Showing papers on "Cyclopropane published in 1995"

Synthesis of Prototypical Fullerene Cyclopropanes and Annulenes. Isomer Differentiation via NMR and UV Spectroscopy

Abstract: Parent cyclopropanes, annulenes, and related derivatives of C{sub 60} and C{sub 70} have been prepared and subjected to extensive spectroscopic scrutiny. Photolysis of the diazomethane adduct of C{sub 60} afforded the C{sub 61}H{sub 2} cyclopropane 2, accompanied by the isomeric annulene 4. Reaction of dimethyldiazomethane with C{sub 60} led to C{sub 63}H{sub 6} annulene 11, which upon heating was smoothly converted to the cyclopropane isomer 10 with an activation energy of 25 {+-} 1 kcal-mol{sup -1}. Addition of diazomethane to a toluene solution of C{sub 70} generated a 12:1:2 mixture of pyrazolines. Photolysis of the mixture gave the C{sub 11}H{sub 2} cyclopropanes 6 and 7, whereas thermolysis furnished the isomeric annulenes 8 and 9. Criteria have been developed for distinguishing fullerene cyclopropane derivatives from the corresponding annulenes via NMR and UV spectroscopy. 57 refs., 12 figs., 1 tab.

Completely Spirocyclopropanated Macrocyclic Oligodiacetylenes: The Family of “Exploding” [n]Rotanes†‡

Abstract: A general approach to the family of completely spirocyclopropanated macrocyclic polydiacetylenes, that is, cyclic dehydrooligomers of 1,1-diethynylcyclopropane 4, is reported. The characterized examples of these “exploding” [n]rotanes are for n = 5, 6, 7, 8, 9, 10, and 12. X-ray crystal structure analyses for the hydrocarbons with n = 5, 6, 7, and 8 disclose a strong electronic interaction between the cyclopropane and the acetylene units leading to a significant shortening of the distal and lengthening of the proximal cyclopropane bonds. While the five-sided compound 18 can occur as a planar or envelope-shaped molecule, depending on the solvent from which crystals are grown, the six- (19), seven- (20), and eight-sided (21) molecules all have chair conformations. While the butadiyne units in 18 and 19 are bent slightly outwards, those in the seven- and eight-sided molecules 20 and 21, respectively, are bent distinctly inward. All these compounds are extremely high-energy molecules: when struck with a spatula or a pestle, they go off with a puff to yield black soot.

Carbenoid Reactions in Rhodium(II)‐Catalyzed Decomposition of Iodonium Ylides

Abstract: The intermediacy of metallocarbenes in decomposition reactions of iodonium ylides with [Rh2(OAc)4] was established by comparison with reactions of the corresponding diazo compounds. The sensitivity of the RhII-catalyzed intermolecular cyclopropane formation from substituted styrenes and bis(methoxycarbonyl)(phenyliodono)methanide (1a) or dimethyl diazomalonate (1b) is identical. The Hammett plot (with σ+) has a slope of −0.47. Iodonium ylides and diazo compounds afford the same products in [Rh2(OAc)4]-catalyzed cyclopropane formations, cycloadditions, and intramolecular CH insertions, and exhibit the identical selectivity in intramolecular competitions for cyclopropane formation and insertion. The intramolecular CH insertion of the ylide 20c, when carried out in the presence of a chiral catalyst ([Rh2{(−)-(S)-ptpa}4]), results in formation of 21a having an ee of 67%, identical to the ee obtained with the diazo compound 20b.

Hydrophobic concave surfaces and cavities by combination of calix[4]arenes and resorcin[4]arenes

Abstract: A general approach to the family of completely spirocyclopropanated macrocyclic polydiacetylenes, that is, cyclic dehydrooligomers of 1,1-diethynylcyclopropane 4, is reported. The characterized examples of these 'exploding' [n]rotanes are for n = 5, 6, 7, 8, 9, 10, and 12. X-ray crystal structure analyses for the hydrocarbons with n = 5, 6, 7, and 8 disclose a strong electronic interaction between the cyclopropane and the acetylene units leading to a significant shortening of the distal and lengthening of the proximal cyclopropane bonds. While the five-sided compound 18 can occur as a planar or envelope-shaped molecule, depending on the solvent from which crystals are grown, the six- (19), seven- (20), and eight-sided (21) molecules all have chair conformations. While the butadiyne units in 18 and 19 are bent slightly outwards, those in the seven- and eight-sided molecules 20 and 21, respectively, are bent distinctly inward. All these compounds are extremely high-energy molecules: when struck with a spatula or a pestle, they go off with a puff to yield black soot.

Carbon-carbon bond activation in cyclopropane by energetic forms of hydrogen on the Ni(100) surface

Abstract: Carbon-carbon bond activation is important in catalytic processes, and is observed here at a low temperature during reactions of adsorbed cyclopropane with both gas phase atomic hydrogen and bulk hydrogen in the presence of surface hydrogen on the Ni(100) surface. The large energies possessed by gas phase atomic hydrogen may allow substantial activation barriers to be overcome in the absence of thermal energy. Therefore, we propose that ring-opening may be initiated by reaction with energetic forms of hydrogen to produce propyl at a low temperature on the surface. 25 refs., 3 figs.

Stereoselective cyclopropanation of 1-azadienes with Fischer carbene complexes

Abstract: 1-Aza-1,3-dienes, including a 1-aza-1,3,5-triene, are cyclopropanated by pentacarbonyl(methoxy)phenylchromium to give trans-cyclopropaneimines stereoselectively; their transformation into cyclopropane aldehydes and five-membered heterocycles is also described.

Preparation of Novel Lipophilic GABA analogues containing cyclopropane rings via cyclopropanation of N‐silylated unsaturated amines

Abstract: Novel lipophilic γ-amino acids 8 and 9 analogous to GABA containing cyclopropane rings are synthesized by Rh(II)-catalyzed reaction of diazo compounds 2 and 3 with N,N-bis(trimethylsilyl)allylamine (1) and subsequent hydrolysis. In addition, routes are described leading to the vinylogous GABA analogue 15 and to the unusual tricyclic γ-amino acid 20.

Diastereoselective iodocarbocyclization reaction of 8-phenylmenthyl allylmalonate: An efficient preparation of a synthetic intermediate of cyclopropane amino acids

Abstract: The iodocarbocyclization of allylmalonate using (−)-8-phenylmenthol as a chiral auxiliary proceeded with high diastereoselectivity to give the iodomethylcyclopropane dicarboxylic ester in good yield.

Synthesis and X-ray crystallographic structure of the right-hand hemisphere of halicholactone and neohalicholactone

Abstract: The synthesis and X-ray crystallographic structure of the right-hand hemisphere of the marine natural products halicholactone and neohalicholactone, which contains a nine-membered lactone and cyclopropane ring, is reported.

gem-Dialkoxy Effect in Radical Cyclizations To Form Cyclopropane Derivatives: Unusual Oxidation of a Dialkoxyalkyl Radical

Abstract: Recently we published an example of the synthetic utility of the gem-dialkoxy effect for the efficient preparation of strained four-membered rings by radical cyclizat i ~ n . ~ As shown below, under similar conditions cyclization of the ethyl 6-bromo-2-hexenoates la-c gave either the acyclic 2a-c or cyclic 3a-c products. The dihydrido substrate l a gave only the acyclic product 2a. gemdimethyl substitution, e.g., lb, increased the amount of cyclic product 3b to 25% of the mixture under normal conditions and to nearly 90% under catalytic conditions. However, the diethoxy substrate IC cyclized completely under normal conditions to give only the cyclobutanone ketal 3c, thus indicating the preparative power of the gem-dialkoxy e f f e ~ t . ~ We speculated that this gemdialkoxy effect might also allow for the formation of

Ring opening of cyclopropanecarboxylates using samarium(II) diiodide (SmI2)-HMPA-THF system

Abstract: Cyclopropane ring of 2- and 2,3-substituted cyclopropanecarboxylates (1) was regioselectively cleaved using SmI 2 -HMPA-THF system in the presence of tert-BuOH to give 4- and 3,4-substituted butyrates (2) in moderate to good yields

Cascade reactions with chiral michael acceptors; synthesis of enantiomerically pure tricyclo[3.2.1.02,7]‐ and bicyclo[3.2.1]octanes

Abstract: Starting from the easily accesible chiral aldehyde 2, we obtained enantiomerically pure (Z)- and (E)-α,-chloro-α,β-unsaturated esters 4c in good yields by olefination reactions. (Z)-and (E)-4c were allowed to react with the kinetically controlled generated lithium dienolate Li-7 to give the enantiomerically pure tricyclo[3.2.1.02,7]octanes 8 and 9, respectively. The „push-pull” substituted cyclopropane moiety of 8 was opened to give solely bicyclo[3.2.1]octane 10.

Asymmetric cyclopropanation by reaction of a γ-chloromethylated chiral vinylic sulfoxide with allylmagnesium bromide

Abstract: An optically active vinylic sulfoxide bearing a leaving group at the γ-position was stereoselectively transformed into a chiral cyclopropane by means of a Michael addition with an allylmagnesium bromide. The Michael induced ring-closure reaction requires both allylmagnesium bromide as a uncleophile and chloride as a leaving group for high diastereoselectivity. The absolute stereochemistry of the cycloadduct was confirmed by X-ray analysis.

Cyclopropane derivatives and method of preparing the same

Abstract: A cyclopropane derivative represented by formula (I): ##STR1## wherein B represents a group of a purine derivative. Another aspect of the invention provides (3-oxa-2-oxobicyclo 3,1,0!hexan-1-yl)methanol and a method of preparing a cyclopropane derivative of formula (I) from this compound by replacing the hydroxyl group of the compound with a leaving group followed by reaction with a purine derivative.

A New Nucleophilic Ring Opening of an Activated Cyclopropane and a Formal Synthesis of (±)-Carbovir

Abstract: The reaction of bicyclo[3.1.0]hexane 1, possessing a doubly activated cyclopropane ring, with acetic acid and potassium acetate in DMSO proceeded smoothly to give the adduct 2 in good yield. A formal total synthesis of the potent anti-HIV agent (±)-carbovir (9) was done by converting 2 into a known precursor 8 in 8 steps via allyl alcohol 7 including the regioselective introduction of a double bond (4 to 5) and attachment of the nucleobase using the Mitsunobu reaction (7 to 8).

Asymmetric synthesis of enantiopure (+)-(1S,2S) methyl 2-formylcyclopropanecarboxylate and corresponding (Z) and (E) didehydroamino acid derivatives

Abstract: The title cyclopropane aldehyde and didehydroamino acids have been synthesized from (S)-(+)-γ-butyrolactone-γ-carboxylic acid as a chiral precursor.

Modern valence-bond description of bonding in strained three-membered rings: cyclopropane, aziridine, ethene oxide, phosphirane and thiirane

Abstract: Spin-coupled (SC) theory was used to investigate the electronic structure of five three-membered rings: cyclopropane (C 3 H 6 ), aziridine (C 2 H 4 NH), ethene oxide (C 2 H 4 O). phosphirane (C 2 H 4 PH) and thiirane (C 2 H 4 S). It is demonstrated that the nuclear frameworks of all five molecules are held together by bent bonds, which gives firm support to one of the two alternative classical models suggested for describing bonding in three-membered rings: the one due to Coulson and Moffitt (the other model, suggested by Walsh, involves the idea of a two-electron three-centre bond). The quantitative character of the SC results is corroborated by the performance of full-valence valence bond (VB) calculations within the active space defined by the SC orbitals. It is argued that the close values of the SC and generalized VB with perfect pairing and strong-orthogonality constraints (GVB-PP-SO) energies for all five systems considered indicates that there is nothing too unusual to distinguish the bonds in three-membered rings from more routine σ bonds, say in saturated hydrocarbon chains, except that due to the particular geometry of the nuclear framework the participating orbitals have to bend in order to reduce strain and to be able to overlap more efficiently.