2,4-Pentadienyl acetates

It is known that tr-allylpalladium acetate is converted into allyl acetate by reductive elimination when it is treated with CO[242,243]. For this reason, the carbonylation of allylic acetates themselves is difficult. The allylic acetate 386 is carbonylated in the presence of NaBr (20-50 mol%) under severe conditions, probably via allylic bromides[244]. However, the carbonylation of 5-phenyl-2,4-pentadienyl acetate (387) was carried out in the presence of EtiN without using NaBr at 100 °C to yield methyl 6-phenyl-3,5-hexadienoate (388)[245J. The dicarbonylation of l,4-diacetoxy-2-butene to form the 3-hexenedioate also proceeds by using tetrabutylphosphonium chloride as a ligand in 49% yield[246]. 

Carbonylation of the 3-phenylallyl acetate 258 under somewhat severe conditions in the presence of tertiary amine and acetic anhydride affords the naphthyl acetate derivative 260. This interesting cyclocarbonylation is explained by the Friedel-Crafts-type cyclization of the acylpalladium 259 as an intermediate [118,119]. Even 5-phenyl-2,4-pentadienyl acetate (261) is cyclocarbonylated to afford 2-phenylphenyl acetate (262) [120],... 

A route to aryl aroates consists of catalyzed reaction from a mixture of ArX, ArOH, CO, and a base. Phenol derivatives are formed in a cyclocarbonylation reaction of 2,4-pentadienyl acetates. ... 

Finally, it would be interesting to compare the above reactions with the related Pd-catalyzed carbonylation of 2,4-pentadienyl acetates to give phenol derivatives (Scheme The latter reaction is supposed to proceed via intramolecular acylpalla-... 

Vinyl-3-alkenols. rj -Pet from pentadienyl acetates with lO and methyl benzoate. The compli mild base to give the dienols. 

Vinyl-3-alkenols. yj -Pentadienylchromium complexes can be generated from pentadienyl acetates with (OC)iCr(PhCOOMe), which is obtained from CrCCO) and methyl benzoate. The complexes condense with aldehydes in the presence of a mild base to give the dienols. 

Andersson and Backvall observed that the regiochemistry of the palladium-catalyzed reactions of malonate esters with pentadienyl acetates depends on the... 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

An alternative approach for generating the pentadienyl carbocation that is needed for the Nazarov cyclization has been demonstrated by de Lera and co-workers [20, 21] (Eq. 13.18). Vinylallene acetal 56 is converted to a ca 1 1 mixture of cyclopentenes 57 and 58 upon exposure to toluenesulfonic acid in acetone at room temperature. The reaction presumably involves initial generation of carbocation 59 that undergoes conrotation to give 60. Intramolecular trapping of the carbocation by the pendant hydroxyl group leads to the observed product. Depending on whether the conrotation in 59 takes place clockwise or counterclockwise, E- (57) or Z-(58) products are formed. 

The mechanism of the catalysis in the rearrangements of 4-acetoxyhepta-2,5-dienes was investigated by using the 170-labeled acetate 362 in the presence of Pd° and Pdn catalysts187. It was shown by 170 NMR spectroscopy that the reaction catalyzed by Pd° affords a 1 1 mixture of the dienes 363 and 364 which results from the Pd-coordinated pentadienyl species intermediate and 170-acetate (equation 132). By using two Pdu-catalysts, viz. (RCN PdCL (R = Me, Ph) [Pdn(l)] and (PhjP Pd [i.e. Pdn(2)], two rearrangement products 365 and 366, respectively, were obtained. The heterocyclic 1,3-dioxanium cations 367 and 368 were assumed to be intermediates of these isomerizations (equation 133). 

Another rare kind of 6-electron ionic cycloaddition is that between a pentadienyl cation and an alkene. A telling example is the key step 2.66 — 2.67 in a synthesis of gymnomitrol 2.68, where the nature of the pericyclic step is heavily disguised, but all the more remarkable for that. Ionization of the acetal gives the cationic quinone system 2.66. That this is a pentadienyl cation can be seen in the drawing of a canonical structure on the left, with the components of the pericyclic cycloaddition emphasized in bold. Intramolecular [4+2] cycloaddition takes place, with the pentadienyl cation as the 4-electron component and the cyclopentene as the 2-electron component. Th is reaction is an excellent example of how a reaction can become embedded in so much framework that its pericyclic nature is obscured. 

A versatile method for the preparation of allyl complexes of transition elements involves addition of allyl halides or acetates to low oxidation state coordinatively unsaturated, or potentially unsaturated, complexes (214). A few attempts have been made to extend the method to halogenopentadienes. l-Chloro-5-phenyl-2,4-pentadiene adds to Pd2(dba)3 (dba = dibenzoylac-etone) to give the >/3-pentadienyl complex 61 [Eq. (30)] (215). A similar palladium complex has also been obtained from PdCl2, and 2,5-dimethyl-2,4-hexadiene in isopropanol (216). 

Substitution of complexed dienols (244) or dienol acetates with carbon or heteroatom nucleophiles, in the presence of a Lewis acid, occurs with retention of configuration (Scheme 69). (Alkyl aluminum reagents act as both nucleophile and Lewis acid in this process). This reaction is believed to proceed via stereospecific ionization, with anchimeric assistance from the iron, to generate the transoid pentadienyl cation (247) followed by attack of the weak nucleophile on the face opposite to iron. The cross-conjugated pentadienyl cation can also be generated the substitution of (2-acetoxymethyl-l,3-butadiene)Fe(CO)3 (193) has previously been discussed (Section 6.1.1). 

As described in Section 6.3.4, activation of coordinated dienols (by protonation) or coordinated dienol acetates (by treatment with Lewis acid) affords the corresponding (pentadienyl)iron cations (248) (Scheme 69). Since the acyclic pentadienyl cationic complexes do not have the geometrical constraints of their cyclic counterparts, they can potentially adopt either a cisoid ( U ) or transoid ( S , sickle) conformation. Nearly all of the (pentadienyl)iron cations prepared appear to be in the cisoid conformation by H NMR spectroscopy. Only a single, sterically biased transoid pentadienyl cation (269) has been spectroscopically observed, but not isolated (equation 66). Owing to their potential to undergo U to S conformational inversion in solution, and owing to the considerably higher reactivity of the less stable S conformer, the (pentadienyl)iron cations are considerably more sensitive to moisture than their cyclic counterparts. 

A synthesis of cyclopentenones somewhat related to those shown earlier was found from conjugated 1,3-enynes (equation 19) Cyclopentenones are obtained by the hydrolysis of the enol acetates. This transformation involves a 1,3-migration of the acetate to form pentadienyl cation and the formation of the gold carbene after a Nazarov-type cyclization. DFT calculations support this mechanism and provide interesting insight into the mechanism of the final stages of the process. 

A recent application of the furan-carbonyl photocycloaddition involved the synthesis of the mycotoxin asteltoxin (147)." Scheme 16 shows the synthetic procedure that began with the photoaddition of 3,4-dimethylfuran and p-benzyloxypropanal to furnish photoaldol (148), which was epoxidized with MCPBA to afford the functionalized product (149) in 50% overall yield. Hydrolysis (THF, 3N HCl) provided the monocyclic hemiacetal which was protected as its hydrazone (150). Chelation-controlled addition of ethylmagnesium bromide to the latent a-hydroxy aldehyde (150) and acetonide formation produced compound (151), which was transformed through routine operations to aldehyde (152). Chelation-controlled addition of the lithium salt of pentadienyl sulfoxide (153) followed by double 2,3-sigma-tropic rearrangement provided (154) as a 3 1 mixture of isomers (Scheme 17). Acid-catalyzed cyclization of (154) (CSA/CH2CI2) gave the bicyclic acetal (155), which was transformed in several steps to ( )-asteltoxin (147). ... 

A interesting variation on this theme employing the isomeric enynol acetates (Scheme 24) has been developed by Rautenstrauch. The cyclizations are induced by a Pd" catalyst in warm acetonitrile. The proposed mechanism is intriguing. Reaction is initiated by an anchimerically assisted palladation to (35) followed by opening the dioxolenium ion to a pentadienylic cation (36). The closure of (36) is analogous to the silicon-directed Nazarov cyclization in the ejection of the Pd" electrofuge from (37). Both secondary and tertiary acetates can be employed as well as both acyclic and monocyclic systems. 

The diester 87 with the same tetracyclic skeleton as 83 had previously been prepared by Paquette et al. via a domino Diels-Alder reaction of 5,5 -bicyclo-pentadienyl 84 with dimethyl acetylenedicarboxylate (Scheme 20) [73]. The key precursor 84 was obtained by iodine-induced oxidative coupling of the copper cyclopentadienide derived from the sodium derivative. The diester 85 formed along with 86 was transformed into a bissilyl bisenol ether by reductive cleavage of the central bond in the succinate moiety with sodium in the presence of trimethylsilyl chloride. Subsequent hydrolysis of the bisenol ether - actually a bisketene acetal - gave the dienic tetraquinacenedicarboxylate 87. This compound served as the key intermediate in the first synthesis of dodecahedrane 88 [74]. 

Pentadienyl)trimethylsilane, in its reaction with pivaloyl chloride in the presence of titanium tetrachloride, acylates to give a mixture of products, chiefly derived from reaction at the e-carbon, with y-acylation also observed (equation 22). The latter mode of reaction is more prevalent if the e-carbon is further substituted, presumably because of steric hindrance. This electrophile showed lower selectivity for the e-carbon than other electrophiles, including aldehydes and acetals. 

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