Cyclopentadiene selective reduction

Diazene 19 was synthesized in the manner portrayed below. Thus, treatment of anhydride 20 with sodium borohydride selectively reduces one carbonyl to a methylene unit. Reduction of the resulting lactone with DIBAL followed by a Wittig reaction and oxidation with PCC afforded aldehyde 22. When treated with cyclopentadiene in the presence of diethylamine in methanol, 22 undergoes a smooth and efficient conversion to fulvene 23. Diels-Alder cycloaddition to the azodicarboxylate 24 proceeded rapidly, a characteristic of reactions with this electron deficient chlorinated dienophile [8]. Selective reduction of the endocyclic n bond using diimide generated in situ, followed by the electrochemical reductive cleavage of the biscarbamate led to diazene 19 [6]. 

Selective reduction of unsaturated endoperoxides. 2,3-Dioxabicyclo[2.2.1]-heptane (3) serves as a model of the prostaglandin endoperoxide (PGG2), involved in the biosynthesis of prostaglandins and also of thromboxanes and PGl2 (prostacyclin). One successful synthesis of 3 involved sensitized photooxygena-lion of cyclopentadiene to give the unstable heat-sensitive endoperoxide 2, which... 

Brimble and coworkers176 studied the asymmetric Diels-Alder reactions of cyclopentadiene with chiral naphthoquinones 272 bearing different chiral auxiliaries. The highest endo and facial selectivities were obtained using zinc dichloride as the Lewis acid catalyst and (—)-pantolactone as the chiral auxiliary. Thus, the reaction between cyclopentadiene and 272 afforded a 98 2 mixture of 273 and 274 (equation 76). The chiral auxiliary was removed easily by lithium borohydride reduction. 

Acryloyl and crotonoyl amides (9) derived from the sultam (8), available from ( + )-l, are somewhat more reactive dienophiles than the i orresponding esters (3) derived from 2. Diels-Alder reactions of 9 with cyclopentadiene catalyzed by TiCU or C2H,AIC1, proceed in high yield, with high endo selectivity ( 99%) ind diastereoselectivity of —95% de. The chiral auxiliary is removed by reduction witfi LiAIH4 to furnish 8 and the chiral alcohol in 89-95% yield.-... 

Similar, but less easily accessible, dienoj iles (397) were reported to undergo preferential si face additions of cyclopentadiene at 0 C when melted by AJCb ot Et2AlCl (Scheme 96, Table 26). This dia-stereodifferentiation was most significant with AlCb-coordinated (397c) (entry 3) and still good, but much less endo selective, in the absence of AlCb (entry 5). Treatment of endo products (398) with I2 in aqueous DME and reduction of the iodolactone with i/MeOH furnished (l/ ,2/ ,4/ )-5-norbomenecar-boxylic acid (68%) and the recovered pyrrolidine auxiliary (63%). 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

Thermal cycloadditions of chiral vinylsulfoxides B with cyclopentadiene, 1,3-cyclohexadiene and 1,3-butadiene have been examined in toluene at high temperature leading to the adducts in moderate selectivities. The electron-deficient sulfur residue activates the alkene unit and serves as a temporary substituent that can be easily removed by reduction or transformed into a variety of other functionalities. An increase in reactivity and excellent selectivities are observed in the presence of additional electron-withdrawing substituents at the alkene unit as shown for the examples and in the table below. At 20°C in dichloromethane the cycloadditions of C with cyclopentadiene proceed smoodily and highly stereoselective. In reactions of menthyl-3-(3-trifluoromethyl-2-pyridylsulfinyl)acrylate A with dienes, e.g. 2-methoxyfliran or cyclopentadiene, high diastereoselectivities (>98 2) were obtained, too. 

Bishomocubanes have been used to provide a synthesis of the fluxional C.oH 10 isomer hypostrophene (984) in 12% overall yield. Treatment of cyclopentadiene with ethyl nitrite and ethoxide gives (980). Deoximation and selective acetalization gives (981), which on irradiation and then hydrolysis gives (982) which was reduced, mesy-lated, and substituted with iodide to give (983). Reduction by Na-K alloy of the 1,4-di-iodide gives hypostrophene. A [2 2] cycloaddition in the hypostrophene... 

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