Cycloadditions allyl esters

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

An interesting [3 + 2] cycloaddition reaction, of imines with TMS allyl esters catalyzed by Pd complexes, has been developed and produces pyrrolidine rings in good to excellent yields under mild conditions (equation 170)611. 

Some examples of photolytic intramolecular [2 + 2] cycloadditions which lead to the formation of lactones and lactams are sketched in Scheme 9.9. The allyl ester and N-monosubstituted amide do not undergo cydization, probably because of the high energy of the required E conformers. Cydization of the homoallyl ester does,... 

The following transformations demonstrate the characteristie feature of titanium alk-oxide-catalyzed transesterification. The isolated double bond does not enter into conjugation with the active methylene moiety (Eq. 218) [515]. Removal of a sterically demanding chiral auxiliary was possible without affecting the ketoester moiety (Eq. 219) [516,517]. Preparation of an allyl ester was achieved in good yield with retention of the nitrone moiety essential for subsequent cycloaddition (Eq. 220) [518]. 

The diastereoselective [4 + 3] cycloaddition between 2,5-disubstituted furans and vinylthionium ions was achieved (140L4476). The vinylthio-nium ion was generated either from a ort/io-phenylethynylbenzoyl allylic ester in the presence of a gold catalyst or from the allylic alcohol with a Bronsted acid. However, using 2-substituted furans led predominantly to the formation of Friedel-Crafts alkylated products. 

Although 2-[(trimethylsilyl)methyl]allyl esters and halides react with imines to give pyrrolidines (Scheme 22),the cycloaddition reaction with carbonyl compounds requires much more drastic conditions. On the other hand, 2-[(trialkyIstannyl)methyl]allyl acetate can react with aldehydes to give furanyl compound, where the presence of trialkyltin acetate, as a by-product, is crucial to compel the reaction successfully (Scheme 23). In fact, in the presence of the hialkyltin acetate, even the 2-[(trimethyl silyl)methyl]allyl ester undergoes [3 + 2] cycloaddition reactions with aldehydes (Scheme 24) and ketones successfully.Similarly, InCls is also an efficient additive for the cycloaddition reaction and both aldehydes and ketones are suitable as sub-strates. ... 

A family of interesting polycychc systems 106 related to pyrrolidines was obtained in a one-pot double intermolecular 1,3-dipolar cycloaddition, irradiating derivatives of o-allyl-sahcylaldehydes with microwaves in toluene for 10 min in presence of the TEA salt of glycine esters [71]. A very similar approach was previously proposed by Bashiardes and co-workers to obtain a one-pot multicomponent synthesis of benzopyrano-pyrrolidines 107 and pyrrole products 108 (Scheme 37). The latter cycloadducts were obtained when o-propargylic benzaldehydes were utihzed instead of o-allyhc benzalde-hydes, followed by in situ oxidation [72]. 

Scheme 6.186) [347]. The condensation of O-allylic and O-propargylic salicylalde-hydes with a-amino esters was carried out either in the absence of a solvent or - if both components were solids - in a minimal volume of xylene. All reactions performed under microwave conditions rapidly proceeded to completion within a few minutes and typically provided higher yields compared to the corresponding thermal protocols. In the case of intramolecular alkene cycloadditions, mixtures of hexa-hydrochromeno[4,3-b]pyrrole diastereoisomers were obtained, whereas transformations involving alkyne tethers provided chromeno[4,3-b]pyrroles directly after in situ oxidation with elemental sulfur (Scheme 6.186). Independent work by Pospisil and Potacek involved very similar transformations under strictly solvent-free conditions [348]. 

Acetoxymethyl)allyl]trimethylsilane (5) in the presence of a Pd(0) reagent, for instance, acts as an equivalent of trimethylenemethane in cycloadditions to electron-deficient alkenes such as a,p-unsaturated ketones, esters, nitriles, sulphones and lactones [7] (Scheme 6.6). 

Tamura et al. (170-172) discovered that, when reactions of ester-substituted nitrones with allylic alcohols are performed in the presence of an equimolar amount of titanium tetraisopropoxide under heating or at room temperature, transesterification takes place to form new nitrones bearing an inner alkene dipolarophile. The resulting nitrone substrates undergo regio- and stereoselective intramolecular cycloaddition reactions to give the ring-fused isoxazolidines (Scheme 11.52). This tandem transesterification/[3 + 2]-cycloaddition method leads to the selective... 

Achiral ester-substituted nitrones as well as chiral nitrones can be employed in diastereoselective asymmetric versions of tandem transesterification/[3 + 21-cycloaddition reactions, as shown in Scheme 11.54 (174). High diastereoselectivity and excellent chemical yields have been observed in the reaction with a (Z)-allylic alcohol having a chiral center at the a-position in the presence of a catalytic amount of TiCl4- On the other hand, the reaction with an ( )-allylic alcohol having a chiral center at the a-position, under similar conditions, affords very low selectivities. Tamura et al. has solved this problem with a double chiral induction method. Thus, high diastereoselectivity has been attained by use of a chiral nitrone. 

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