I can opt out at any time by clicking the "unsubscribe" link at the end of each newsletter. Further information about data processing and your corresponding rights. We only use this information to personally address you in your newsletter. Keep up! Thieme emails bring you the latest medical and scientific resources. Forgot password? Review as guest. Houben-Weyl Methods of Organic Chemistry. Add to cart. Ar represents in each case identically or differently monosubstituted to trisubstituted phenyl, thienyl, pyridyl or pyrimidinyl, where as substituents there may be mentioned:.
By the process according to the invention, compounds of the formula I are very particularly preferably prepared in which. Ar represents identically or differently monosubstituted or disubstituted phenyl, where as substituents there may be mentioned:. These radical definitions can be combined together in any manner, that is also between the respective ranges and preferred ranges. In formula II , Ar preferably represents or particularly preferably represents those meanings which have already been specified above as preferred or particularly preferred for Ar in connection with the description of the compounds of the formula I.
Ar 1 preferably represents identically or differently monosubstituted to pentasubstituted phenyl or in each case identically or differently monosubstituted to trisubstituted thienyl, pyridyl or pyrirnidinyl, where as substituents there may be mentioned:.
Allylsilanes in the synthesis of three to seven membered rings: the silylcuprate strategy
Ar 1 particularly preferably represents in each case identically or differently monosubstituted to trisubstittited phenyl, thienyl, pyridyl or pyrimidinyl, where as substituents there may be mentioned:. Ar 1 very particularly preferably represents identically or differently monosubstituted or disubstituted phenyl, where as substituents there may be mentioned:. The compounds 1- 4-methoxyphenyl2-hydroxy-ethanone oxime and 1- 5-bromothienyl hydroxy-ethanone oxime Khimiya i Tekhnol, Elementoorg. Preparation Example. The process described under 1 is carried out in the presence of Raney nickel or Raney cobalt.
These are well-known catalysts which are described, e. The process according to the invention described under 1 is preferably carried out in the presence of a diluent. Diluents which are useful are all conventional organic solvents inert under the reaction conditions. These include alcohols, such as methanol, ethanol, n-propanol, iso-propanol, n-butanol or iso-butanol; hydrocarbons, such as benzene, toluene, xylene, hexane, cyclohexane or ethers, such as diethyl ether, diisopropyl ether, tert-butyl methyl ether, tert-amyl methyl ether, 1,2-dimethoxyethane, tetaahydrofiran and dioxane.
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However, under the reaction conditions it can also be advantageous to use mixtures of water and one of the diluents specified above. The reaction temperatures can be varied in a relatively broad range when the process according to the invention described under 1 is carried out. The process according to the invention described under 1 is carried out in the presence of hydrogen and generally under pressure, the hydrogen pressure being between one and bar, preferably between 15 and bar. The process according to the invention described under 1 is carried out under basic reaction conditions, preferably in the presence of ammonia.
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This can be used if appropriate in the gaseous or liquid state. The work-up can proceed by conventional methods, for example by filtering off the catalyst and removing the solvent under reduced pressure. The crude product thus obtained can be purified by recrystallization or chromatography.
The amino alcohols can also be purified by formation of their salts, e. The 2-aminoarylethanols specified above under 2 are generally defined by the formula Ia. X preferably represents fluorine, chlorine, bromine, C 1 -C 4 -alkyl or C 1 -C 4 -alkoxy,. Y preferably represents C 1 -C 4 -halogenoalkoxy or halogen-substituted C 3 -C 6 -cycloalkoxy,. The 2-aminoarylethanols of the formula Ia are novel and are subject-matter of the invention. They can be obtained by the process 1 according to the invention.
The 2-arninoarylethanols of the formula I to be prepared by the process according to the invention can be used as starting materials for the preparation of pesticides cf. EP After the mixture is cooled, it is depressurized, filtered and the solvent is removed under reduced pressure. A dark oil remains which slowly crystallizes.
In a similar manner and according to the general specifications for preparation, the compounds of the formula I listed below in Table 1 are obtained: STR The mixture is poured onto ice-water and filtered. By analogy with Example II-1 and in accordance with the general specifications for preparation, the compounds of the formula II listed below in Table 2 are obtained: STR A unique feature of the reaction is the invariable formation of an exocyclic double bond by loss of the silicon group.
The methylenecyclopentanol moiety is present in the skeleton of some naturally occurring terpene families.
Recent work has shown that the nature of the silyl group may cause important modifications in the mechanism pathway and therefore may change the final outcome. This is the case of analogous allylsilanes bearing the bulky t-butyldiphenylsilyl group, which give 3-cyclopentenols maintaining the hindered silyl group.
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Silylcupration of acetylenes is also a powerful tool for cyclopentane annulations. Terminal alkynes 17—19 bearing electron-withdrawing groups in appropriate positions undergo silylcupration-ring formation, when treated with higher order cyanocuprates as PhMe 2 Si 2 CuCNLi 2. Epoxidation of the oxoallylsilanes obtained from the "silylcuprate methodology" provides a rapid access to epoxyallylsilanes. Despite its synthetic potential, the cyclization of epoxyallylsilanes has not been widely reported. Although Baldwin's rules predict that 5- exo attack, leading to cyclopentanols, is favoured over 6- endo attack, none of the former cyclization mode is observed when epoxyallylsilanes 23—27 are submitted to Lewis acidic conditions.
The diastereoselectivity of the reaction depends on the Lewis acid used.
Houben-Weyl Methods of Organic Chemistry
Boron and aluminum-based catalysts show a preference for the cis isomer 29b and 30b whereas TiCl 4 gives almost exclusively trans isomers 29a and 30a. Cyclization of epoxyallylsilanes containing the bulky t-butyldiphenylsilyl group takes place without loss of silicon giving cyclohexenols bearing the t-butyldiphenylsilyl group. Formation of the spirocycle proceeds by a two-step pathway involving firstly, Me 3 Al-catalysed intramolecular cyclization of the oxoallylsilane and subsequent formation of a methylenecyclopentanolate, and then cyclopropanation.
This unique mechanism enables the construction of hydroxylated bi-tri- and tetracyclic skeletons, bearing the spiro-cyclopropane moiety, from open chain allylsilanes in just one step. The use of reagents different from organoaluminun compounds resulted in poor efficiency and low stereoselectivity.
Unfortunately, this route cannot be used to synthesize spiro[ 2 , 5 ]octanes from epoxyallylsilanes of the type 23 , due to the high reactivity of the epoxide group towards Me 3 Al, the latter giving S N 1 attack resulting in the formation of methyl alcohols to a great extent.
Future work will show if cyclopropanating reagents with a weaker Lewis acid character can be appropriate to direct the reaction toward the synthesis of spiro[ 2 , 5 ]octanes, an structural moiety of interest in the synthesis of natural products. Alcohols as 40 containing an allylsilane unit, which can be readily obtained by reaction of epoxides with the silylcuprate 2 , are excellent synthons for cyclobutane ring-formation.
A different approach, starting from acetylenes instead of allenes and using silyl- or stannylcuprates followed by addition of an epoxide as electrophile, led to substituted cyclobutenes after cyclization of the vinylsilane or vinylstannane intermediate.
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The key step is the syn addition of the tin cuprate to the acetylene, which controls the cis stereochemistry required for cyclization. The use of nitriles and imines as electrophiles in the silylcupration of allene provides new alternatives for carbocyclization. Whatever is the reason, this tandem process allows the introduction of two silylated functions, which display a markedly different reactivity.
Similarly, silylcupration of imines [ 25 ] provides a simple and efficient route for the preparation of seven membered carbocycles with different substitution patterns. Other allylsilane-based strategies have been recently developed to build up cycloheptane derivatives. In summary, the metallocupration Si-Cu and Sn-Cu of allenes and acetylenes has proven to be extremely useful for the construction of cyclic structures ranging from three to seven membered rings, through processes which imply addition of the intermediate silylcuprate to an electrophile enone, epoxide, nitrile, imine, etc followed by Lewis-acid catalysed intramolecular cyclization, where the electrophile used determines the type of process and the size of the ring.
This paper is dedicated to Professor Miguel Yus Astiz on occasion of his 60 th birthday.