Acetoxy-1,3-butadienes, Diels-Alder reaction

The presence of the catalyst can also favor multiple Diels-Alder reactions of cycloalkenones. Two typical examples are reported in Schemes 3.6 and 3.7. When (E)-l-methoxy-1,3-butadiene (14) interacted with 2-cyclohexenone in the presence of Yb(fod)3 catalyst, a multiple Diels-Alder reaction occurred [21] and afforded a 1 1.5 mixture of the two tricyclic ketones 15 and 16 (Scheme 3.6). The sequence of events leading to the products includes the elimination of methanol from the primary cycloadduct to afford a bicyclic dienone that underwent a second cycloaddition. Similarly, 4-acetoxy-2-cyclopenten-l-one (17) (Scheme 3.7) has been shown to behave as a conjunctive reagent for a one-pot multiple Diels-Alder reaction with a variety of dienes under AICI3 catalysis, providing a mild and convenient methodology to synthesize hydrofluorenones [22]. The role of the Lewis acid is crucial to facilitate the elimination of acetic acid from the cycloadducts. The results of the reaction of 17 with diene... 

Table 5.1 High pressure Diels-Alder reactions of (E)-l-acetoxy- (18a) and (E)-l-methoxy butadiene (18b) with acrylic and crotonic dienophiles...

Uncatalyzed Diels-Alder reactions between l-(trimethysiloxy)- or 1-acetoxy-l,3-butadiene and sugar-derived nitroalkenes having D-galacto or D-manno configurations proceed with complete regioselectivity. Diastereofacial selectivity is also complete with the D-galacto dieno-phile, whereas it is only moderate with the D-manno (Eq. 8.30).51... 

Chiral boron catalysts had already been widely used in a variety of reactions before they were applied in Diels-Alder reactions220. Boron catalysts were first employed in the Diels-Alder reactions of quinones with electron-rich dienes. Kelly and coworkers221 found that stoichiometric amounts of a catalyst prepared from BH3, acetic acid and 3,3 -diphenyl-l,l/-bi-2-naphthol (344) catalyzed the reaction of 1-acetoxy-l,3-butadiene (341) with juglone (342) to afford cycloadduct 343 with 98% ee (equation 96). The reaction was supposed to proceed via a spirocyclic borate complex in which one face of the double bond of juglone was effectively shielded from attack by the diene. 

A number of other specific reactions have been studied. For example, Diels-Alder reactions of the unsaturated 5(4//)-oxazolone derived from piperonal with 2-ferf-butyldimethylsilyloxy-1,3-butadiene, piperylene, 1-acetoxy-1,3-butadiene, and Danishefsky s diene have been described. In these cases, the results are variable and are dependent on the diene with poor yields often obtained even at high temperatures. Moreover, the stereochemical outcome of these reactions has not been determined. " ... 

The 5,6-double bond in activated pyrimidines such as 2-acylamino-6-acetyl-4(lH)-pyrimidinones (427) undergo Diels-Alder reactions to yield on heating with l-acetoxy-3-methylbutadiene (428) hydroquinazolines (429). The yields are modest, but the regiochemistry is clean. With 1,3-butadiene and with isoprene the cycloadducts were formed in low yields (83JOC3627). 

Diels-Alder reactions.1 A key step in a synthesis of 4-demethoxydaunomy-cinone (5) involves a regioselective Diels-Alder reaction of anthracene-1,4,9,10-tetrone (2) with (1) to give the desired adduct (3) in 79% yield. A similar reaction with 2-acetoxy-1,3-butadiene affords only a 58% yield of the corresponding adduct. The product (rather unstable) is aromatized and hydrolyzed to give 4 in high yield. 

Dichlorobis(diisopropoxy)titanium(IV). Titanium(IV) chloride. Zinc iodide. DIELS-ALDER REACTIONS 2-Acetoxy-I-methoxy-3-trimethylsilyloxy-1,3-butadiene. 4-Acetoxy-1 -trimethylsilyl-1,3-butadiene. Benzyl irans-l,3-butadiene-l-carbamate. 1,3-Bis(/-butyldimethylsilyloxy)-2-aza-1,3-diene. 2,3-Bis(trimethylsilyl)methyl-1.3-buladiene (10-1,3-Dimethoxybutadiene 4-I)iniethyhnnino 1,1,2... 

Table 4. NLE in the asymmetric Diels-Alder reaction of 1-acetoxy-l,3-butadiene and methacrolein catalyzed by MS-free BINOL-Ti ( ).

Easily prepared from glycols, enones have been investigated as dienophiles. They react with butadiene under Lewis acid catalysis to form chiral cyclohexenes used in the synthesis of compactin analogs [353]. Levoglucosenone has been used in a Diels-Alder reaction with acetoxy-butadiene to construct a part of the indole alkaloid reserpine [354], and in synthetic studies toward tetrodotoxin [355]. Analogs of the anthracycline rhodomycinone have been similarly prepared [356]. [4 + 2]-Cycloaddition of the same enone with silyloxydiene allowed the creation of the fused ring system present in actinobolin [357]. 

The use of alumina and silica to promote Diels-Alder reactions was first reported by Hudlicky [24]. The reactions of 1-acetoxy-l,3-butadiene with tetrafluoro-p-benzoquinone or / -benzoquinone were considerably accelerated when they occurred inside the pores of the adsorbents. Several other silica- or alumina-catalyzed Diels-Alder reactions, dealing either with investigation of standard model reactions [25] or of reactions of more synthetic utility, were subsequently described. For example, neutral alumina catalyzed the intramolecular reaction of a tetraene to the verrucarol skeleton [26] (Scheme 3). The catalytic action was observed in an attempt to isolate the tetraene by column chromatography. A variety of Diels-Alder reactions were accelerated by silica or the more basic Florisil (Mg0.Si02) in the absence of solvent [27-30]. Table 3 gives some examples of these reactions, which are thought to be catalyzed via multicentered donor-acceptor interactions of the substrates with the active sites of the surface of the adsorbents. 

Excellent selectivities were observed in Lewis acid-promoted Diels-Alder reactions. In general, cyclopentadiene or 1,3-butadiene were chosen as dienes (see, table column 4). Anthracene and other dienes, as for example (5) or (6), were examined in isolated cases as indicated in the following table. 1-Acetoxy-1,3-butadienes (5) and diene (6) were employed in natural product syntheses furnishing the desired cycloadducts as single diastereomers. 

Recently, Overman, et al. reported an organocatalytic Diels-Alder reaction of 2-acetoxy-1,3 -butadiene 40 and acrolein (28f) by MacMillan catalyst, ent-30-HOTf, Scheme 3.11 [21], It was noteworthy to use water-saturated nitromethane as the reaction solvent. The adduct 41 was transformed to a tetracycle with the skeleton of the ring A-D of daphnicyclidin-Type alkaloids (42). 

Diels-Alder reaction of 1-acetoxy-1,3-butadienes with a,/3-unsaturated acid followed by Pd-catalyzed elimination gave various substituted 1,3-cyclohexadienes. 

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