Opening of cyclopropanes


The growing importance of cyclopropane derivatives (A. de Meijere, 1979), as synthetic intermediates originates in the unique, olefin-like properties of this carbocycle. Cyclopropane derivatives with one or two activating groups are easily opened (see. p. 69f.). Some of these reactions are highly regio- and stereoselective (E. Wenkert, 1970 A, B E. J. Corey, 1956 A, B, 1975 see p. 70). Many appropriately substituted cyclopropane derivatives yield 1,4-difunctional compounds under mild nucleophilic or reductive reaction conditions. Such compounds are especially useful in syntheses of cyclopentenone derivatives and of heterocycles (see also sections 1.13.3 and 4.6.4).  [c.76]

Cyproterone Acetate. Cyproterone, the free alcohol of cyproterone acetate, is an antiandrogen. Cyproterone acetate (41) is both an antiandrogen and a progestin. It is used in prostatic carcinoma treatment and for the control of Hbido. In females, it is used in conjunction with ethynylestradiol for the control of acne and hirsutism (65). Cyproterone acetate may be recrystallized from diisopropyl ether (66). It may be synthesized from 17-acetoxyprogesterone (42). The triene (67) reacts with diazomethane to give a pyrazoline pyrolysis, to effect loss of nitrogen, provides a cyclopropane (43) and oxidation with perbenzoic acid leads to an epoxide. Treatment with hydrochloric acid results in both the desired opening of the epoxide and the undesired cleavage of the cyclopropane. The cyclopropane is re-formed through treatment with coUidine to afford cyproterone acetate (41) (68-70).  [c.212]

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates.  [c.32]

Selectivity is influenced by temperature. Hydrogenolysis of 22 to 23 was carried out at 5°C to prevent opening of the cyclopropane ring (S2).  [c.162]

The cyclopropane ring is necessarily planar, and the question of conformation does not arise. The C—C bond lengths are slightly shorter than normal at 1.5 A, and the H—C—H angle of 115° is opened somewhat from the tetrahedral angle. These structural  [c.146]

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed  [c.282]

The last group of reactions uses ring opening of carbonyl or 1-hydroxyalkyl substituted cyclopropanes, which operate as a -synthons. d -Synthons, e.g. hydroxide or halides, yield 1,4-disubstituted products (E. Wenkert, 1970 A). (1-Hydroxyalkyl)- and (1-haloalkyl)-cyclopropanes are rearranged to homoallylic halides, e.g. in Julia s method of terpene synthesis (M. Julia, 1961, 1974 S.F. Brady, I968 J.P. McCormick, 1975).  [c.69]

On the other hand, the Pd-catalyzed ring opening of the vinylcyclopropane 59 bearing two EWGs takes place to give the 7r-allylpalladium intermediate 60, which reacts with amines[50] and carbon nucleophiles to afford 61[5I], The (l,3-butadienyl)cyclopropane 62 substituted by two EWGs forms the 7r-allyl-palladium complex 63 by ring opening, and recyclizes to form the tive-mem-bered ring compound 64[52], This is another example of the cleavage of allylic C—C bonds. In these ring-opening reactions, the formation of a stabilized carbanion is a driving force of the 7r-allylpalladium intermediate formation. The optically active cyclopentene 66 was prepared by the asymmetric vinylcy-clopropane-cyclopentene rearrangement using the chiral sulfoxide 65 as a chiral source with 89% stereoselectivity[53].  [c.300]

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state.  [c.153]

Substituted cyclopropanes readily available from addition of carbenoids to C = C double bonds (see p. 74ff.) have become interesting d -reagents, since electron donating and withdrawing, groups strongly facilitate ring opening by acids or bases. In the first example shown below ring opening occurs in a homoallylic rearrangement after the addition of a cyclopropyl anion to a carbonyl compound has taken place (E.J. Corey, 1975B), in the second the d -synthon is generated directly by Lewis acid promoted ring opening (E. Nakamura, 1977).  [c.14]

Highly strained polycycloalkanes Consisting of several intercoimected cyclopropane or cyclobutane rings undergo thermal and hydrogenolytic ring opening with great ease (D. Kauf-mann, 1979). Those bonds which release the largest amount of strain energy are cleaved selectively.  [c.79]

C—C double bond and opened selectively at the less hindered 9of-position. After protection of the alcohol group with t-butyldimethylsilyl chloride only one etiolate is formed with the non-nucleophilic base potassium tert-butoxide. The enolate to the bridgehead carbon does not form (Bredt s rule). The carbanion then substitutes the bromine atom and a tricycloheptanone is formed in the second step. This compound is susceptible to homo-Michael type attack at the most labile bond of the highly strained cyclopropane. The cuprate reagent with the strongly nucleophilic alkenyl substituent and the less nucleophilic acetylide reacts at -78 °C to give the desired norbomanone in the third step. The following conventional steps of the synthesis involve Baeyer-Villiger oxidation of the ketone, DIBAL reduction of the lactone, and Wittig reaction with the resulting aldehyde.  [c.276]

Hydrogen bromide adds to acetylene to form vinyl bromide or ethyHdene bromide, depending on stoichiometry. The acid cleaves acycHc and cycHc ethers. It adds to the cyclopropane group by ring-opening. Additions to quinones afford bromohydroquinones. Hydrobromic acid and aldehydes can be used to introduce bromoalkyl groups into various molecules. For example, reaction with formaldehyde and an alcohol produces a bromomethyl ether. Bromomethylation of aromatic nuclei can be carried out with formaldehyde and hydrobromic acid (6).  [c.291]

The addition of nucleophiles to cyclic fluoroolefins has been reviewed by Park et al. [2 ]. The reaction with alcohols proceeds by addition-elimination to yield the cyclic vinylic ether, as illustrated by tlie reaction of l,2-dichloro-3,3-di-fluorocyclopropene Further reaction results in cyclopropane ring opening at the bond opposite the difluoromethylene carbon to give preferentially the methyl and ortho esters of (Z)-3-chloro-2-fluoroacrylic acid and a small amount of dimethyl malonate [29] (equation 8).  [c.731]

The mechanism of the Knorr reaction has been studied in detail by several labs using low temperature NMR methods. Using low temperature flow NMR, a reaction of the 1,1-diacetyl cyclopropane with hydrazine was studied by Salivanov and coworkers for the mechanism of the cyclopropane ring opening with nucleophiles. They observed an immediate formation of the intermediate A at -70°C within 10 seconds followed sequentially by formation of B and C. Because the signal intensity for intermediate B was small and constant ( quasi stationary ) over the period of the reaction, it suggested a complex combination of consecutive reactions with at least the 3 intermediates A-C before forming the product. Similar intermediates A-C were observed for unsymmetrical diketones reacted with hydrazines or mono N-alkyl hydrazines. In addition, key intermediates 3,5-dihydroxypyrazolidine 14, 15, and the hydrazone 16 have been isolated.  [c.293]

An a priori determination of the direction of ring opening is not always easy, for it is difficult to decide which of several controlling factors is operative. Various generalities concerning the direction of opening have been suggested. Cyclopropanes carrying only phenyl substituents are cleaved exclusively at the bonds adjacent to the phenyl substituent (57), whereas alkyl substituents favor cleavage at the bond opposite the substituent [I22a). Electron-attracting substituents are often cleaved at the bond adjacent to the substituent 63,79,94,146,154), but there are exceptions 113,176).  [c.174]


See pages that mention the term Opening of cyclopropanes : [c.206]    [c.114]    [c.329]    [c.338]    [c.77]    [c.299]    [c.535]    [c.122]    [c.163]    [c.428]    [c.174]   
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Organic reactions in steroid chemistry  -> Opening of cyclopropanes