Cyclobutane alkenyl

Radialenes represent the biggest and best known subset of the radialene family this is not surprising in view of the fact that more methods to prepare them exist than for any other class of radialenes. The major strategies are the transformation of appropriate cyclobutane derivatives, the thermal or Ni(0)-catalyzed cyclodimerization of butatrienes or higher cumulenes and the cyclotetramerization of (l-bromo-l-alkenyl)cuprates. 

The Lewis acid catalyst 53 is now referred to as the Narasaka catalyst. This catalyst can be generated in situ from the reaction of dichlorodiisopropoxy-titanium and a diol chiral ligand derived from tartaric acid. This compound can also catalyze [2+2] cycloaddition reactions with high enantioselectivity. For example, as depicted in Scheme 5-20, in the reaction of alkenes bearing al-kylthio groups (ketene dithioacetals, alkenyl sulfides, and alkynyl sulfides) with electron-deficient olefins, the corresponding cyclobutane or methylenecyclobu-tene derivatives can be obtained in high enantiomeric excess.18... 

Recently it has been shown that radical anionic cyclization of olefinic enones effectively compete with intramolecular [2 -I- 2]-cycloaddition to form spirocy-clic compounds [205, 206], 3-Alkenyloxy- and 3-alkenyl-2-cyclohexenones 235 are irradiated in the presence of triethylamine. As depicted in Scheme 46 two reaction pathways may operate. Both involve electron transfer steps, either to the starting material (resulting in a direct cyclization) or to the preformed cyclobutane derivative 239, which undergoes reductive cleavage. The second... 

Chromium and tungsten carbene complexes containing an alkynyl or alkenyl substituent afford moderate to high yields of cyclobutene or cyclobutane complexes respectively via formal [2 - - 2]cycloaddition with... 

Pt(II) to the alkyne of the substrate likely triggers all these events. The cycloisomerization might undergo a metallacyclic intermediate that proceeds to eliminate /3-H. The formation of cyclopropanes is presumably succeeded via alkenyl platinum carbene followed by platina(IV)cyclobutane intermediates. The extension using formal metathesis of the enynes includes two transformations, the formation of 1,3-diene moieties and the stereoselective tetrasubstituted aUcene derivatives via O C allyl shift, both leading to diverse structural motifs and serving as the key step in the total synthesis of bioactive targets (Scheme 83). 

An exciting recent development has been the discovery that titanocene-mediated reactions of oo-carbonyl epoxides [36] and o>alkenyl epoxides [37] can produce functionalised cyclopropane and cyclobutane derivatives. For example, Fernandez-Mateos and co-workers discovered that treatment of... 

Alkenes. Titanocene bis(triethyl phosphite), which is prepared in situ from titanocene dichloride, triethylphosphite, and Mg, promotes carbonyl olefmation with gem-dichlorides and dithioacetals [e.g., l,l-bis(phenylthio)cyclobutane ] including those derived from enals (to give 1,3-dienes). Enol ethers-and alkenyl sulfides are obtained in the analogous reaction with dithioorthoformates and trithioorthoformates. Cross-coupling of dithioacetal and thiolesters furnishes predominantly (Z)-alkenyl sulfides. ... 

The reaction of a-lithioalkylselenoxides, at —78 with 2,2-disubstituted cyclobu-tanones affords55) the corresponding P-alkoxy selenoxides, which, as expected S5), produce I-(l-hydroxy)-l-(r-alkenyl) cyclobutanes after sequential acid hydrolysis... 

Photochemical [2+2] cycloaddition reactions and [2+2] cycloaddition reactions of ketenes have been widely used for the preparation of cyclobutane derivatives. The thermal [2+2] cycloaddition reaction is known to proceed between highly electrophilic and nucleophilic alkenes alkenes having cyano, fluoro, and trif-luoromethyl groups react with electron-rich alkenes such as alkenyl ethers and sulfides [8]. As for the catalyst-mediated [2+2] cycloaddition reactions, Lewis acids are known to promote [2+2] cycloadditions [9,10,11,12,13,14,15,16,17,... 

However, the apphcability of this cycloaddition is rather limited because of the side reactions such as ene reactions, conjugate addition reactions, and ring opening reactions of the produced cyclobutane derivatives. There was no general Lewis acid-catalyzed [2+2] cycloaddition reaction, until Takeda [25] and Narasaka et al. [26,27,28,29,30,31,32,33] found that alkenyl sulfides react with a wide variety of electron-deficient olefins in the presence of a Lewis acid. 

Alkenyl sulfides are known to react with some labile electron-deficient olefins such as methyl vinyl ketone in the presence of AlCl3 to form cyclobutanes [25]. In the present chiral titanium-promoted asymmetric reaction, alkenyl sulfides can also be employed as electron-rich components. 2-Ethylthio-l-propene (7a) reacts with 2a in the presence of a catalytic amount of the chiral titanium reagent, giving the diastereomeric [2-1-2] cycloaddition products 8a and 9a in 51% (>98% ee) and 19% (79% ee) yields, respectively (Scheme 7 and Table 2). Although 2-ethylthio-l-propene (7a) is known as a good ene component in the reaction with carbonyl compounds, 3-(3-(methoxycarbonyl)-5-ethylthio-5-hex-enoyl)-l,3-oxazolidin-2-one, an ene product, is obtained only in 16% yield as a side product. 

Abstract The integration of conservation of orbital symmetry and the orbital overlap effect serves as a powerful tool to reliably predict the stereochemical course of pericyclic reactions as exemplified in this chapter. The orbital overlap factor has been discussed with a variety of examples such as the thermal fragmentations of cyclopropanated and cyclobutanated r .v-3,6-dimethyl-3,6-dihydropyridazine, and [1,5] sigmatropic shifts in c/.v-2-alkenyl-1 -alkylcyclopropanes and civ-2-alkenyl-1 -alkylcyclobutanes. 

Stereogenic centers on the tether may induce high selectivities, especially when these centers also belong to the starting enone or to a cycloalkenyl chain. The high stereocontrol observed results in an important discrimination between the transition states involved in the cyclization step. When the alkenyl chain is linked to the cyclenone at an asymmetric carbon, the approach to excited enone is restricted. Here, the cyclobutane and the tether usually have relative cis stereochemistry in the product [123-130] (Scheme 20). 

The first catalytic asymmetric version of [2 + 2] cycloaddition reaction was realized by Narasaka in 1989 using chiral titanium catalyst derived from TADDOL. It was found that the reaction of ketene dithioacetal with acryloyloxazolidinone derivatives proceeded smoothly in the presence of 10 mol% of TADDOL-TiCl2 to give the cyclobutane derivatives in high yields (64-96%) and good to excellent enantioselectivities (80-98%) [183]. The reaction is presumed to proceed via a carbonyl substrate chelated TADDOL-TiCb intermediate although the exact reaction mechanism is unclear. Moreover, alkynyl, alkenyl, and 1,2-propadienyl... 

Self-sensitized photooxygenation of Cgo-fused cyclobutanes containing an oxidizable alkene moiety" or alkenyl-substituted fullerene adducts followed by reduction also results in the formation of allylic alcohols. Whereas in the former case formation of both the three and erythro forms of the allylic alocohols is observed, in the latter case the reaction occurs regioselectively by the preferential abstraction of an allylic hydrogen next to the fullerene group. The hydrogen atom linked to the fullerene skeleton is unreactive under the reaction conditions. 

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