Cyclopropane moieties cycloaddition

In addition to siloxy- and alkoxy-substituted VCPs, alkyl- and H-substituted VCPs are also effective in the intermolecular [5 + 2]-cycloaddition reaction (Scheme 11). In general, an increase in the steric bulk of the cyclopropane substituent (H vs. Me vs. Pr1) leads to increased reaction rates, putatively through preferential population of the more reactive as-oid arrangement of the vinyl and cyclopropane moieties.43... 

In the presence of phosphane-free Ni(0) catalysts, substituted methylenecyclopropanes dimerize at low temperatures giving formal [2 + 2] and [3 -I- 2] cycloadducts 24 and 25, respectively. The chemoselectivity of [2 -t- 2] cycloaddition depends on the substitution pattern of the substrate and is restricted to systems without further substituents at the exocyclic double bond. [3 + 2] Cycloaddition and acyclic product formation depend on further substituents at the cyclopropane moiety. In general, the product is obtained as a mixture. The combined yield resulting from cycloaddition is higher in the presence of electron-deficient alkenes, such as dimethyl ( )-but-2-enedioate. ... 

The accumulation of the cycloaddition product is related to its thermal stability in regard to nitrogen elimination. Here, elimination of nitrogen is even more pronounced because of two reasons the presence of the double C-C bond instead of a cyclopropane moiety (Scheme 11) and because it can produce corresponding furan derivatives. Furan is actually one of the rare aromatic heterocyclic compounds that easily participates in Diels-Alder reactions as a moderately active diene. Therefore, it is also reasonable to postulate that the furan derivative obtained after elimination of nitrogen is more reactive than 2,5-bis(trifluoromethyl)-l,3,4-oxadiazole. Additionally, the cycloadduct with a second molecule of cyclooctyne would be a final product of the cycloaddition reaction. To explore this possibility further, a semiempirical study of cycloadduct stability and activation barrier needed for cyclooctyne to react with furan was performed. 

Bicyclic lactone synthesis by Rh-catalyzed cycloisomerization reactions of ester-tethered substrates was well studied by Saito et al. [49]. For instance, formal (5-f2) cycloaddition reaction of alkyne 75 bearing vinyl cyclopropane moiety smoothly proceeded to give oxa-bicyclo[5,3,0]octene compoxmd 76 in good yield (Scheme 34). 

The two articles in this current volume describe recent developments with small ring compounds which have not teen compiled in such a context before. T. Hirao discusses selective transformations initiated by transition derivatives in the construction of functionally substituted five-, six- and seven-membered rings as well as open-chair compounds. Cycloadditions onto methylene- and alkylidene-cyclopropane derivatives, described by A. Goti, F. M. Cordero and A. Brandi, not only yield products with spirocyclopropane moieties which can be desirable as such or as potential mimics of gem-dimethyl groupings, but also intermediates which can undergo further transformations with ring-opening of the cyclopropane units. 

Hexamethyl[3]radialene (25) does not undergo Diels-Alder-reactions with the typical electron-poor dienophiles, probably because of the full substitution at the diene termini. With TCNE, however, a violet-blue charge-transfer complex is formed which disappears within 30 min at room temperature to form a 1 1 adduct (82% yield) to which structure 55 was assigned9. Similar observations were made with tris(2-adamantylidene)cyclopropane (34), but in this case cycloaddition product 56 (81% yield) was identified its allenic moiety is clearly indicated by IR and 13C NMR data12. 

The cyclopropane diester (800) bearing a vicinal acetylenic moiety, when treated with Co2(CO)s, affords the formation of the dicobalt hexacarbonyl complex (801). It undergoes a smooth cycloaddition with a,N -diphenylnitrone, in the presence of Sc(OTf)3, to form the corresponding dicobalt hexacarbonyl complex of tetraydro-l,2-oxazine (802). De-complexation of adduct (802) gives 6-ethynyl-tetrahydro-l,2-oxazine (803) (Scheme 2.332) (856). 

The reaction course is shown in Scheme 4. Enyne 12 reacts with 2 to give vinyl carbene complex 17, which is in a state of equilibrium with vinyl ketene complex 21. [2+2] Cycloaddition of the ketene moiety and alkene part in 21 gives cyclob-utanone 22. On the other hand, the vinyl carbene complex 17 reacts with the alkene intramolecularly to produce metalacyclobutane 18. From metalacyclob-utane 18, reductive elimination occurs to give cyclopropane derivative 23. Ret-... 

Vaska s complex catalyzed the transformahon of aUenylcyclopropane into 2-alkenylidenecyclohex-3-enone under conditions of pressurized CO (Scheme 11.25) [38]. In this reaction, the jr-coordination to internal oleflnic moiety of the aUene brings the metal closer to the cyclopropane ring. Release of the cyclopropane ring strain then facilitates the oxidative addition of vinylcyclopropane moiety along with C-C bond cleavage, such that metallacyclohexene is obtained a subsequent carbonyl insertion and reductive elimination then provides the product Hence, the reaction can be recognized as a [5+1] cycloaddition of vinylcyclopropane and CO. 

With respect to the large number of unsaturated diazo and diazocarbonyl compounds that have recently been used for intramolecular transition metal catalyzed cyclopropanation reactions (6-8), it is remarkable that 1,3-dipolar cycloadditions with retention of the azo moiety have only been occasionally observed. This finding is probably due to the fact that these [3+2]-cycloaddition reactions require thermal activation while the catalytic reactions are carried out at ambient temperature. A7-AUyl carboxamides appear to be rather amenable to intramolecular cycloaddition. Compounds 254—256 (Scheme 8.61) cyclize intra-molecularly even at room temperature. The faster reaction of 254c (310) and diethoxyphosphoryl-substituted diazoamides 255 (311) as compared with diazoacetamides 254a (312) (xy2 25 h at 22 °C) and 254b (310), points to a LUMO (dipole) — HOMO(dipolarophile) controlled process. The A -pyrazolines expected... 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

Among the variety of terpenes which were synthesized with an intramolecular meta photocycloaddition as key step, triquinane derivatives were particularly well studied. Two isomers of these compounds are readily accessible via intramolecular meta cycloaddition in position 1,3 (Sch. 9). The following cyclopropanation reaction of intermediate O controls which of the angular or linear isomers is formed (Sch. 17) [67]. As this step is almost always unselective, both isomers are concomitantly formed. However, one of the isomers can be obtained predominantly via an additional photochemical equilibration step (compare Sch. 11). The triquinane frame is obtained by rupture of the distant C-C bond of the cyclopropane in connection to the 10-membered ring moiety. 

Cyclopropanation of steroids (363) at the 1,2-position with malonate in the presence of potassium t-butoxide and tetramethylguanidine takes place with formation of the di(alkoxycarbonyl)cyclopropane derivatives (364) accompanied by steroidal cyano-alkoxycarbonylcyclopropanes (365). When the reaction is carried without tetramethylguanidine only minor amounts of cyclopropyl steroids are isolated The formation of the cyano compound is explained by an initial nucleophilic displacement of bromine by the tetramethylguanidine moiety, followed by several steps (equation 119). Although the reaction of equation 120 cannot be classified as a MIRC reaction sensu strictu the formation of cyclopropa[c]cinnolines (367) via an intramolecular 1,1-cycloaddition of nitrilimines is mentioned here. Treatment of o-vinylphenyl-substituted chloroglyoxylate... 

Recently, this methodology has been expanded towards the synthesis of 1-substituted vinylcyclopropanes. Thus, heating 0.5-1 equivalent of the appropriate cyclopropenone acetal in a dry aromatic solvent to 80-150°C for 30-48 h in the presence of an electron-deficient alkene afforded a cyclopropane. This was isolated after hydrolysis of the ketene acetal moiety. The [1-1-2] cycloaddition proceeded with a high degree of regio- (>92 8) and stereoselectivity (> 84 16), depending on the substituents (see table below). ... 

The methylenecyclopropane moiety can also be linked to the tether with its cyclopropane ring as in 5 so that compounds 6 result in the intramolecular [2+4] cycloaddition. 

Besides the addition of classical reagents to cyclopropanones vide supra), there are some reactions of cyclopropanones which lead to functionalized cyclopropanes via such reactions as cycloadditions with various reagents. Cycloadditions of the 2 + 2 - 4 type with the carbonyl moiety of 2,2-dimethylcyclopropanone (1) and dimethylketene and 1,1-dimethoxyethene give spiro compounds 2 and 3, respectively. ... 

With an acceptor-substituted alkene moiety tethered to the molecule, the intermediate silyl enol ether may undergo an intramolecular [2-I-2] cycloaddition.The silyl-assisted addition of hydrogen halides to cyclopropanes is not restricted to ketones with carbonyl groups as activating function or iodide as nucleophile. Esters and other acid derivatives underwent similar reactions when treated with iodotrimethylsilane alone or in the presence of an additional catalyst such as mercury(II) or zinc(II) chloride.Subsequent treatment of the y-iodo ester with potassium carbonate in tetrahydrofuran gave the respective y-butyrolactones in good yield. 

If the [3-h 2]-cycloaddition reaction of a methylenecyclopropane is performed with a remote alkene or alkyne moiety within the same molecule, products 2 of the bicyclo[3.n.0]-type (n < 3) or the bicyclo-[n.3.0]-type (n > 3) can, in principle, be obtained. The former of these product types can be considered as unhkely, as an alkene or alkyne separated by only one or two atoms from the methylenecyclopropane would give rise to a strained annulated ring, i.e. a cyclopropane or a cyclobutane. It is known from investigations of photochemically initiated intramolecular [2 3-2] cycloadditions of alkenes that the minimum size required for a spacer between 7t-systems is three atoms in a reaction of a non-crossed type. There are actually a few examples of photoinduced intramolecular cycloadditions of the crossed type involving substrates with short spacers (n =... 

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