Allyl acetate, 2- cycloaddition

Aldehydes take part in the cycloaddition to give the methylenetetrahydrofuran 178 by the co-catalysis of Pd and Sn compounds[115]. A similar product 180 is obtained by the reaction of the allyl acetate 179, which has a tributyltin group instead of a TMS group, with aldehydesfl 16]. The pyrrolidine derivative 182 is formed by the addition of the tosylimine 181 to 154[117]. 

RhClCO(dppp) 2] for the sequential construction of an enyne precursor, starting from a malonic acid derivative and allylic acetate, which was converted in situ to the cycloaddition product with excellent yields. Obviously, the Pd complex catalyzes the allylic substitution reaction, while the rhodium catalyst is responsible for the PKR (Eq. 6). 

Schkeryantz and Pearson (59) reported a total synthesis of ( )-crinane (298) using an intramolecular azide-alkene cycloaddition (Scheme 9.59). The allylic acetate 294 was first subjected to an Ireland-Claisen rearrangement followed by reduction to give alcohol 295, which was then converted into the azide 296 using Mitsunobu conditions. Intramolecular cycloaddition of the azide 296 in refluxing toluene followed by extrusion of nitrogen gave the imine 297 in quantitative yield. On reduction with sodium cyanoborohydride and subsequent reaction with... 

The addition of simple ester or ketoenolates to TT-allylpalladium complexes may constitute the second step of an ingenious [3 + 2] cycloaddition reaction. One substrate that undergoes this process is 2-(tri-methylsilylmethyl)allyl acetate (5). The mechanism proposed involves initial formation of a 2-(tri-methylsilylmethyl)allylpalladium cation followed by desilylation by the acetate liberated in the oxidative addition (Scheme 1). The dipolar intermediate can be envisioned as an T]3-trimethylenemethane-PdL2 species (6) or, less likely, an -complex (7). 

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). 

The presence of five-membered rings such as cyclopentanes, cyclopentenes, and dihydrofurans in a wide range of target molecules has led to a variety of methods for their preparation. One of the most successful of these is the use of trimethylenemethane [3 + 2] cycloaddition, catalysed by pal-ladium(O) complexes. The trimethylenemethane unit in these reactions is derived from 2-[ (trimethylsilyl)methyl]-2-propen- 1-yl acetate which is at the same time an allyl silane and an allylic acetate. This makes it a weak nucleophile and an electrophile in the presence of palladium(0). Formation of the palladium 7t-allyl complex is followed by removal of the trimethylsilyl group by nucleophilic attack of the resulting acetate ion, thus producing a zwitterionic palladium complex that can undergo cycloaddition reactions. 

These examples demonstrate that a selective Heck-Diels-Alder sequence with two different alkenes is only possible either in a stepwise manner, if an alkene reacts much faster in the Heck reaction than in the subsequent cycloaddition so that the 1,3-diene can be isolated, or as a real cascade reaction if one alkene is more reactive and thus selectively reacts as a coupling partner, whereas the other one is a better dienophile. Both concepts have been used by Kollar et al. for the annelation of cyclohexene rings onto the steroidal skeleton 26 (Scheme 4) [28-30]. At 60 °C the cycloaddition was sufficiently suppressed so that the Heck coupling product 29 could be isolated and subsequently subjected to Diels-Alder reactions with different dienophiles. For a domino reaction with both methyl acrylate and dimethyl fumarate (28) present in the reaction mixture, the conditions had to be precisely adjusted so that the mixed products 31 and 32 were formed predominantly along with only small amounts of the products of a twofold reaction of either 27 (R = CC Me) or 28 with 26. These conditions also proved suitable for a cascade reaction of 26 involving allyl alcohol 27 (R = CH2OH) or allyl acetate 27 (R = CH2OAc) and dimethyl fumarate (28). 

A recent example of an intermolecular [3 + 4] cycloaddition starts with an allylic acetal, as shown in Eq. (167) [419,420]. Other Lewis acids, for example AlEt Cl3 (n = 0-3), TMSOTf, TfOH, SbCL, SnCL were less effective. Although the exact nature of the transition state is still uncertain, the stereochemistry of the product might be explained on the basis of the rule of endo addition with the least hindered approach of the diene. The possibility of asymmetric synthesis starting with the same substrate with a chiral acetal moiety has been mentioned (see Table 15). 

The two double bonds in 2,3-dimethoxycarbonylnorbornadiene are almost equally active. Furthermore, the reactions with methylenecyclopropane are stereoselective leading exclusively to the exo-isomers. Both observations are in striking contrast to the results obtained in the Pd(0) catalyzed cycloadditions of 2-[(tri-methylsilyl)methyl]allyl acetate with norbomadiene derivativesl97). 

Another interesting point is the regioselectivity of these [3+2]-cyeloadditions. Whenever a 2-substituted methylenecyclopentane is detected in a codimerization, catalyzed by a Pd(0) compound, the substitution pattern is the same as found in [3 -f 2]-cycloadditions starting with 2-[(trimethylsilyl)methyl]allyl acetates 183) (Eq. 85). 

The oxidative addition of an allylic acetate having an allylsilane structure (18) to a Pd(0) complex provides a (trimethylenemethane)palladium species (19), which undergoes [3+2] cycloaddition to a variety of electron-deficient olefins (eqs (115) and (116)) [146]. 

Diastereoselectivity in the cycloadditions of lb is slightly lower with cyclohexene (80% de). styrene (80% de) and allyl acetate (80% de)32. 

The two double bonds in 2,3-dimethoxycarbonyInorbomadiene are almost equally active. Furthermore, the reactions with methylenecyclopropane are stereoselective leading exclusively to the exo-isomers. Both observations are in striking contrast to the results obtained in the Pd(0) catalyzed cycloadditions of 2-[(tri-methylsilyl)methyl]allyl acetate with norbornadiene derivatives The last observations automatically lead to the conclusion that non-activated alkenes also could undergo these reactions. Indeed it was found that ethylene, norbornene, norbornadiene and allene react with methylenecyclopropane to give cycloadducts (Scheme 7). The reason for the limitation to these alkenes lies in the ability of methylenecyclopropane to compete successfully with alkenes in n-complexation to the metal. Thus cyclodimerization of methylenecyclopropane is much faster than codimerization with other alkenes, which give less stable rt-com-plexes with Pd(0). 

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