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Pyrazolines => diazoalkanes

Diazoalkane cydoadditions to alkenes produce 1-pyrazolines as the initial cydo-adducts which are not so quite stable that these undergo spontaneous 1,3-proton... [Pg.278]

Diazoalkanes add readily to the double bond of esters of vinylphosphonic acid, giving the pyrazoline derivatives (100), which can lose nitrogen to give esters of cyclopropylphosphonic acids. In a similar reaction, acyl-phosphonic acid esters (101) were converted to epoxy-derivatives (102). A -Phenylsydnone adds to diethyl prop-l-ynephosphonate, giving the pyra-zole (103). The addition of cyclopentadiene to dimethyl vinyl phosphate leads to an exojendo quotient of 1.2, but with hexachlorocyclopentadiene only e/ii/o-isomer is formed. ... [Pg.117]

Moffett and coworkers203 reported the synthesis of several 4-/3-D-ribofuranosylpyrazoles, such as 284(a-c), by 1,3-dipolar cycloaddition of diazoalkanes to the alkenic C-glycosyl compound 283, followed by dehydrogenation of the resulting pyrazolines. In view of the known biological activities of several nucleosides containing the... [Pg.183]

The transition metal-catalyzed reaction of diazoalkanes with acceptor-substituted alkenes is far more intricate than reaction with simple alkenes. With acceptor-substituted alkenes the diazoalkane can undergo (transition metal-catalyzed) 1,3-dipolar cycloaddition to the olefin [651-654]. The resulting 3//-pyrazolines can either be stable or can isomerize to l//-pyrazolines. 3//-Pyrazolines can also eliminate nitrogen and collapse to cyclopropanes, even at low temperatures. Despite these potential side-reactions, several examples of catalyzed cyclopropanations of acceptor-substituted alkenes with diazoalkanes have been reported [648,655]. Substituted 2-cyclohexenones or cinnamates [642,656] have been cyclopropanated in excellent yields by treatment with diazomethane/palladium(II) acetate. Maleates, fumarates, or acrylates [642,657], on the other hand, cannot, however, be cyclopropanated under these conditions. [Pg.115]

Cyclohexadiene 45 was converted to 46 by what has proven to be a general method for preparation of the cyclohexa-2,4-dien-l-one ring system.2 Fragmentation of the aziridinyl imine in 46 at 110 °C gave an intermediate diazoalkane which underwent an intramolecular 1,3-dipolar cycloaddition to give the pyrazoline 47. At 140 °C, pyrazoline 47 expelled N2 and rearranged to the tricyclic ketone 48. The development of this and related bicyclizations29 illustrated a practical synthetic equivalence of an intramolecular diene-carbene 4-1-1 cycloaddition in the cyclohexa-2,4-dien-l-one series. [Pg.5]

With diazoalkanes 34, as should be expected in view of the high polarity of its double bond, the chloro ester 1-Me demonstrated its reasonably good dipolaro-philicity. Indeed, mixtures of the regioisomeric pyrazolines 35 and 36 were obtained in good yields [26a, 33], when a solution of 1-Me was treated with any of the diazoalkanes 34a-e (Scheme 8). [Pg.158]

Cyclic diazoalkanes have been used to generate spirocyclopropanes via ring contraction of pyrazolines formed in the initial cycloaddition step. Two novel examples are the transformation of 3-diazo-2-nitromethylenepiperidine (16) into 5-aza-spiro[2.5]octane 17 (45) and the conversion of 1-diazo-2-methylenecyclo-propane (18) into methylene-spiro[2.2]pentane 19 (46) (Scheme 8.6). Related reactions have also been reported for diazocyclopropane (47) and diazospiropen-tane (48). [Pg.546]

An interesting preparation of aliphatic diazoalkanes (R R C = N2 R, R = alkyl) involves the photolysis of 2-alkoxy-2,5-dihydro-1,3.4-oxadiazoles (see Scheme 8.49). When the photolysis is carried out in the presence of an appropriate dipolarophUe, the diazo compounds can be intercepted (prior to their further photolysis) by a [3 + 2] cycloaddition reaction (54). As an example, 2-diazopropane was intercepted with A-phenylmaleimide (54) and norbornenes (55) to give the corresponding A -pyrazolines. [Pg.547]

The cycloaddition of several diazoalkanes with (2-arylvinyl)-[(—)-(8-phenyl-menthyloxy] methylene chromium complexes 72 gave the A -pyrazolines 73 with high diastereoselectivity. These compounds were converted into pyrazolinecarbox-ylates 74 by A-protection and metal decomplexation (98) (Scheme 8.17). It is... [Pg.554]

The first effective enantioselective 1,3-dipolar cycloaddition of diazoalkanes catalyzed by chiral Lewis acids was reported in the year 20(X) (139). Under catalysis using zinc or magnesium complexes and the chiral ligand (R,/ )-DBFOX/Ph, the reaction of diazo(trimethylsilyl)methane with 3-alkenoyl-2-oxazolidin-2-one 75 (R = H) gave the desilylated A -pyrazolines (4S,5R)-76 (R =Me 87% yield, 99% ee at 40 °C) (Scheme 8.18). Simple replacement of the oxazohdinone with the 4,4-dimethyloxazolidinone ring resulted in the formation of (4R,5S)-77 (R = Me 75% yield, 97% ee at -78 °C). [Pg.555]

In a novel total synthesis of the tricyclic sesquiterpene (—)-longifolene, an intramolecular diazoalkane cycloaddition to a cyclohexadienone ring followed by thermal ring contraction of the resulting pyrazoline gave the tricychc vinylcyclo-propane 261 and this constitutes the key steps in this synthesis (314) (Scheme 8.63). The interesting features of this sequence are the separation of dipole and dipolarophile by five atoms and the formation of a seven-membered ring in the cycloaddition step. [Pg.595]

Olefins have been added to diazoalkanes to prepare 1-pyrazolines, a class of cyclic azo compounds. [Pg.152]

In the course of an extensive study of the chemistry of azo compounds, Overberger et al. [47a] prepared 1-pyrazolines, which may be considered cyclic azo compounds, by the addition of diazoalkanes to styrene. The reaction is considered a stereospecific 1,3-dipolar addition and, in the reaction between />-methoxyphenyldiazomethane and />-methoxystyrene both trans-3,5-bis-(p-methoxyphenyl)pyrazoline (m.p. 129°C, dec.) and c/s-3,5-bis(/>-methoxy-phenyl)pyrazoline were prepared and separated [47b]. [Pg.164]

Diazoalkane Method of pyrazoline decomposition Products and yields... [Pg.955]

RC=CKWG) yields 2,4-di-EWG-substituted pyrroles in the presence of copper catalyst but 2,3-di-EWG-substituted pyrroles in the presence of a phosphine catalyst.74 The 3 + 2-cycloaddition of diazoalkanes to (6 )-3-p-tolylsulfinylfuran-2(5//)-one produces diastereoisomeric pyrazolines in almost quantitative yield and with des >98%. (g) The sulfinyl group is responsible for the complete control of the n-facial selectivity in all these reactions.75 The Rh(II)-catalysed intramolecular 1,3-dipolar cycloaddition reaction of diazoamides (57) with alkenyl and heteroaromatic n -bonds yields pen-tacyclic compounds (59), via the ylide (58), in good to excellent yields and in a (g) stereocontrolled manner (Scheme 15).76... [Pg.361]

A similar conclusion based on experiments in which 9-diazofluorene was decomposed thermally in the presence of maleic and fumaric esters is unreliable, since these olefins could react directly with the diazoalkane giving the observed cyclopropanes by way of intermediate pyrazolines, and the appropriate kinetic cheek was not carried out. [Pg.196]

Reaction of 4-chloro-2-pyrazolin-5-ones with potassium carbonate proceeds along two pathways, leading to diketo derivatives of pyrazoIo[l,2-a]pyrazoIe (syn- and anti- dioxabimanes )- A mechanism involving the intermediate formation of diazacyclopen-tadienone (185) which reacts with its potential extended diazoalkane functionality in dipolar cycloaddition fashion to give syn-dioxabimanes (major product) and anri-dioxabimanes (minor product) has been proposed (Scheme 6) (80JA4983). [Pg.1000]

Cycloaddition of diazoalkanes to 6-6 bonds was among the first reactions tested on the carbon cages41 and affords isolable fullerene-fused pyrazolines as primary adducts. Photochemically induced extrusion of N2 from the latter yields 6-6-closed methanofullerenes having the functionalized bond embedded in a cyclopropane substructure, whereas the thermal process affords 6-5 open homofullerenes in which a methylene group is bridging the open junction between a six- and a five-membered ring.15-17... [Pg.50]

Access to alkyl substituted derivatives of the homotropylidene complexes is provided via the / -tropone iron complex by reaction with diazoalkanes, followed by mild thermolysis of the 3+2 pyrazoline adduct to give the corresponding homotropone complexes (equation 149)217,218 jYiQ 8,8-dimethyl derivative was used as starting material for the preparation of the fluxional ( -2,8,8-trimethylbicyclo[5.1.0]octa-2,4-dienylium)Fe(CO)3 cation com-plex More recently (homotropone)Fe(CO)3 was used for the synthesis of unique chiral 1,2-homoheptafulvene iron complexes . ... [Pg.555]

The nitrogen extrusion from 1-pyrazolines and 3H-pyrazoles giving cyclopropanes and cyclopropenes, respectively, has been extensively reviewed The cyclopropane synthesis from 1-pyrazolines can be executed thermally as well as photochemically, but the latter method generally gives substantially better results than the former. The major side reaction observed in the thermal process is the production of olefins, which arise in the migration of a substituent from the C(4) to C(3) position. A retro-1,3-dipolar addition producing a diazoalkane and an olefin has been observed in certain cases. The decomposition of 3-acyl- or 3-alkoxycarbonyl-1-pyrazolines is catalyzed by acids, such as perchloric acid and boron trifluoride and by Ce The stereochemical course... [Pg.310]


See other pages where Pyrazolines => diazoalkanes is mentioned: [Pg.180]    [Pg.279]    [Pg.294]    [Pg.87]    [Pg.957]    [Pg.262]    [Pg.180]    [Pg.180]    [Pg.956]    [Pg.957]    [Pg.1102]    [Pg.11]    [Pg.164]    [Pg.31]    [Pg.109]    [Pg.109]    [Pg.999]    [Pg.87]    [Pg.30]    [Pg.305]    [Pg.95]    [Pg.543]   
See also in sourсe #XX -- [ Pg.1006 ]




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