1.4- Pentadien

In this manner, self-condensation of acetaldehyde is rninimi2ed and yields in the range of 77—85% are obtained. However, even with these precautions a detectable amount of 5-phen5l-2,4-pentadienal [13466-40-5] is invariably formed. 

Compound 95, a 5-chloro-2,4-pentadien-l-one is a doubly vinylog-ous acid chloride. The addition of j8-chlorovinyl ketones to acetylenes was known to afford such compounds as 95 366,867 valence... 

Pentadiene is much more reactive in Diels-Alder reactions than 2,4-pentadienal. Why might this be ... 

Ordinary Grignard reagents react with a, -unsaturated carbonyl compounds and afford both 1,2-adduct and 1,4-adduct. However, methylsulfonyhnethylmagnesium bromide or p-tolylsulfonylmethylmagnesium bromide gave only 1,2-adducts in the reaction with conjugated carbonyl compounds such as crotonaldehyde, cinnamaldehyde, trans-4-phenyl-3-buten-2-one, benzalacetophenone and l,5-diphenyl-2,4-pentadien-l-one. 

It is notable that Table 11 contains examples of intramolecular cyclopropanation of an acrylate. It was found that Cu(acac)2 was not an efficient catalyst for this transformation cosiderable improvement was achieved by using catalytic amounts of Cu(acac)2 and excess CuS04 186). A similar observation was made with (2,4-pentadien-l-yl)diazoacetates or diazomalonates t91). 

Treatment of 1-pyridinium sulphonate with sodium or potassium hydroxide generates sodium or potassium salts of 5-hydroxy-2,4-pentadienal (glutaconaldehyde), which are starting materials for a variety of transformations (equation 178)171b 301. For example, the reaction of the potassium salt with a carbon electrophile has been used for the preparation of a dienol aldehyde (equation 179)mb which was an intermediate in the total synthesis of a mutagen, (S)-3-(dodeca-l,3,5,7,9-pentaenyloxy)propane-l,2-diol. 

An example is the structure of the derivative 18 formed by reacting the labelled oxypyrrolinyl with 2,4-pentadienal. 

The regioselectivity of Michael additions of thiolates to 2,4-dienones can be altered drastically by variation of the reaction conditions and addition of Lewis acids to the reaction mixture. Lawton and coworkers examined the reaction of 2-mercaptoethanol with l-(3-nitrophenyl)-2,4-pentadien-l-one and observed a high regioselectivity in favor of the 1,6-addition product at 45 °C (equation 42)123,124. Lowering of the reaction temperature caused an increase in the amount of 1,4-adduct, and at —40°C, a product ratio of 40 60 was found. These events suggest that kinetic control favors the 1,4-addition product whereas the 1,6-adduct is thermodynamically more stable. If, however, the reaction was carried out with a complex of the dienone and titanium tetrachloride, only the 1,4-adduct was isolated after hydrolytic workup123. Obviously, this product is trapped as a metal chelate which prevents formation of the 1,6-adduct by retro-Michael/Michael addition. In the absence of the chelating Lewis acid, the 1,4-addition product can indeed be converted... 

Structurally rather complicated target molecules can be synthesized with the aid of thi-olate 1,6-addition reactions to acceptor-substituted dienes as well. For example, a richly functionalized proline derivative with a 2,4-pentadienal side chain was converted into the corresponding 6-phenylthio-3-hexen-2-one derivative by 1,6-addition of phenylthiolate, treatment of the adduct with methyl lithium and oxidation (equation 46)127. The product was transformed into acromelic acid A, the toxic principle of clitocybe acromelalga ichimura. Similarly, the 1,6-addition reaction of cesium triphenylmethylthiolate to methyl 2,4-pentadienoate served for the construction of the disulfide bridge of the macrobicyclic antitumor depsipeptide FR-901,228128. 

Alternatively, bromo trienyne 66, prepared by the Wittig reaction of TMS-capped propargyl ylide with , -5-bromo-2,4-pentadienal, could be coupled with dienyl zinc reagent 67, as illustrated in equation 3657. Subsequent desilylation followed by treatment with trimethyl aluminum in the presence of catalytic Cp2ZrCl2 afforded the alane of tetraenyne 68 which, on exposure to chloroformate, gave essentially all- polyene ester 69. 

The diastereoselective formation of dienol tricarbonyliron complexes on treating rf-2,4-pentadienal)Fe(CO)3 with functionalized zinc-copper reagents has been investigated (equation 49)66. Cyano-substituted complexes undergo intramolecular nucleophilic additions when treated with lithium diisopropylamide (LDA) as shown in equation 50. 

Jung and Nichols have reported a tandem [l,2]-Brook/[l,6]-retro-Brook rearrangement of a l-(trimethylsilyl)-2,4-pentadien-l-ol anion to the aldehyde 181 (equation 112) . Representative examples of [1,5]- and [l,6]-retro-Brook rearrangements are summarized in Table 11. 

Uraneck and Smith (45) recently have described that the product of 2,4-pentadiene-l-ol and sec-butyl-lithium can be used to synthesize polymers with hydroxyl end groups. However, some of the sec-butyl-lithium ( - 137 ) remains unreacted. 

If both thermodynamic and kinetic factors operate simultaneously in favor of open-chain forms like 460 and 463, then 2//-pyran 459 may not be identifiable. This may be the reason why attempts to prepare unsubstituted 2//-pyran (4) from cis-2,4-pentadienal (466) have failed (for other attempts to prepare 4, see reference 386). Thus even if 466 had been generated by thermolysis of epoxycyclopentene 465, no traces of 4 were detected.28 Moreover, 466 seems to be thermodynamically more stable than cyclic form 4 according to ab initio MO calculations.56... 

Methyl-2,4-pentadien-l-ol, AH75 3-Methyl-2,4-pentadlenyl acetate, A0O6... 

The final aldol reaction used in our synthesis of spongistatin 1 was one of the more remarkable reactions of this type our group has witnessed over the years. The aldol union of ketone 64 with ( )-4-chloro-2,4-pentadienal 65 required the creation of the (475) stereochemistry in the resultant alcohol 66. Formally, this would require 1,5-syn induction from the ketone 64, which is opposite to that observed previously for boron aldol reactions with simple [i-alkoxy methyl ketones. However, ketone 64 is densely packed with stereocentres, and predicting the influence of these remote centres on the reaction outcome was not possible with any degree of certainty. It was hoped that should 64 display undesirable 1,5-anti bias, this may be overturned by appropriate choice of Ipc ligands on boron. 

Uraneck and Smith [125] reported that the product of 2,4-pentadien-l-ol and sec-butyllithium [Eq. (41)] can be used to prepare polymers with terminal hydroxyl groups. 

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