Cyclopentadiene, Michael reactions

The preparation of 5-ACETYL-l,2,3,4,5-PENTAMETHYLCYCLO-PENTADIENE is of value in the synthesis of pentamethyleyclo-pentadiene and many pentamethylcyclopentadienyl metal carbonyl derivatives that are more soluble in organic solvents than those derived from cyclopentadiene. Simple preparations of 5,6-DIHYDRO-2-PYRAN-2-0NE and 2-//-PYRAN-2-ONE make these hitherto rather inaccessible intermediates available for cycloaddition and other reactions. The already broad scope of the Michael reaction has been widened further by including an efficient preparation of ETHYL (E)-3-NITROACRYLATE. Workers in the field of heterocyclic chemistry will find a simplified method for the preparation of 2,3,4,5-TETRA-HYDROPYRIDINE of help. 

The absolnte confignrations of both the Diels-Alder and the Michael addncts with R = Ph were elncidated by transformation into known compounds. In the case of the D-A reaction, the (2S,3S) enantiomer was formed in excess, indicating that cyclopentadiene attacks from the Si face of the dienophile. Similarly, nucleophilic attack to the enone in the Michael reaction was shown to occur from the Si face. These results suggest that the Re face of the bound substrate is shielded efficiently by the DNA in both reaction classes (Fig. 14). 

Scheme 3.13 Michael reaction of cyclopentadiene or anthrone with a,P-unsaturated aldehydes.

In 2004, Fossey and Richards [21] reported the synthesis of 2,6-bis(2-oxazolinyl) phenylplatinum(I I) NCN-Pincer complexes (63a-d) (Scheme 16.20). These cationic complexes were employed as Lewis acid catalysts for the Michael reaction between methyl vinyl ketone (49c) and ethyl cyanoacetate (64) and Diels-Alder reaction between acrylonitrile (67) and cyclopentadiene (14). The highest rates of the Michael reactions were observed when Pt-oxazolines (63a) and (63b) were used as catalysts, and the lowest rates of the Diels-Alder reaction was observed when Pt-oxazoline (63c) was used as a catalyst. Introduction of a nitro substituent on para position as shown for (63c) resulted in higher Lewis acidity but reduced catalytic activity. 

What product would you expect to obtain from the base-catalyzed Michael reaction of (a) l,3-diphenylprop-2-en-l-one (Section 19.5A) and acetophenone and (b) 1,3-diphenylprop-2-en-l-one and cyclopentadiene Show all steps in each mechanism. 

The optically active a-sulfinyl vinylphosphonate 122 prepared in two different ways (Scheme 38) is an interesting reagent for asymmetric synthesis [80]. This substrate is an asymmetric dienophile and Michael acceptor [80a]. In the Diels-Alder reaction with cyclopentadiene leading to 123, the endo/exo selectivity and the asymmetry induced by the sulfinyl group have been examined in various experimental conditions. The influence of Lewis acid catalysts (which also increase the dienophilic reactivity) appears to be important. The 1,4-addition of ethanethiol gives 124 with a moderate diastereoselectivity. 

The first propynoate molecule undergoes a carbon—carbon bond-forming reaction with the zirconacyclopentadiene. The second molecule of the propynoate then donates a proton to the zirconacycle to open the ring. Further intermolecular Michael addition to the resulting carbon—carbon double bond produces the cyclopentadiene compounds. Investigations using deuterated propynoate were clearly indicative of the reaction mechanism shown in Eq. 2.47. 

The reacuons of electrophihc tnflyl sources with nucleophiles were investi gated The reaction of tnfhc anhydride with an organolithium reagent is not synthetically promising because of ditnflylation and other side reactions [20] When phenyllithium reacts with tnfhc anhydnde, dimerization products and acetylenic Michael diadducts are observed [20] (equation 17), but using the sodium salt of the alkynes instead of the lithium salt provides the alkynyl tnfluoromethyl sulfones [21] (equation 18) (Table 7) Alkynyl tnfluoromethyl sulfones are of synthetic interest, because they show a pronounced reactivity toward nucleophiles in addition reactions and cyclopentadiene in Diels-Alder reactions [21] (Table 7)... 

Stable at room temperature for a short while. Detailed NMR experiments have demonstrated the non-aromatic property of this brr-electron system [8]. In contrast to the isomeric benzene, fulvene 1 has localized double bonds [9]. Therefore, it behaves like a diene or like a dienophile, and it readily oligomerizes at temperatures above 0°C [7]. In contrast, terminally disubstituted fulvene derivatives such as dimethylfulvene 11 which had been obtained as early as 1906 by Thiele et al. from cyclopentadiene and acetone in methanol in the presence of potassium hydroxide [10], are perfectly stable compounds. A rather efficient new synthesis of 11 and the monosubstituted fulvene derivative 12 by the reaction of cyclopentadiene with unsaturated carbonyl compounds in the presence of pyrrolidine (Scheme 2), has been described by Griesbeck [11]. Dimethylfulvene 11 in this transformation arises via Michael addition of the cyclopenta-dienide ion to the a,j9-unsaturated ketone mesityloxide and a subsequent retro-... 

Such important processes as alkylation of arylacetonitriles, cyclopentadiene hydrocarbons, aldehydes and ketones, esters, sulfo-nes etc., condensation of carbanions with aldehydes and ketones, the Knoevenagel, Darzens, Michael and related reactions as well as many reactions involving sulfonium and phosphonium ylides have been successfully carried out under these conditions. 

Endocyclic dienes ranging in ring size from four to seven have also been used in the intramolecular Diels-Alder reaction. Cyclobutadienes are prepared from the corresponding metal complexes [37]. Cyclopentadienes are most readily prepared by alkylation of cyclopentadienyl anion [38, 39]. They are also available by Michael addition to fulvene derivatives and by aminal exchange [40, 41]. 

Iminium catalysis is another key catalytic concept in aminocatalysis. Initialworkwas disclosed by MacMillan for the Diels-Alder reaction of cyclopentadiene and a,P-unsaturated aldehydes [12], but it was rapidly extended to Michael additions (including Friedel-Crafts reactions). Now iminium catalysis has been established as a general mode for nucleophilic addition to a,]3-unsaturated carbonyl compounds. 

CJglycerol trinitrate On the other hand, equimolar amounts of H CHO reacted with nitromethane to give 2-nitro[l- C]ethanol (115). Dehydration of 115 furnished 1-nitro-[2- C]ethylene (116). a valuable Michael acceptor and Diels-Alder dienophile. Its reaction with cyclopentadiene, for example, gave predominantly e t/o-2-nitro[3- C]-norcamphane (117). Subsequent catalytic reduction of the double bond, Fe-reduction of the nitro group and concluding acyloxy-deamination converted 117 into exo-[ C]norbornyl acetate (118). ... 

---