Allyl carbonates, 2- cycloaddition

Chemoselectivity in the cycloaddition of 2-methylenecycloheptenone (174) changes on addition of In(acac)3. The allylic carbonate 175 reacts with the ketone 174 in the presence of In(acac)3 to give the methylenetetrahydrofuran 176, and the methylenecyclopentane 177 is obtained in its absence[l 13], The cycloaddition of ynones to produce the methylenetetrahydrofuran proceeds smoothly only in the presence of In(acac)3 (10 mol%)[114]. 

Besides the formation of carbenes from diazo compounds and the hydroformyla-tion, rhodium (as described previously for palladium) has also been used as catalyst in domino processes involving cycloadditions. Thus, Evans and coworkers developed a new Rh(I)-catalyzed [4+2+2] cycloaddition for the synthesis of eight-membered rings as 6/2-105 using a lithium salt of N-tosylpropargylamines as 6/2-104, allyl carbonates and 1,3-butadiene (Scheme 6/2.22) [221]. The first step is an al-... 

Interestingly, treatment of the allylic carbonate 23, which had proven problematic in the previous study, under analogous reaction conditions with the copper enolate derived from 24 furnished the a,/9-disubstituted ketone. Subsequent ring-closing metathesis furnished the 1,2-cyclohexenes 25a/25b in 75% overall yield favoring the trans-dia-stereomer 25a (2° 1°=30 1, ds=10 l) [14]. Overall, this reaction provides an alternative approach to an exo-selective Diels-Alder cycloaddition, and indicates that a-substituted enolates are even more tolerant nucleophiles than the unsubstituted derivatives. 

The reaction of PhCH=C(CN)2 with the hydroxy allylic carbonate in the presence of catalytic amounts of Pd2dba3 in CHCI3 and (o-tolyl)3P in THF at 100°C gives the cycloaddition product, the oxepane, in 31% yield with low diastereoselectivity (dba = dibenz[ , ] anthracene) <2001JOC7142>. 

The 7r-allyl complex 165 is formed from the allyl carbonate 164 bearing an EWG. The dipolar molecule 166 is generated by deprotonation with the alkoxide. The frve-membered ring with exomethylene 169 is formed via 168 by the [3+2] cycloaddition with the alkene 167 [84], This cycloaddition proceeds under neutral conditions. For example, the reaction of the reagent 170 with 5,5-dimethylcyclopentenone gave rise to the bicyclic ketone 171 [85],... 

The palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a [42] and aziridine 20b [39] with the activated olefin 4a for the formation of five membered cyclic ether 21a and pyrrolidine derivative 21b has also been reported in our laboratories. The mechanistic issue is very much similar to that discussed in Scheme 9. Pd(0) catalyst added oxidatively to 20 to produce the 7r-allylpalladium complex 22. The Michael addition of a hetero nucleophile in 22 to the activated olefin 4a gives 23 which undergoes intramolecular nucleophilic attack on the inner 7r-allylic carbon atom to give the cy-clized products 21 and Pd(0) species is generated (Scheme 10). Similarly, the palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a with the N-losylimincs 24 is also known (Scheme 11) [43]. Intermolecular cycloaddition of vinyl epoxides and aziridines with the heterocumulenes such as isocyanates, carbodiimides and isothiocyanates is also known [44,45]. Alper et al. reported the regio- and enatioselective formation of the thiaolidine, oxathiolane, and dithiolane derivatives by the palladium-catalyzed cyclization reaction of 2-vinylthiirane with heterocumulenes [46]. 

Enantioselective cycloadditions of allylic carbonates 23a,b have also been examined with chiral phosphine ligands which led to reasonably high ee s up... 

The model correctly predicts (rationalizes) the observed preference for formation of Ae anti raAer than Ae syn product in the cycloaddition reactions of a wide variety of chiral allylic eAers, Aus successfully laying to rest years of frustrated discussion. - It also correctly predicts Aat as Ae size of R increases (Me - Ph < Et < Pr < Bu ), Ae preference for transition state structure (18 ) should increase leading to enhanced anti stereoselectivity (Table 15). At first, Ais prediction seems strange. However, once it is realized Aat, like Ae Felkin-Anh model for nucleophilic addition to a carbonyl, Ae outside position is actually more sterically demanAng Aan Ae inside, Aen Ae prediction is sensible on purely steric grounds. Thus, Ae angle d approach of Ae nitrile oxide oxygen to Ae alkene actually places it nearer Ae outside Aan Ae inside substituent located at Ae allylic carbon. ... 

Cyclopentenones. A [2 + 2 + l]cycloaddition involving an alkene, an allylic carbonate, and carbon monoxide occurs under the influence of the Ru(I) complex. 

VinyItetrahydrofurans were successfully synthesized via a palladium(0)-catalyzed cycloaddition of electron-deficient alkenes with allylic carbonates, as illustrated below 1J0C7142>. Another Pd(Il)-catalyzed cyclization of allylsilanes led to the formation of 2-vinyltetrahydrofurans <01CEJ4097>. 

Other resnlts obtained by Tsnji s group up to 1987 using allyl carbonates are carbony-lations to afford /3,y-nnsatnrated esters " (Scheme 3) (see Sect. VI.3), cycloadditions of trimethylenemethane palladium complex to afford methylenecyclopentanes (Scheme 3), and cycloadditions of zwitterions to afford substituted vinylcyclopentanes (Scheme 4) (see Sect. V.2.5.2). 2-Butene-1,4-diol dicarbonates afford a variety of cyclic strnctnres by reaction with dinncleophiles nnder Pd(0) catalysis (see Sect. I). The first case reported seems to be the formation of the vinylcyclopropane of Scheme 44" ... 

The TT-allylpalladium complexes 241 formed from the ally carbonates 240 bearing an anion-stabilizing EWG are converted into the Pd complexes of TMM (trimethylenemethane) as reactive, dipolar intermediates 242 by intramolecular deprotonation with the alkoxide anion, and undergo [3 + 2] cycloaddition to give five-membered ring compounds 244 by Michael addition to an electron-deficient double bond and subsequent intramolecular allylation of the generated carbanion 243. This cycloaddition proceeds under neutral conditions, yielding the functionalized methylenecyclopentanes 244[148], The syn-... 

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