Stereochemical course, diene

Evidence for that stereochemical course comes from the rearrangement of meso-3,4-dimethylhexa-1,5-diene 4, which yields the E.Z-configured diene 5 almost quantitatively. With a transition state of boatlike geometry, a Z,Z- or E,E-configured product would be formed." ... 

Alkoxyallenes turned out to be excellent starting materials also for the synthesis of highly functionalized 1,3-dienes, two examples being depicted in Schemes 8.61 and 8.62. As described by Kantlehner et al., 1,3-dienes such as 238 were obtained from methoxyallene derivative 50 by condensation with CH-acidic compounds 237 [135]. Hoppe and co-workers explored the stereochemical course of the allene Claisen rearrangement under Johnson s conditions, e.g. the reaction of 239 with trimethyl orthoacetate, which furnished intermediate 240 followed by rearrangement to the isomeric dienes 241a,b [136],... 

The 1,3-diene moiety in 227 which included the carbon atoms and CVC was oxidized to the l,4-dihydroxy-2-ene moiety in 238 that was further exploited to functionalise the A-ring as well as for the annulation of the C-ring (Scheme 37). The transformation of 227 into 238 was realized by a diastereoselective epoxidation of 227 to afford a vinyl epoxide (241) that was subjected to the conditions for a Palladium(O)-catalysed allylic substitution with the acetate ion [126]. The mechanism and the stereochemical course of the allylic substitution may be explained as depicted in Scheme 37. Sn2 ring opening of the protonated vinyl epoxide 241 by an anionic Pd complex proceeded with a (3Si) topicity to the r-allyl Pd com-... 

Blechert et al. succeeded in intermolecular CM of terminal alkyne and terminal alkene. A reaction carried out in CH2CI2 at RT in the presence of 5-7mol% Ic gives a mixture of ( )- and (Z)-isomers (Table 2). Because of the nonselective stereochemical course, a silyl-protected ally alcohol is employed and the resulting metathesis product is deprotected and oxidized to afford the desired diene having an -configuration (Equation (13)). 

J. Jnrczalc, A. Gotfbiowski, and J. Raczko, Influence of the N-protecting group on the stereochemical course of [4+2]cycloaddition of l-ethoxy-3-[(trimethylsilyl)oxy] buta-l,3-diene to... 

Introduction of a silyl group, such as Me3Si and (t-Bu)Me2Si, on the oxygen of a propar-gylic alcohol181,182 or on vinylallenes183 affects the feasibility and the stereochemical course of intramolecular Diels-Alder reactions with an alkynyl diene, maleic anhydride or TV-methylmaleimide. A possible explanation involves steric effects. 

In the first attempt the tetracyclic intermediate (33) was prepared from 3,4-methylenedioxy-ta-nitrostyrene upon Diels-Alder addition of butadiene followed by zinc and hydrochloric acid reduction to an amine eventually converted into 33 by formaldehyde and hydrochloric acid. All other experiments, designed to anticipate the addition of a C6 diene (in order to introduce at once also the C2 carbon unit for ring D formation) or of a four-carbon diene with different functionality, failed. The structure of 33 is based on spectroscopic data, on considerations on the accepted stereochemical courses of this type of... 

Reactions of 32a with acyclic dienes (butadiene and piperylene) yielded the cyclohexadienes derived from desulfinylation of the resulting adducts, precluding the deduction of any information about the stereochemical course of these reactions [45]. From the results of the reaction of racemic sulfoxide 34 with the acyclic diene 35, Alexandre et al. [48] were able to isolate and characterize bicy-clic adduct 36 as the only adduct because its desulfinylation required 30 min in CH2C12 under reflux (Scheme 17). This result proved that the exo-selectivity of these reactions with acyclic dienes is much higher than that with cyclic dienes. Moreover, this paper shows that the reactivity of 3-sulfinyl acrylates is lower than that of the 2-sulfinyl derivatives, because the former are not able to react with 35. 

In the original paper there is no discussion of the stereochemical course of this reaction. Nevertheless, the stereochemistry of the major adduct exo(f)-48 suggested that the favored approach of the diene must take place toward the less hindered face of dienophile, with the sulfoxide adopting the spatial arrangement of conformation 47A (Scheme 24), which would be expected to exhibit a similar 7r-facial selectivity to that of the rotamers with the sulfinyl oxygen in an s-trans arrangement. This behavior is similar to that observed for 44 (see Fig. 4)... 

Pradilla et al. [140] have recently produced a nice paper showing that enan-tiopure hydroxy 2-p-tolylsulfinyl butadienes 158 (Scheme 78) undergo a highly face-selective Diels-Alder cycloaddition with AT-phenyl maleimide and phenyl-triazolidine dione, presumably controlled by the chiral sulfur atom (dienophile approach from the upper face of diene). Complementary 7r-facial selectivity (dienophile approach from the bottom face of diene) is displayed by related enantiopure sulfonyldienes 158 (Scheme 78). The authors suggest that the behavior of 158 is a consequence of the predominant influence of the chiral sulfur with respect to the hydroxylic carbon (the only chiral center in 158 ) on the stereochemical course of the cycloadditions. According to their explanation, dienes will adopt conformations similar to those depicted in Scheme 78, with the chiral centers employing their stereochemistry to maximum effect due to 1,3-allylic strain (which is considered as the main directing effect of these cycload-... 

From the absolute configuration of the products, determined by X-ray diffraction, the stereochemical course of the cycloaddition can be inferred for the (Z)-isomer the diene approaches the plane of the thiocarbonyl 5-oxide group from the CRe,Ssi face which is the least hindered face in the conformation shown here85. 

We noted previously that photochemical electrocyclic reactions take a different stereochemical course than their thermal counterparts, and we can now explain this difference. Ultraviolet irradiation of a polyene causes an excitation of one electron from the ground-state HOMO to the ground-state LUMO. For example, irradiation of a conjugated diene excites an electron from il/2 to and irradiation of a conjugated triene excites an electron from i/f j to (/ 4 (Figure 30.6). 

The above powerful effect has been shown to have its origin in the superior orbital overlap in 4a-like TS wherein the breaking ctc n bonds are aligned parallel to the orbital axes of the cyclopropane ring (dashed lines in 4a). In the alternate 4b-like TS, orbital overlap is unsatisfactory because the axes of the involved orbitals are essentially perpendicular. Thus, the cyclopropane has synthetically compelled the choice of one pathway and controlled the stereochemical course of the reaction at the sites of the two newly generated it bonds. Such a feature was observed from the cyclobutane derivative 7 as well as it fragmented to the .y, .y-diene 8 with a stereospecificity that was too high to measure. However, the reaction 7 8 was... 

The results for the rearrangement of 36 are contrasted by the finding8 4 that mevo-divinyl derivatives such as 39 should cleanly lead to ( , )-dienes. However, this result also differs from the stereochemical course usually observed in Cope rearrangements of mevo-dienes. Therefore, it seems more likely that in this case an epimerization prior to rearrangement may occur. 

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