Diene syntheses orientation

The total synthesis of palytoxin (1) is a landmark scientific achievement. It not only extended the frontiers of target-oriented synthesis in terms of the size and complexity of the molecules, but also led to new discoveries and developments in the areas of synthetic methodology and conformational analysis. Among the most useful synthetic developments to emerge from this synthesis include the refinement of the NiCh/CrC -mediated coupling reaction between iodoolefins and aldehydes, the improvements and modifications of Suzuki s palladium-catalyzed diene synthesis, and the synthesis of A-acyl vinylogous ureas. 

The structure and stereochemistry of the adducts formed in this reaction depend on the mutual orientation of the diene and dienophile which, in its turn, is determined by the form of the transition compound. According to modern ideas, the transition state in the diene synthesis is a cyclic complex in which the system of double bonds of the diene and the dienophile are located in parallel planes A and B (Fig. 3a) and the four reaction centers are present in one plane. 

PbOj anode, 40 155-156 oxygen evolution, 40 109-110 PCE, catalytic synthesis of, l,l,l-trifluoro-2,2-dischloroethane, 39 341-343 7t complex multicenter processes of norboma-diene, 18 373-395 PdfllO), CO oxidation, 37 262-266 CO titration curves, 37 264—266 kinetic model, 37 266 kinetic oscillations, 37 262-263 subsurface oxygen phase, 37 264—265 work function and reaction rate, 37 263-264 Pd (CO) formation, 39 155 PdjCrjCp fCOljPMe, 38 350-351 (J-PdH phase, Pd transformation, 37 79-80 P-dimensional subspace, 32 280-281 Pdf 111) mica film, epitaxially oriented, 37 55-56... 

Michael additions of organocopper reagents to acceptor-substituted dienes have found widespread application in target-oriented stereoselective synthesis [12]. For... 

Systematic studies of topochemical reactions of organic solids have led to the possibility of asymmetric synthesis via reactions in chiral crystals. (A chiral crystal is one whose symmetry elements do not interrelate enantiomers.) (Green et al, 1979 Addadi et al, 1980). This essentially involves two steps (i) synthesis of achiral molecules that crystallize in chiral structures with suitable packing and orientation of reactive groups and (ii) performing a topochemical reaction such that chirality of crystals is transferred to products. The first step is essentially a part of the more general problem of crystal engineering. An example of such a system where almost quantitative asymmetric induction is achieved is the family of unsymmetrically substituted dienes ... 

Dieb-Alder catalyst. The key step in a recent total synthesis of androstanes is a SnCVcatalyzed Diels-Alder reaction of 1 with the (Z)-dienophile 2. The geometry of the diene favors addition ami to the C,0-methyl group, and the catalyst promotes the desired enrfo-orientation. A1C1, and BF3 ctherate are less suitable for additions involving aliphatic bifunctional dienophiles. The initial adduct a can be isolated, but in only 15-20% yield. The synthesis of the androstane 4 is completed by ketalizatioh of 3 followed by a novel cyclization affected with dimsylsodium. ... 

In contrast to the sulfinyl acrylates, the behavior of the enantiomerically pure sulfinyl enones as dienophiles has been very little studied. The first report in this field was due to Maignan et al. [53], who described the synthesis of several sulfinyl enones and the reaction of 3-p-tolylsulfinyl butenone 47 with cyclopenta-diene. The reaction required 12 h at room temperature to reach completion, and a 60 40 mixture of the two exo adducts was obtained (Scheme 24). This result suggested that the endo-orientating character of the carbonyl group is much higher than that of the sulfinyl one, thus resulting in only exo-adducts (endo with respect to the carbonyl group). By contrast, the 7r-facial selectivity is very low. 

Cyclic ketone dianions obtained by the condensation of 1,4-dilithio-1,3-dienes with carbon monoxide can be used for the one-pot synthesis of a variety of 3-cyclopentenone derivatives20. Treatment of the dilithio species with 2 equivalents of benzyl bromide gave the corresponding 2,5-dibenzyl 3-cyclopentenone in 73% yield. A single-crystal structure analysis revealed that the two benzyl groups are in a trans orientation. Several examples of the one-pot synthesis of 3-cyclopentenone derivatives using this method are summarized in Table 7. 

The electron-donating nature of this diene confers high reactivity and orientational specificity in its reaction with unsymmetrical dienophiles.5 This fact, coupled with the readily available conversion to the a,(3-un-saturated ketone from the imparted functionality, makes l-methoxy-3-trimethylsiloxy-1,3-butadiene (2) a potentially very valuable reagent in organic synthesis. The general reaction scheme is illustrated below ... 

In a study on vitamin D2 synthesis [ 124], a sterically very hindered sulfone 273 (Scheme 90) with the axially oriented sulfonyl group was transformed into an anion (NaHMDS) and subjected to reaction with diene-aldehyde 274 used in excess. After deprotection, vitamin D2 275 and its unstable C7-C8 Z-isomer 276, were obtained in 70% yield, ratio of 72/28, respectively. Some BT-sulfone 277 generated by epimerization of 273 was also isolated. An attempt to carry out the reaction with the reverse allotment of the fimctional groups (sulfone 278 with ketone 279) failed. With regard to the isomer ratio 275 and 276, it is of interest that the classical Julia reaction of phenylsuhbne corresponding to 277 and aldehyde 274 occurs with somewhat lower selectivity towards the -isomer [125]. 

Nucleophilic 1,4- and 1.6-additions of cuprates and other organometallic reagents to acceptor-substituted dienes have been utilized extensively in target-oriented stereoselective synthesis - . Schollkopf and coworkers reported the diastereoselective 1,6-addition of a bislactim ether-derived cuprate to 3,5-heptadien-2-one (90% ds equation 17). The corresponding reactions of dienoates were conducted with the lithiated bislactim ether and proceeded with diastereoselectivities of >99% ds (equation 18) the adducts could be converted easily into diastereo- and enantiomerically pure amino acid derivatives. 

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