Twisted Diels-Alder reaction

Interestingly, we were intrigued by the ESI mass spectrum of the compound, as the observed base peak consisted of [M-S02+Na]+. This led us to explore a thermal retro-Diels-Alder reaction that could afford the desired enone 69. It is noteworthy that the chemistry of cyclic enol-sulfites would appear to be an under-explored area with a few references reporting their isolation being found [57]. At last, we were also able to prepare epoxy ketone 70 from 69 in three steps, albeit epoxidation did not take place unless the TES group was removed. Spartan models reaffirmed our initial conformational assessment of enone 69 and epoxy ketone 70, which contain sp3-hybridized C8a and s/r-hybridized C8b (p s e u d o-. v/r - h y b r i d i zed C8b for 70) at the AB-ring junction (Fig. 8.12) and displayed the desired twisted-boat conformation in A-ring. 

DFT calculations on the intramolecular Diels-Alder reaction of penta-l,3-dienyl acrylates predict stereoselectivities that are in good agreement with the experimen- (g) tal results.85 Another DFT study at the B3LYP/6-31G(d) level of the intramolecular Diels-Alder cycloaddition of 5-vinylcyclohexa-1,3-dienes has been reported. Reaction rates are influenced by dienophile twisting and substituent effects.86 The intramolecular dehydro-Diels-Alder reactions of ynamides (79) provides a new synthesis of benzo[fc]-, tetrahydrobenzo[fc]-, naphtho[l,2-/j -, and dibenzo[a,c]carbazoles... 

The increased stereoselectivity of decatrien-3-ones relative to the n(matrien-3-ones may also be the consequence of cooperativity between twist asynchronicity and endo stabilization that both favor the c -fused product. The decatrien-3-one transitirm state is considerably less strained than the nonatrien-3-one transition state, and the uncatalyzed decatrienone cyclizations occur at or near ambient temperature. Consequently, one expects greater endo stabilization, and hence also greater cis stereoselectivity, in the decatrienone intramolecular Diels-Alder reactions. 

In the attempted synthesis of twisted polycycle 1,2,3,4-tetraphenylfluorenoM, 9-g/i]quinoline, R.A. Pascal Jr. et al. used the Combes quinoline synthesis to assemble the azaaceanthrene core. Oxidation with DDQ was followed by a Diels-Alder reaction with tetracyclone (tetraphenylcyclopentadienone) to afford the corresponding cycloadduct. However, the last decarbonylation step of the sequence failed to work even under forcing conditions, presumably due to steric hindrance. 

May and coworkers proposed the plausible biosynthetic root to communesin A (70) shown in Fig. (12) (part 1) [50]. A quinone methide (75) derived from tryptamine and the related natural product, aurantioclavine (76), undergo a Diels-Alder reaction to form a polycyclic intermediate (77). This highly twisted lactam (77) should be easily cleaved by the residual primary amine to produce spiro lactam (78). Reduction of 78 and aminal closure afford a common intermediate (79) of communesins. Epoxidation and acylation of 79 afford 70. Expansion... 

A new approach to determination of partial rate constants for reactions of conformers has been developed on the basis of constituents of the Gibbs energy of activation for reactions of a series of conformationally heterogeneous substrates. The proposed model allows solving of formal kinetic tasks under conditions of thermodynamic control and in the absence of diastereoisomeric products. The p-values and partial rate constants have been determined for the chair and twist conformers of a series of 2-substituted l,3-dioxacyclohept-5-enes in the model Diels-Alder reaction with l,2,4>5-tetrazine-3,6-dicarboxylate in two solvents. In dioxane, the chair conformer reacts 3.4 times faster and, in acetone, 1.4 times faster than does the twist conformer <1996RJC477>. 

The twisted alkene is unstable and rotates to the much more stable cis alkene even at 20 °C. It can rotate because the overlap between the p orbitals is weak as they are not parallel. Trapping in a Diels-Alder reaction preserves the trans stereochemistry. 

Another example of a Diels-Alder reaction of benzoquinone 24 with a substituted cyclopentadienone 23 is the synthesis of a molecular ribbon (Scheme 7) [26]. The compound is twisted in two directions and does not adopt a single longitudinal twist as originally intended. Again, no route for the aromatization has been described. 

Jorgensenandcoworkers examined chiral bisoxazoline/copper(Il)-substrate complexes (substrate = glyoxal or methyl glyoxylate) in regard to hetero Diels-Alder reactions carried out in their labs [132] Ab initio calculations showed a 17-electron bisoxazoline/copper(II)-substrate complex to be most stable and most reactive. Furthermore, the optimized 17-electron bisoxazoline/copper(II)-substrate complexes of t-Bu-BOX (13) and Ph-BOX (15) showed that the substrate molecule was twisted 40-45° out the plane of the ligand-copper(II) plane. This observation is in good agreement with X-ray structures solved fcc the complexes of interest. Another report by Alvarez and coworkers focused on quantitative chirality analysis of molecidar subunits of spirocyclic bisoxazoline/copper(ll) complexes [133]. 

The highly twisted Ti-conjugated macrocycle (184) with two allq nyl moieties underwent intramolecular [2 + 2] photocycloaddition to give the thiophene-fused bisdehydro[12]annulene (185) in 61% yield. Photolysis of the bis(dithienyl)ethynes (186) in the presence of iodine resulted in sequential electrophilic and photochemical cyclizations to yield the tetrathienonaphthalenes (183) in one pot reactions. The compounds (187) showed a significant potential as a cruciform scaffold for nanostructured Ji-electron materials. The alkenyl-substituted bis(dithienyl)ethynes (188) underwent p-benzoquinone photosensitized double 5-exo-cyclization to give the diarylated dithienofulvalenes (189). The atylpyridinylethynes (190) and (191) in aqueous HCl solution underwent photo-dehydro-Diels-Alder reaction to afford... 

Repeat your analysis for Z,Z-hexa-2,4-diene, and again calculate the energy to twist the diene into the same conformation as seen in the Diels-Alder transition state (Z,Z-hexa-2,4-diene+TCNE). Compare the two twisting energies , and rationalize the observed relative rates for the two cycloaddition reactions. 

It is quite likely that 2H- thiin is very similar in structure to the dioxide though perhaps with a widened C(6)—S(l)—C(2) angle and consequently less twisted dialkene this is supported by the Diels-Alder reactivity it displays (albeit with difficulty), while the sulfone (34) is inert to similar reactions. Structures (36) and (37) are tentatively suggested as possible pseudo chair and pseudo boat conformations for the AH-thiin system. 

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