1.4- Diacetoxy-2-cyclohexene

When a palladium(II)-hydroquinone system is used as the mediator4 in the anodic oxidation of 1,3-cyclohexadiene in acetic acid, either trans- or cis- 1,4-diacetoxy-2-cyclohexene is formed with rather high selectivity, though the possible formation of 1,2-diacetoxylated compound is not discussed. 

Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyciohexadiene was converted into a mixture of isomeric 1,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCl and benzoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, //mv-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and m-l,4-diacetoxy-2-cycIohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cw-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

By incorporating the quinone molecule into the macrocycle, a more efficient palladium-catalyzed aerobic 1,4-oxidation was developed [69]. Thus, with catalytic amounts of 48 and Pd(OAc)2, 1,3-cyclohexadiene was oxidized to 1,4-diacetoxy-2-cyclohexene at more than twice the rate achieved with a system having the... 

MPa O2). The role of the encapsulated [Co(salophen)] complexes is to catalyze the aerobic oxidation of hydroquinone to p-benzoquinone, which in turn oxidizes Pd(0). For the oxidation of 1,3-cyclohexadiene to l,4-diacetoxy-2-cyclohexene, the most active catalyst system involved the encapsulated complex [Co(tetra-tert-butyl-salophen)], which afforded product yields of 85-95% after 3 h at room temperature with greater than 90% trans-selectivity. This complex displayed significantly higher activity than the encapsulated [Co(salophen)] complex (72% yield in 3h) and the analogous homogeneous complex (86% yield in 5h). The increased activity of the t-butyl substituted catalyst was attributed to distortion of the bulky complex by the... 

Then, pseudo-p-DL-gulopyranose (14) was synthesized by hydroxylation of 2,5-di-hydroxy-3-cyclohexene-l-methanol triacetate (12), which was prepared by Diels-Alder cycloaddition of 1,4-diacetoxy- 1,3-butadiene (10) and allyl acetate (11), with osmium tetroxide and hydrogen peroxide and successive acetylation as the pentaacetate (13). Analogous hydrolysis of 13 in ethanolic hydrochloric acid afforded the free pseudosugar 14 in 33% yield from 12 [2] (Scheme 7). 

To a stirred solution of DIB (3.2 g, 10 mmol) in dichloromethane (40 ml) the silyl ether (1.7 g, 10 mmol) in dichloromethane (20 ml) was added at room temperature, under argon. After 6 h, the reaction mixture was concentrated and the residue extracted with pentane the extract was concentrated and then distilled to give 1-trimethylsilyloxy-2-acetoxy-cyclohexene (1.78 g, 78%), b.p. 55°C (0.2 mmHg). A by-product (7%) was 1,2-diacetoxy-l-trimethylsilyloxy-cyclohexane. 

Diacetoxylation of 1,3-dienes.1 Palladium-catalyzed oxidation of 1,3-cyclo-hexadiene with benzoquinone (used in catalytic amounts with Mn02 as the external oxidant) in acetic acid gives a 1 1 mixture of cis- and trans-, 4-diacetoxy-2-cyclohexene. Addition of LiCl or LiOAc has a profound effect on the stereochemistry. Oxidation in the presence of lithium acetate results in selective fraws-diacetoxylation, whereas addition of lithium chloride results in selective cw-diacetoxylation (equation I).2... 

Acyl esters of vicinal diols are obtained by the reaction of alkenes with metal carboxylates [436]. Lead tetraacetate in acetic acid at 70 °C converts 1,2-dihydronaphthalene to rranj-l,2-diacetoxy-l,2,3,4-tetrahy-dronaphthalene in 72% yield [436]. The reaction is not always stereospecific. Cyclohexene treated with thallium triacetate gives a mixture of diastereomers in varying ratios, depending on reaction conditions, and byproducts as a result of rearrangements (equation 89) [411],... 

Yildiz-Unveren, H.H. and Schomacker, R. (2005) Hydroformylation with rhodium phosphine-modified catalyst in a microemulsion comparison of organic and aqueous systems for styrene, cyclohexene and l,4-diacetoxy-2-butene. Catal. Lett., 102, 83. 

One example of such a reaction was reported in 1971 by Brown and Davidson [55], who studied oxidation reactions of 1,3- and 1,4-cyclohexadiene. These authors observed that reaction of 1,3-cyclohexadiene with p-BQ in acetic acid in the presence of catalytic amounts of Pd(OAc)2 produced l,4-diacetoxy-2-cyclohexene of unknown configuration. At the time. Brown and Davidson were uncertain about the mechanism, and suggested possible involvement of radicals. A related palladium-catalyzed 1,4-diacetoxylation of butadiene employing as an oxidant and a heterogeneous Pd-Te catalyst has been developed and commercialized by Mitsubishi Chemicals [56]. 

In the catalytic cycle of the palladium-BQ-based 1,4-oxidation of 1,3-dienes, BQ is reduced to HQ. The diacetoxylation reaction is conveniently performed with p-BQ in catalytic amounts employing Mn02 as the stoichiometric oxidant In this process, the HQ formed in each cycle (cf. Scheme 11.8) is reoxidized to p-BQ by Mn02. For example, the catalytic reaction of 1,3-cyclohexadiene using catalytic amounts of both Pd(OAc)2 and p-BQ with stoichiometric amounts of Mn02 in acetic acid in the presence of lithium acetate afforded a 93% yield of trons-l,4-diacetoxy-2-cyclohexene (>91% trans) [57]. The corresponding reaction in the presence of lithium chloride gave cis-l,4-diacetoxy-2-cyclohexene in 79% yield (>96% cis). 

A soln. of frans,frans-l,4-diacetoxy-l,3-butadiene and methyl acrylate stabilized with hydroquinone heated slowly to reflux in xylene, and the product isolated after 37 hrs. methyl 2j, 5) -diacetoxy-3-cyclohexene-la-carboxylate. Y 93%. E. E. Smissman et al., Am. Soc. 84, 1040 (1962). 

Startg. m. vigorously stirred and refluxed 25 min. with 5%-Pd-BaS04 in abs. methanol containing cyclohexene -> 3y -methoxy-3a,9a-oxido-lla,18-diacetoxy-14j -hydroxy-20-oxo-J -pregnene. Y 80%. E. Gossinger et al., Helv. 54, 2785 (1971). 

---