Cyclic carbonates, allylation reactions

Propargylic alcohol, after lithiation, reacts with CO2 to generate the lithium carbonate 243, which undergoes oxypalladation. The reaction of allyl chloride yields the cyclic carbonate 244 and PdC. By this reaction hydroxy and allyl groups are introduced into the triple bond to give the o-allyl ketone 245[129]. Also the formation of 248 from the keto alkyne 246 with CO2 via in situ formation of the carbonate 247 is catalyzed by Pd(0)[130]. 

Table 8.17 shows the scope of the reaction of acetylcobalt tetracarbonyl with polyenes. The reactions are regiospecific with the acetyl group adding to the terminal unsaturated carbon atom of the ir-electron system to produce the E-a,p-unsaturated ketones [9]. In the reaction with fulvenes [10], only the 1-acetyl and 1,4-diacetyl derivatives are formed, with no evidence of the 2-isomer. This is an indication of the relative stabilities of the cyclic it-allyl complexes, compared with the exocyclic complex. It has been postulated that, in the reactions of conjugated systems, the initial o-allyl adduct proceeds to the products via the it-allyl complex (cf Scheme 8.1), whereas in the case of unconjugated tt-systems, the initial o-adduct is more stable and tends to undergo a further carbonylation reaction. 

A study of the Pd-catalyzed resolution of allylic substrates with thiols was undertaken to see whether they could function as nucleophiles [11]. The cyclic carbonates rac-laa and roc-lba and the acyclic carbonates rac-3aa and rac-3ba were selected as substrates and 2-pyrimidinethiol as nucleophile (Schemes 2.1.4.5 and 2.1.4.6). The reactions of carbonates rac-laa and rac-lba with 2-pyrimidinethiol... 

As shown by Table 2.1.4.7, the highly enantio-enriched cyclohexenyl and cyclo-heptenyl sulfones 2aa and 2ba, respectively, were obtained in high yields by using Pd(0)/L (1.5 mol%) and BPA (4.5 mol%) (entries 1 and 2). In the case of the eight-membered cyclic carbonate roc-lca the selectivity of the kinetic resolution was so high that even after a prolonged reaction time the conversion of the allylic... 

The reaction is believed to proceed via a c-allyl copper complex, in which the carbon-copper bond is formed at the "/-position, anti to the acetate leaving group. Reductive elimination of copper led to pure "/-substitution. With cyclic aliphatic allylic acetates, the selectivity is generally lower because the o-allyl copper complex can isomerize to the ir-allyl complex with loss of regioselectivity. 

Various unsaturated compounds, such as CO2, isocyanates and aldehydes, undergo Pd-catalysed cycloaddition with vinyl epoxides. Reaction of CO2 with 127 affords cyclic carbonates 128 with retention of the configuration at C(3), offering a method of cis hydroxylation of epoxides [66], and has been used for the synthesis of the side-chain unsaturated (—)-exo-brevicomin (129) [67], The tetrahydrofuran 131 was prepared by [3+2] cycloaddition of activated alkenes such as benzylidene malononitrile (130) with vinyl epoxide via Michael addition and allylation [68],... 

Beckwith has observed moderate diastereoselectivity in this reaction involving cyclic systems. The cyclizations of allyl and homoallyloxycarbonyloxy radicals are potentially useful as radical based alternatives for an overall oxidation or hydrolysis of a double bond, and also various further transformations of the cyclic carbonates can lead to synthetically useful products.62b In contrast, simple alkoxycarbonyloxy radicals 91a add intermolecularly to ethyl vinyl ether to give, ultimately, carbonates of glycoaldehyde derivatives 91b, Scheme 37.62a... 

Allene carboxylic acids have been cyclized to butenolides with copper(II) chloride. Allene esters were converted to butenolides by treatment with acetic acid and LiBr. Cyclic carbonates can be prepared from allene alcohols using carbon dioxide and a palladium catalyst, and the reaction was accompanied by ary-lation when iodobenzene was added. Diene carboxylic acids have been cyclized using acetic acid and a palladium catalyst to form lactones that have an allylic acetate elsewhere in the molecule. With ketenes, carboxylic acids give anhydrides and acetic anhydride is prepared industrially in this manner [CH2=C=0 + MeC02H (MeC=0)20]. 

Cyclic iodo carbonates. Allylic and homoallylic alcohols are converted regio-and stereoselectively into five- and six-membered cyclic iodo carbonates, respectively, by carboxylation of the alkoxide followed by reaction with iodine. These products can be converted into cis-1,2- or 1,3-diols as illustrated.2... 

Alfylations. A direct preparation of allyltin reagents from allyl halides is accomplished by treatment with SnCl. Allytributylstannane is activated by SnClj in MeCN for reaction with carbonyl compounds and imines. Allylic alcohols and cyclic carbonates can also be used as allylating agents for amines and aldehydes, respectively. 

Chiral allylic cyclic carbonates such as 551 or 552 undergo excellent regioselective alkylation reactions with soft nucleophiles in the presence of palladium(O) in refluxing THF to provide ( )-allylic alcohols. The reaction of 4-0-benzyl-2,3-isopropylidene-L-threose (167) with the appropriate ylid, followed by deprotection of the isopropylidene ring with acidic resin and cyclic carbonate formation, provides a good overall yield of either 551 or 552. Ring opening with diethyl malonate in the presence of tetrakis(triphenylphosphine)palladium(0) provides in excellent yield the allylic alcohols 553 or 554, where the diastereoselectivity exceeds 99%. This reaction represents an efficient method of 1,3-chirality transfer [180] (Scheme 122). 

These chiral allylic cyclic carbonates are also capable of reacting with carbon, oxygen, or sulfur nucleophiles to provide products whose regio- and diastereoselectivities depend on the nature of the nucleophile. The reaction of 551 with phenol in the presence of palladium(O) and triethylamine provides in 79% yield 555, whereas similar reaction with sodium benzene-sulfinate furnishes the ( )-allylic alcohol 556 in 80% yield. However, sodium thiophenoxide, under conditions that do not lead to catalyst poisoning, attacks proximal to the oxygen atom with inversion to afford the threo-P-hy oxy sulfide 557 in 74% yield. This reaction is not observed in the absence of a palladium catalyst [181] (Scheme 123). 

Reaction with sodium borohydride was equally stereoselective, the stereochemistry of the resulting diol being evident from the ready formation of the cyclic carbonate in F9. An additional advantage of this derivative arose from a timely report by Sutherland [72] that allylic deoxygenation with compounds such as these proceeded more smoothly than with the corresponding acetates [73]. Indeed, reaction of F9 with a Pd(0) catalyst led smoothly to FIO and, as expected on the basis of steric and conformational grounds, epoxidation occurred exclusively from the convex surface to give Fll. 

Most allylic substitution reactions have been conducted witti derivatives of allylic alcohols, such as acetates, phosphates, and carbonates. These reactions occur with allyl electrophiles displaying a wide structural variation. The allylic electrophiles can be cyclic or acyclic, substituted with aliphatic or aromatic groups, substituted at one or both termini, and substituted or unsubstituted at the central carbon. As discussed in more detail below, these substituents affect the regioselectivity of the substitution process. 

As a final set of examples, enantioselective allylic substitution of unstabilized eno-lates to form a new stereocenter at the enolate carbon have been developed through the decarboxylative reactions of allyl enol carbonates. - - These reactions are enantioselective versions of reactions closely related to those in Equation 20.18 and Scheme 20.4, and two examples are shown in Equations 20.60 and 20.61. In these cases, a new stereocenter is formed at the a-carbon of the enolate nucleophile. Most of these reactions have been conducted with allyl enol carbonates that generate cyclic ketone enolates, but enantioselective reactions of acyclic allyl enol carbonates have also been reported. Although allyl enol carbonates undergo decarboxylation faster than the 3-keto ester isomers, the 0-allyl p-keto esters are more difficult to prepare, and enantioselective allylations starting with p-ketoesters have been reported. - Decarboxylative reactions of amines and a-amino acids have been conducted to form allylic and homoallylic amines (Equation 20.62), respectively, and enantioselective decarboxylative allylations of amides have been reportedIridium-catalyzed enantioselective decarboxylative allylation of amides starting with 0-allyl imides has also been reported. ... 

The other significant variation of the prototypical Kulinkovich reaction is the so-called Kulinkovich-de Meijere reaction, where de Meijere extended the substrates from esters to amides. Other carboxylic acid derivatives including (cyclic) carbonate, imides, and nitriles also react with the key Kulinkovich intermediate. Szymoniak developed an efficient new synthesis of cyclopropanes via hydrozirconation of allylic ethers (e.g., using Cp2Zr(H)Cl) followed by addition of a Lewis acid (e.g., BFa OEta). Casey et al. further investigated the stereochemistry of this interesting cyclopropanation reaction using deuterated allylic ethers. ... 

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" ... 

J. PALLADnJM(0)-CATALYZED REACTIONS OF ALLYLIC CYCLIC CARBONATES... 

One method to prepare cyclic carbonates of the structural type shown in Scheme 44 is by direct reaction of 1,2-diols with a phosgene synthetic equivalent, but they can also be prepared by Pd(0)-catalyzed reaction of vinylepoxides with carbon dioxide. Trost and Granja have found that the behavior of vinylepoxides and their related carbonates in Pd(0)-catalyzed allylations is different from the stereochemical viewpoint. ... 

A few Pd(0)-catalyzed reactions of aUyUc carbonates cannot be classified under Sects. A-O. Cationic Tj -allylpalladium complexes can be reduced by tin(II) chloride to afford a tin-allyl intermediate provided with nucleophilic reactivity (umpolung), which reacts with carbonyl groups (Scheme 62). Tm(II) chloride has been used by Masuyama and co-workers in Pd(0)-catalyzed reduction of aUylic carbonates,allylic cyclic carbon-ates, and even allylic alcohols,followed by reaction with carbonyl compounds. Related results with cyclic carbonates have been reported by Kang and co-workers Diethylzinc and samarium(II) iodide have been used as the reducing reagent. Protona-tion of the allylsamarium intermediate gives overall reduction of the C—O into C—H bond.f ... 

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