Diels-Alder reaction s. Diene

Diels-Alder reaction s. Diene syntheses l,l -Dienamines... 

Dieckmaiin cyclization 21, 910 22, 898/0 Diels-Alder reaction s. Diene syntheses Dienamines s. a. Allenylamines... 

In most of the successful Diels-Alder reactions reported, dienes containing no heteroatom have been employed, and enantioselective Diels-Alder reactions of multiply heteroatom-substituted dienes, e.g. Danishefsky s diene, are rare, despite their tremendous potential usefulness in complex molecular synthesis. Rawal and coworkers have reported that the Cr(III)-salen complex 15 is a suitable catalyst for the reaction of a-substituted a,/ -unsubstituted aldehydes with l-amino-3-siloxy dienes [21] (Scheme 1.28, Table 1.12). The counter-ion of the catalyst is important and good results are obtained in the reaction using the catalyst paired with the SbFg anion. 

Simple dienes react readily with good dienophiles in Diels-Alder reactions. Functionalized dienes are also important in organic synthesis. One example which illustrates the versatility of such reagents is l-methoxy-3-trimethylsilyloxy-1,3-butadiene (.Danishefsky s diene) 1 Its Diels-Alder adducts are trimethylsilyl enol ethers which can be readily hydrolyzed to ketones. The /j-mcthoxy group is often eliminated during hydrolysis. 

In the Diels-Alder reaction, the dienes had to have an s-cis conformation about the central single bond so that they were already in the shape of the product. Many useful 1,3-dipoles are actually linear and their 1,3-dipolar cycloadditions look very awkward. We shall start with the nitrile oxides, which have a triple bond where the nitrone had a double bond. 

In the Diels-Alder reaction a diene (which must be capable of being s-cis) reacts with a dienophile, which must have an electron-withdrawing group conjugated to an alkene. 

Table 25 Lewis Acid Catalyzed Diels-Alder Reactions of Dienes to N-Acryloyl-(S)-proline Benzyl Ester (391)...

Conjugated ketenes. a-Diazo-y.S-unsaturated P-ketoesters undergo rearrangement to ketenes which enter Diels-Alder reactions as dienes toward electron-rich alkenes. Substituted phenols are acquired. 3-Acyloxy-2-chloro-2-cyclohexenones are formed when 2-diazocyclohexane-l,3-diones are treated with acid chlorides in the presence of Rh COAc),. ... 

The brief report of Jacobsen s total synthesis starts with a detailed retrosynthetic analysis. The compound was broken into four pieces 21a after removal of the phosphate. The unsaturated lactone 24 (M is a metal) could be made by an asymmetric oxo-Diels-Alder reaction from diene 22 and ynal 23. The epoxide 25 provides a second source of asymmetry. One cis alkene comes from an alkyne 26 and the rest from a dienyl tin derivative 27. 

In order to react in the Diels-Alder reaction, the diene must be in the s-cis conformation. ( -1,3-Pentadiene is very strained in this conformation because its methyl group points toward the double bond. As a result, (E)-1,3-pentadiene, which can attain this conformation much more readily, is more reactive. 

On the other hand, the Diels-Alder reaction of dienes 265 with oximinosulfonates 266 (derived from Meldrum s acid) in the presence of dimethylaluminum chloride afforded the cycloaddition adduct, which was then reacted with alkoxide and oxidizing agent (NCS) to give the desired pyridines 267. ... 

Danishefsky s interest in the development of synthetically useful Diels-Alder reaction methodology has recently extended to two different dienophiles. First, he studied the reactivity of enyne dienophiles to show that ynones react preferentially to enones. Substituted styrene 78 is the exclusive product on intermolecular Diels-Alder reaction of diene 76 with dienophile 77 followed by a retro Diels-Alder reaction (to generate 2-methylpropene) and two desilylations. ... 

The dienes in both (a) and (b) are fixed in the s-trans conformation and therefore are not capable of participating in Diels-Alder reactions. The diene in (c) is fixed in the s-cis conformation and therefore has the proper orientation to participate in Diels-Alder reactions. 

After an appropriate time. Professor Gometz assayed the reaction mixmre by H NMR spectroscopy and was disappointed to find that only a small portion (S-10%) of the desired enamine B had formed. Ever the optimist, he ran the Diels-Alder reaction with diene A anyway, using the reaction mixture containing what he knew to be only a small amount of B. He was delighted and astonished to obtain an 80% yield of cycloadducts. Your task is to explain how Professor Gometz could get such a good yield of products when only a small amount of enamine B was present in the reaction mixmre. Hint See Problem 16.9. 

A brief note has appeared on a new approach to S-amino-acids based on Diels-Alder reactions between dienes and JV-(2,2,2-trichloroethylidene)-carbamates. ... 

For a diene to be reactive in a Diels-Alder reaction, a diene must be able to adopt an s-cis conformation. 

One example of an early PGF synthesis is Robinson s 1917 landmark synthesis of tropinone 96 [80] (Scheme 13.15). By the combination of succinaldehyde 93, methylamine 94, and acetone dicarboxylic acid 95, tropinone resulted from what can be assumed to be a biomimetic process [81]. Another classic PGF synthesis is Danishefsky s 1968 preparation of patchouli alcohol (100) [82]. The atom-economical first step of this sequence, the Diels-Alder reaction of diene 97 and methyl vinyl ketone (98), is one of but not the only reason this example was chosen. Not to take away from the beauty of Danishefsky s synthesis, functionality contained within patchouli alcohol is minimal and as a result limited PG usage would be expected. Also, during the decades of the two syntheses the... 

Baran s synthesis of 1,9-dideoxy-pre-axinellamine is outlined in Scheme 13.24 [115]. To form the densely functionalized cyclopentane ring of the natural product family, a cyclohexane to cyclopentane ring contraction strategy was utilized. Diels-Alder reaction of diene 144 and dimethyl fumarate (145) produced 146 and set the relative stereochemistry of three stereocenters. After conversions of the esters of 146 to azides, the TIPS was exchanged for a PMB group. This unproductive PG manipulation was necessary as the... 

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