Cyclopentenes carbonyl compounds

Simple sugar dehydration/fragmentation products furans, pyrones, cyclopentenes, carbonyl compounds, acids... 

The McMurry reaction (the reductive dimerization of carbonyl compounds giving olefins after treatment with the low-valent titanium reagents TiCla/Mg and TiCla/Zn in THF at 40°C) (CR891513) is a key step in the synthesis of photochromic cyclopentene derivatives. Later, it has been shown that TiCU, which is easier to handle than TiCls, can be used. The cyclization products are formed in 50-60% yields (98S1092,03EJO155). The advantages of this method are that the reactions can be scaled and inexpensive starting materials can be used. 

The alternative to this 0,0-acetal formation is the sequence of addition and El reaction. As a matter of fact, this is familiar from the transformation of alcohols with carbonyl compounds, but only occurs in some (very rare) cases. This is illustrated by Figure 9.31 using acid-catalyzed transformations of ethanol with two /3-diketones as an example. Here, enol ethers, namely 3-ethoxy-2-cyclopentene-l-one and 3-ethoxy-2-cyclohexene-l-one, respectively, are... 

A procedure for the large-scale conversion of alkenes to unsaturated carbonyl compounds using singlet oxygen has been published, whereby the conversion of cyclopentene to cyclopentenone can be carried out on a molar scale in 60% yield. 

Flowers et al. have dealt with the thermal gas-phase reactions of methyl-oxirane, other methyl-substituted oxiranes, and ethyloxirane. The kinetics of the processes have been compared. Pyrolysis of these compounds is a first-order, homogeneous, nonradical process the reaction rate is not affected by radical scavengers. A biradical mechanism holds. The thermochemical behavior of cyclopentene oxide and cyclohexene oxide is similar. The primary products are the corresponding carbonyl compounds and unsaturated alcohols. Two mechanistic possibilities have been discussed they are obtained from a common biradical intermediate or the alcohol is formed directly from the oxirane in a concerted manner. Thermolysis of spirooxiranes leads to ketone derivatives via biradicals with homolytic bond cleavage (Eqs. 376, 377). ... 

The reaction of allenylsilanes with electron deficient ir-systems constitutes a powerful and general method for the tegio- and stereo-selective preparation of substituted cyclopentenes. As first reported, (trimethylsilyl)allenes function as three-carbon nucleophiles in TiCU-promoted (trimethylsilyl)cyclopen-tene annelations (Scheme 31). The annelation process involves the polarization of an a,p-unsaturated carbonyl compound by TUTU to generate an alkoxyallylic cation. Regiospecific electnqrhilic substitution of this cation at C-3 of the allenylsilane produces a silicon-stablizied vinyl cation. A 1,2-shift of the tri-... 

It is not always necessary to start with gem-dihalides for the synthesis of aldehydes or ketones. There is a process by which carbonyl compounds can be obtained from monohalides, sometimes in excellent yield for instance, when 3-chlorocyclopentene is stirred vigorously with aqueous sodium dichromate solution at 0°, a chromium complex of 2-cyclopenten-l-one is formed, and this is decomposed to give a 60-68% yield of the ketone if 50% sulfuric acid is dropped in carefully with cooling 513 and 1,2-cyclopentanedione can be obtained in 80% yield by dropwise addition of aqueous iron(m) chloride solution to 2-chlorocyclopentanone in water with rapid stirring at 100°.539... 

Cyclopentenes. Zirconacyclopentadienes prepared from two alkynes react with CO in the presence of BuLi to give substituted cyclopentenones. With CuCl as catalyst, the reaction of zirconacyclopentadienes with p-iodo-a,p-unsaturated carbonyl compounds provides cyclopentadienes. In the cases of 3-iodo-2-cycloalkenones, the products are spirocycles. 

The reaction between lithio TMS-diazomethane and carbonyl compounds, which generates alkyUdenecarbenes, has been studied further. Aliphatic ketones give cyclopentene derivatives, while aldehydes and aryl alkyl ketones give alkynes. The diazoalkene intermediates from aliphatic ketones can be trapped with diisopropylamine, producing... 

A (trimethylsilyl)cyclopentene annulation method has been developed that leads to a regiocontrolled approach to the synthesis of five-membered rings/ Thus, it was shown that (trimethylsilyl)allenes could be persuaded to react at -78 C with unsaturated carbonyl compounds in the presence of titanium tetrachloride (Scheme 16). The methodology is presently being applied to polyquin-ane natural product synthesis. 

Synthesis of Five-Membered Carbocycles. 1-Substituted al-lenylsilanes react with Q ,/3-unsaturated carbonyl compounds in the presence of titanium tetrachloride to produce cyclopentenes. For example, carvone and 1-methyl-l-(trimethylsilyl)allene react smoothly to give a ds-fused bicyclic system (eq 7). ... 

Zinc and Mercury.—A remarkable new C—C bond-forming reaction involves a high-yield trimolecular condensation between alkyl halides, activated alkenes, and carbonyl compounds promoted by Zn in MeCN [equation (1)]. Cyclic bifunctional products are obtained when halogeno-ketones react under these conditions with acrylonitrile or methyl acrylate. Cyclopentene derivatives are... 

A second common reaction of sulphonium ylides is Michael addition to ajS-unsaturated carbonyl compounds, giving cyclopropanes. This reaction forms the basis of a cyclopentene synthesis starting from sulphonium allylide (14). Other recent examples include the preparation of spiro[cyclopropane-l,4-A -pyrazolin]-5 -one derivatives (15) and cyclopropyl-ulose (16). Un-... 

P,y-Unsaturated aldehydes are a family of carbonyl compounds that have been considered ODPM unreactive for many years. This conclusion was based on literature precedents that indicated that the normal photochemical reactivity of such compounds, both on direct and sensitized irradiation, was decarbonylation to the corresponding alkene. > f> Thus, compounds 5, 6, 7, and 8 are representative examples of a series of steroidal and cyclic P,y-unsaturated aldehydes, studied by Schaffner et al. about 20 years ago, that undergo decarbonylation on irradiation. Diirr et al. have studied the photochemistry of some aryl-substituted-2-cyclopentene-l-carbaldehydes 9. In this instance, ds-stilbene-type electrocy-clic cyclization and decarbonylation were the only reactions observed. Other authors have also reported the decarbonylation of the acyclic P,y-unsaturated aldehydes 10 and 11 on direct irradiation. 

This hypothesis is supported by Chauvin s report (33) on a catalyst derived from (CO)5W=C(OEt)C4H9. This highly stable carbene-W(O) compound does not display catalytic activity for cyclopentene monomer. When mixed in the dark with TiCl4, a slow evolution of 1 equivalent of CO occurs. Subsequent thermal or photochemical activation produces ah extremely efficient catalyst system. Chauvin demonstrated that a high conversion to polypentenamer is obtainable at a W/cyclopentene ratio of 10 li at 5°C. The role of TiCI4 is not well understood nevertheless, it promotes carbonyl displacement which appears to be essential. 

Since the pioneering work by Sarel and co-workers on the iron carbonyl promoted transformation of vinylcyclopropanes and related compounds [1], a variety of transition metal complexes have been examined to achieve effective activation of the vinylcyclopropane-cyclopentene rearrangement which usually requires pyrolytic conditions. These reactions have been applied to natural product synthesis in some cases and have already been reviewed in several excellent articles [2-4]. 

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