Cyclization 2+2 conrotatory

An example of preferred conrotatory cyclization of four-7c-electron pentadienyl cation systems can be found in the acid-catalyzed cyclization of the dienone 12, which proceeds through the 3-hydroxypentadienyl cation 13. The stereochemistry is that expected for a conrotatory process. 

When colorless crystals of rac-s-trans-3,8-di-tert-butyl-l,5,6,10-tetraphenyl-deca-3,4,6,7-tetraene-l,9-diyne (123) were heated at 140 °C for 2 h, the ben-zodicylobutadiene derivative (126) was produced as green crystals. As shown in the sequence (Scheme 20), 123 is first isomerized to its s-ds-isomer (124), and intramolecular thermal reaction of the two allene moieties through a [2+2] conrotatory cyclization gives the intermediate 125, which upon further thermal reaction between acetylene moieties gives the final product 126 [19,22].This is another example of the crystal-to-crystal reaction. 

Electrocyclization of 1,4-dienes is an efficient process when a heteroatom with a lone pair of electrons is placed in the 3-position, as in 77 (Scheme 20)38. Photoexcitation of these systems generally results in efficient formation of a C—C bond via 6e conrotatory cyclization to afford the ylide 78. These reactive intermediates can undergo a variety of processes, including H-transfer (via a suprafacial 1,4-H transfer) to 79 or oxidation to 80. In a spectacular example of reaction, and the potential it holds for complex molecule synthesis, Dittami and coworkers found that the zwitterion formed by photolysis of divinyl ether 81 could be efficiently trapped in an intramolecular [3 + 2] cycloaddition by the... 

The predicted conrotatory cyclization of octatetraenes was confirmed for the case of the methyl-substituted compounds, which above 16 °C readily formed the cyclooctatrienes shown in equations 13 and 14)14. We conclude this section with an electrocyclic reaction involving ten TT-electrons, that is, the formation of azulene (17) when the fulvene 16 is heated (equation 15)15,16. 

Tius and co-workers elegantly applied a variant of the Nazarov reaction to the preparation of cyclopentenone prostaglandins (Scheme 19.39) [46]. Moreover, it was demonstrated that the chirality of non-racemic allenes is transferred to an sp3-hybridized carbon atom. Preparation of allenic morpholinoamide 214 and resolution of the enantiomers by chiral HPLC provided (-)- and (+)-214. Compound (-)-214 was exposed to the vinyllithium species 215 to afford a presumed intermediate which was not observed but spontaneously cyclized to give (+)- and (—)-216 as a 5 1 mixture. Compound (+)-216 was obtained with an 84% transfer of chiral information and (-)-216 was obtained in 64% ee. The lower enantiomeric excess of (—)-216 indicates that some Z to E isomerization took place. This was validated by the conversion of 216 to 217, where the absolute configuration was established. The stereochemical outcome of this reaction has been explained by conrotatory cyclization of 218 in which the distal group on the allene rotates away from the alkene to give 216. 

Reactions.—Nucleophilic Attack at Carbon. (/) Carbonyls. Methyl arylglyoxylates react with trisdimethylaminophosphine (TDAP) to form m-a/S-dimethoxycarbonyl-stilbene oxides.63 The initially formed zwitterion (61) reacts with a second molecule of the ester to form a fra/ -diphenyl-1,4,2-dioxaphospholan intermediate, which undergoes a concerted symmetry-allowed retrograde n2s + 4 cycloaddition to give a carbonyl ylide, conrotatory cyclization of which leads to the cw-oxirans (62) (Scheme 3). 

Now the 1,3-cyclopentanediyl diradical is constrained to cychze in a disrotatory fashion while the trimethylene species might well close in both disrotatory and con-rotatory ways. Were all other factors constant one could infer that the geometrical restrictions imposed on the 1,3-cyclopentanediyl diradical entailed no significant deduction in rate of cyclization, and thus that conrotatory cyclization of the trimethylene diradical is not strongly preferred under the given reaction conditions and circumstances. 

Cis-stilbene (Zl) also undergoes a conrotatory cyclization reaction into trans-4a,4b-dihydrophenanthrene (DHP, 2), a short living, not isolated product with an absorption in the visible spectrum at 450 nm. In the absence of an oxidizing agent DHP will return to the starting material, by both a thermal and a photochemical ring opening reaction. 

The photocyclization (Scheme 87) is metal ion dependent, and those with low-lying metal d states are not suitable because of quenching of the excited state.250,256 257 This reaction may be explained as a photoinduced 1,16-sigmatropic H shift L,2S + s16a] followed by an 18m conrotatory cyclization... 

Keywords frarcs-diallene, [2+2]conrotatory cyclization, dimethylenecyclobutene... 

Sordo et al. [144] explained the stereoselectivity on the basis of torquoelectronic effects. Low-temperature infrared spectroscopy was also used to identify the reactive intermediates [145]. Two mechanisms were proposed to explain the product distribution in the (3-lactam formation reaction. The ketene mechanism was observed in a low temperature infrared spectroscopy study [145], while the acylation of imine mechanism was believed to be involved in some [122]. Both mechanisms were supported by evidences. It had been hypothesized that cycloaddition of the imine occurs from the least hindered side of the ketene, and this process generates zwitterionic intermediates conrotatory cyclization of these intermediates then produce cis- and //Y/ .v-[S-lactanis. Acylation of the imine by the acid chloride to form /V-acyliminium chloride also produced zwitterionic intermediates (Scheme 10). 

The mechanism of this novel photochemical rearrangement involves a photochemical conrotatory cyclization, a novel 1,9-hydrogen shift, lateral ring opening, and finally photochemical conversion of the dienol ether to the conjugated ketone (Scheme 22). 

Trialkylsilyl vinyl ketenes (72) have been shown to react stereoselectively with a-benzotriazolyl organolithium species to give highly substituted cyclopentenones. The selectivity was found to be kinetic, not thermodynamic, in origin. Several possible mechanisms have been proposed (Scheme 10). It has been suggested that the observed stereoselectivity may result from torquoselectivity in a concerted reaction, or from stereospecific conrotatory cyclization of cation (73), formed stereoselectively because of the interaction shown between the electron-withdrawing group Z and the metal ion.75... 

With this model, we need only apply the method already used to derive the selection rules for electrocyclic reactions (p. 53). From the Coulson equations, we can deduce that in the in conrotatory cyclization of pentadiene, the MO generates a destabilizing C5-C4 secondary interaction, a stabilizing and Fg a destabilizing interaction. The absolute values of these contributions rise steadily because the terminal coefficients increase from Fg to Fg. Therefore, the sign of their sum is given by the HOMO contribution. If R is an attractor, the HOMO is Fg and rotation inwards is favored. If R is a donor, the HOMO is 4T and rotation inwards is disfavored. As the Coulson equations are valid only for polyenes, these conclusions are correct insofar as R can be modeled by a carbon 2p orbital. It follows that the Rondan-Houk theory works better for conjugative than for saturated substituents. 

When refluxed for 15 h in xylene, aryl-substituted /V-(trimethylsilylmethyl)imines rearrange to /V-tri methyl si I y lazi ridines with high c/s-selectivity. It has been demonstrated that the reaction proceeds via an ylid that can be trapped with diethylacetylene-dicarboxylate (DEAD), for example. The more stable frans-geometry of the ylid and conrotatory cyclization would be responsible for the c/.v-aziridines. The authors note that no reaction occurs when the reaction is run in a polar solvent such as THF or acetonitrile, and that treatment of the imine with cesium fluoride in THF leads to protodesilylation only, as does 1 equiv. of water in HMPA.329... 

When the reactant molecules have built-in restrictions, certain alternatives are precluded. A conrotatory cyclization is thermally allowed for butadiene but is not possible in cyclopentadiene, because it would lead to a trans ring junction photochemical excitation must... 

Various types of enamides are now shown to undergo ready photocyclization according to a mechanism of a six -electron conrotatory cyclization via a trans cyclic intermediate B, from which the photocyclized product is formed depending on the reaction condition, either oxidative or nonoxidative. 

Photochemical conversion of stilbenes to phenanthrenes via a six 7t-elec-tron conrotatory cyclization according to an electrocyclic mechanism to the dihydrophenanthrenes and subsequent dehydrogenation is a very famous and useful synthetic reaction (103). 

The conrotatory cyclization of fl//-cw-deca-2,4,6,8-tetraene to traw-7,8-dimethyl-cycloocta-l,3,5-triene has been studied in solvents of different polarity [157]. In agreement with a synchronous eonrotatory ring closure via an isopolar activated complex, the solvent effect is negligible as shown by the relative first-order rate constants in Eq. (5-52). 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

There is ample evidence that these cycloadditions involve the initial formation of a dipolar intermediate (5 Scheme 6). Conrotatory cyclization of (5) leads to the 3-lactam. Intermediates of type (5) can be trapped by another molecule of ketene to yield (d) or by sulfur dioxide to yield sulfone (7). This mechanism also explains the formation of [4 -I- 2] adducts (8) which are sometimes observed with conjugated imines. ... 

The HOMO for butadiene is the conrotatory cyclizations shown here,... 

Photolysis of a-cyanostilbene oxides has been examined in cydoaddition reactions with the dipolarophile dimethyl fumarate. The stereochemistry of the tetra-hydrofuran adduct prepared from the ylides formed in the disrotatory electro-cyclic ring-opening predicted by Woodward and Hoffman points to cis-trans isomerization of the carbonyl-ylide, probably via conrotatory cyclization. 

The addition of an imine to an alkene provides a zwitterion, which subsequently undergoes a conrotatory cyclization to give the four-membered ring. The configuration of the imine determines... 

The formation of the cwdo-ir-allyl structure (XIX), can be explained by trans attack of alcohol followed by a thermally allowed conrotatory cyclization step (X = OR) 183) ... 

As is already obvious from Scheme 23, another potential route to phosphiranes involves the cyclization of bis-ylides. The thermal cyclization of such species has been studied in some depth by Niecke and co-workers <92CB77l> (Equation (71)). The conrotatory cyclization does not work for highly hindered bis-ylides e.g., R1 = Et2N, R2 = R3 = Ph, R4 = R5 = TMS. Some time earlier, Fluck and Braun <86CZ379,89PS(44)29i> had also described some applications of this scheme, e.g., Equation (72). Finally, a similar chemistry has been described by Kolodyazhnyi and Ustenko <92ZOB464, 92ZOB1418) (Schemes 32 and 33). 

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