Cyclohexene system

One recent example of preferential [2+2] cycloaddition of dienes is the reaction of 2-siloxybutadienes with allenecarboxylates to afford cyclobutanes used for the preparation of very hindered cyclohexene systems [22]. 

The Diels-Alder reaction is an organic chemical reaction (specifically, a cycloaddition) between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can also proceed if the alkene is replaced by an alkyne moiety or even if some of the atoms... 

The hydrogens within the octahedral olefin-dihydride intermediate are transferred consecutively with overall cis addition, and the rate-determining step (k9) is olefin insertion to give the alkyl- hydride. Kinetic and thermodynamic parameters for nearly all the steps of Fig. 1 have been estimated for the cyclohexene system. Because the insertion reaction is generally believed to require a cis disposition of the hydride and olefin... 

Cycloalkenes Absorption of the internal double bond in the unstrained cyclohexene system is essentially the same as that of a cis isomer in an acyclic system. The C=C stretch vibration is coupled with the C—C stretching of the adjacent bonds. As the angle a... 

The large difference between the AAG = 5.7 kcal mol 1 found for the cyclohexene system 190/19181 and the AH = 34 kcal mol 1 for the similar ions 196/19720 point to drastic differences in the mode of stabilization of the transition state for the protonation in solution and the free silyl-substituted carbocation in the gas phase. 

Systematic investigation of the stereo- and regio-control of electrophilic additions to cyclohexene systems (steroids in particular) by neighbouring groups allowed formulating certain principles, which govern the reactivities toward representative electrophiles,... 

Since both oxidative splitting of the double bond and aldol condensation represent reliable and general reactions, their sequence serves as an efficient route for the transformation of readily available cyclohexene systems e.g. formed via the Diels-Alder reaction or Robinson annulation) into functionalized cyclopentene derivatives. This standard operational mode is extensively used in total syntheses. One of the numerous examples, the synthesis of helminthosporal 463, the sesquiterpenoid toxin of fungi, is shown in Scheme 2.150. In the initial phases of the synthesis, commercially available (—)-carvomenthone 464 was transformed into 465 via Michael reaction with methyl vinyl ketone to give 466 and subsequent intramolecular aldol condensation. 

In the cyclohexene system (9) carbon atoms i, 2, 3 and 6 are in one plane. Although normal axial and equatorial bonds are present at C(4> and C(s>, the bonds at C(g) and C(6) have less clearly defined character and are termed pseudo-axial or pseudo-equatorial [a and e in (9)]. The effects of conformation on reactions of unsaturated steroids are discussed in Chapters 3 and 4. 

Upon examination of the three fragments of Figure 8-4 as possible starting points for the structural elucidation of the unknown compound, it becomes clear that the isobutyl group is an attractive choice if, of course, additional connections can be made to carbons and protons elsewhere in the molecule. If such connections cannot be made, the methyl protons (S) of the putative cyclohexene system provide another possible starting point. 

Similar to our approach to the cyclohexene system, the next step in this structural determination is to confirm the structure of the third fragment. Unlike the former subunit, the latter is well defined by its COSY and ID spin-coupling data. Nevertheless, the HMBC data are checked for consistency. Hj displays two- and three-bond correlations with Cg and C, respectively, while C7 exhibits connectivities to Hg and the hydroxyl proton J. In addition, two-bond correlations are shown by Hg and Hp with Cg and C6, respectively. The middle carbon (8) exhibits the aforementioned connectivities and another (two-bond) connectivity from OHj. 

Additions to Cyclohexenones. Intramolecular additions in cyclohexene systems also provide access to natural products. Thus the photocyclization of enone (27) affords the adduct (28) which is suggested as a route to the nortaxane or taxane skeleton. Intramolecular reaction of the enone (29) affords two products (30, 70%) and (31, 14%). The identity of the major products was determined by X-ray crystallography and is the result of hemiacetalisation of the original (2-f-2)-adduct (32). The minor product (31) is formed from the major by a yie TTo-Aldol reaction. ... 

Tilidine synthesis is impressively simple. [112] The cyclohexene system is constructed by a Diels-Alder reaction of an enamine, derived from croton-aldehyde, and ethyl atropate. 

In all of these approaches to juvabione, a precursor of the cyclohexene ring is the key intermediate. In Schemes 10.4 and 10.5, an aromatic ring is the precursor of the cyclohexene system, and the reduction of the aromatic system is carried out fairly late in the synthesis. Schemes 10.6 and 10.9 use an optically active natural product, limonene, as the starting material. This provides a useful simplification of stereochemistry, particularly if the product is required in optically active form. The syntheses in Schemes 10.4,10.6, and 10.10 proceed to build up the side chain, step-by-step, from the key cyclic intermediate. In the syntheses in Schemes 10.7, 10.8, and 10.9, the side chain is almost in final form when it is attached to the cyclic intermediate. 

Diels-Alder cycloaddition is an organic chemical reaction between a conjugated diene and a substituted alkene to form a substituted cyclohexene system [62]. Polymer-supported chloroaluminates, poly(l-butyl-4-vinylpyridinium) chloroaluminate, poly(l-butyl-3-vinylimidazolium) chloroaluminate, and poly(l-ethyl-3-vinylimidazolium) chloroaluminate were synthesized, and their activities were tested for the Diels-Alder reaction of cydopentadiene with methyl methacrylate [63]. These polymer-supported heterogeneous catalysts exhibited activities comparable to their corresponding monomeric homogeneous analogs and could be reused with a moderate loss of initial activity up to five runs. 

JuzT jTs"voboda, V. Holub, R. Pick, J. Phase equihbria in the methanol - cyclohexene system Collect. Czech. Chem. 

The Diels-Alder reaction is defined as a [4-1-2] cycloaddition between a conjugated diene and a substituted dienophile (alkene or alkyne) to form a (hetero-)cyclohexene system. Based on the electronic effects of the substituent on the diene and dienophile, Diels-Alder reactions can be classified as normal electron-demand (electron-rich diene reacts with electron-deficient dienophile) or inverse electron-demand (iEDDA, electron-deficient diene reacts with electron-rich dienophile) reactions (Scheme la). In a normal electron-demand Diels-Alder reaction, the electron-deficient dienophile, typically a Michael acceptor, is likely to be attacked by endogenous nucleophiles such as free amino and thiol groups in vivo. For this reason, the use of this reaction in bioorthogonal chemistry apphcations poses a challenge. 

Curran proposed a vinylogous anomeric effect concept to explain the observed rate difference of the Ireland-Claisen rearrangements of dihydropyran 152 and cyclohexene system 155. Oxygen attached to Ce is proposed to stabilise the polar Ireland-Claisen intermediates 160-161 which contribute to a faster rate of reaction. ... 

Among the techniques for C—C bond constmction, the Diels—Alder reaction has attained a preeminent position. It is a conjugate addition reaction of conjugated dienes to substituted alkenes, commonly called dienophiles, to form a substimted cyclohexene system (Fig. 14.10). The simplest example is the reaction of 1,3-butadiene with ethene to produce cyclohexene. 

Diels-Alder reaction [1, 2] (Fig. 2.4) happens between conjugated dienes and double bond substrates to form substimted cyclohexene system. Even if the atoms in some positions are non-carbon heteroatoms, the reaction still can occur normally. The reaction was first discovered by Otto Paul Hermann Diels and Kurt Alder in 1928. They won the 1950 Nobel Prize for chemistry because of this discovery. 

Morisue, T. Noda, K. Ishida, K. Vapor-Uquid equihbria for the benzene - cyclohexene system. J. Chem. Eng. Jpn. 1973, 6, 355-357. 

---