Dienes pericyclic rearrangement

Starting with an indole-containing Zincke aldehyde substrate 91, the above authors [39] were able to effect a similar pericyclic rearrangement, followed by an intramolecular cycloaddition reaction of the resulting diene-amide 92, thus accessing a tetracyclic product 93 which could possibly serve as a key precursor in the synthesis of indole alkaloid analogs (Scheme 26). 

Despite efforts made to improve enantioselectivities by supporting TADDOL-titanium catalyst, the homogeneous conditions remain the best ones. BINOL was also reported as an efficient ligand, however, to our knowledge supported-BINOL-titanium complexes were not studied in the asymmetric Diels-Alder reactions. In contrast, BINOL was extensively studied in the hetero-Diels-Alder reaction and in particular in the pericyclic rearrangement of several aldehydes and electron-rich and highly regioselective Danishefslqr s diene (84) that provide access to 2-substituted-2,3-dihydro-4/f-pyran-4-ones 85 (Scheme 7.50). 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

These reactions, called electrocyclic rearrangements, take place by pericyclic mechanisms. The evidence comes from stereochemical studies, which show a remarkable stereospecificity whose direction depends on whether the reaction is induced by heat or light. For example, it was found for the thermal reaction that cis-3,4-dimethylcyclobutene gave only cw,tran5-2,4-hexadiene, while the trans isomer gave only the trans-trans diene... 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

Selenium dioxide is also an oxygen donor to alkenes. In this case, however, the initial reaction of the double bond is with the selenium center followed by two pericyclic steps. After hydrolysis of the organo-selenium intermediate, the result is a hydroxylation at the allylic carbon position65. Thus, limonene (2) yields racemic p-mentha-l,8(9)-dien-4-ol66. The high toxicity of selenium intermediates and prevalence of many rearrangements has limited the widespread use of the reagent in synthesis. 

It has been established that the course of the sequential pericyclic reaction of cyclopentadienones with acyclic conjugated alkadienes depends on the reaction temperature, thermal treatment at low temperatures affording 3a,4,7,7a-tetrahydroinden-l-one derivatives by way of a Cope rearrangement (see Scheme 38). Roman et al have developed an efficient stereoselective synthesis of enantiomerically pure i-nitrotricyclo[5.2.2.0 ]undeca-3,8-dienes via a tandem consecutive asymmetric Diels-Alder-Cope rearrangement (see Scheme 39). Adducts... 

Generally, when 1,5-dienes are heated, they isomerize in a [3,3]-sigmatropic rearrangement known as a Cope rearrangement17 (not to be confused with the Cope elimination process). The mechanism is a simple six-membered pericyclic process, the chair form being the usual transition state. However, in this case the cyclopropyl moiety forces the geometry of the transition state into boat form 28. 

The ability of the PC double bond within tetraphosphahexadienes to participate in pericyclic reactions initiated interest in the synthesis of additional phosphahexadienes and studies of their properties. Attempts to synthesize 1,6-diphosphahexadiene a via a [1,1 ] Cope rearrangement out of 3,4-diphosphahexa-l,5-diene, derived from l,2-dipotassium-l,2-diphenyl diphosphide and vinylbromide, were not successful when carried out up to the temperature of decomposition (above 120°C). 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

Three enantiomerically pure starting materials ensure remote stereochemical control Tandem iminium ion formation and vinyl silane cyclisation Part IV - Tandem Pericyclic Reactions Electrocyclic Formation of a Diene for Diels-Alder Reaction Tandem Ene Reactions Tandem [3,3]-Sigmatropic Rearrangements Tandem Aza-Diels-Alder and Aza-Ene Reactions... 

Steinhardt SE, Silverston JS et al (2008) StereocontroUed synthesis of Z-dienes via an unexpected pericyclic cascade rearrangement of 5-amino-2, 4-pentadienals. J Am Chem Soc 130 7560-7561... 

The hetero Diels-Alder reaction has also been carried out efficiently in water. For instance, glyoxylic acid undergoes cycloadditions with various dienes [20] although the carbonyl function is almost exclusively present as its hydrate form. Other pericyclic reactions such as 1,3 dipolar [21] or [4+3] cycloadditions [Eq. (1), Table 1], and Claisen rearrangement [22] gave better results when conducted in aqueous media than in organic solvents. 

The Cope Rearrangement A Pericyclic Reaction (Section 20.6B) The Cope rearrangement converts a 1,5-diene to give an isomeric 1,5-diene. The reaction takes place in a single step and involves the redistribution of six electrons in a cyclic transition state. 

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