1. Yield: 2. Addition of p -methoxyphenyltelluriumtrichloride to alkynes, typical procedure. To a mL round bottomed flask equipped with a reflux condenser and a magnetic stirring bar were added p -methoxyphenyltellurium trichloride prepared as described for 2 15 mmol , the alkyne 15 mmol and benzene 50 mL.
The mixture was heated under reflux for 8 h and the reaction was monitored by TLC eluting with a mixture of hexane:ethyl acetate Forgot password? Review as guest. Houben-Weyl Methods of Organic Chemistry. Print with eBook included E-Book. Add to cart.
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