Esters allenyl

In the coupling of the allenyl ester 7 with a terminal alkyne, an electron-deficient phosphine (Ph3P) gave the enyne-conjugated ester 8 as the major product, while an electron-rich phosphine (TDMPP or TTMPP) yielded the non-conjugated enyne esters ( )- and (Z)-9[4],... 

The allenyl moiety (2,3-aikadienyl system) in the carbonylation products is a reactive system and further reactions such as intramolecular Diels-Alder and ene reactions are possible by introducing another double bond at suitable positions of the starting 2-alkynyl carbonates. For example, the propargylic carbonate 33 which has l,8(or 1.9)-diene-3-yne system undergoes tandem carbonylation and intramolecular Diels-Alder reaction to afford the polycyclic compound 34 under mild conditions (60 C, 1 atm). The use of dppp as ligand is important. One of the double bonds of the allenyl ester behaves as part of the dieneflSj. 

In the reaction of disubstituted alkynes, 1,3-migration of the acetate takes place to give allenyl esters that can be versatile substrates, especially for [3,3]-Cope rearrangements.333 1,5-Enynes have proved to be versatile substrates for the preparation of perfumery agents such as sabinol334 and sabina ketone.335 Transannular systems undergo similar reactions.336... 

A fundamentally different approach to the synthesis of 3-pyrrolines is evidenced in the annulation in Eq. 13.50 [58]. Ethyl 2,3-butadienoate 150 reacts with N-sulfony-limine 151 in the presence of triphenylphosphine under very mild conditions to give JV-protected 3-pyrroline 152 in 90% yield. The mechanism that has been postulated is related to that of the Baylis-Hillman reaction. Michael addition of triphenylphosphine to the allenyl ester generates a zwitterion that combines with the imine to give 153 in a non-concerted process. This is followed by ring closure, proton exchange and expulsion of triphenylphosphine to give 152. This annulation is successful only for aromatic or cinnamyl imines [59]. 

In spite of these first successful results, so far Stille cross-couplings have rarely reported employing functionalized stannylated allenes such as easily available donor-substituted allene 80a or allenyl esters such as 81 (Scheme 14.19) [19, 41, 42]. A single palladium-catalyzed annulation reaction with 80b as precursor leading to an a-pyrone derivative was reported [43],... 

Scheme 16.94 Cross-coupling reaction of allenyl esters and terminal alkynes.

The first observations, done by Tsuji et al. in 1980 on substituted acetylenes, were reinvestigated by Brandsma in 1994 and concern the alkoxycarbonylation of these substrates, not to give an acrylate moiety, but instead acetylenic esters [111,112]. The catalytic system involves PdCl2/CuCl2/NaOAc. Copper chloride introduced in stoichiometric amounts is responsible for the CH activation, maintaining the triple bond. Such an activation recalls that observed on allenyl- and propynylhalides that classically gives the allenyl-esters [113]. 

Enzymes such as pig liver esterase have been successfully applied in enantioselective hydrolysis of allenyl esters on a scale of 2 mmoles131. This provides the enantiomerically enriched allene-carboxylic acid as well as the ester of opposite configuration, by what is in fact a catalytic kinetic resolution (6-90% oy). Conversely, partial enantioselective esterification of /1-hydroxy-allenes (3-72% oy) employing lipases has been reported132,133. 

Transfer of chirality in aldol reactions has been attempted using / -allenyl ester enolates. These ambident nucleophiles have an axis of chirality, and such compounds have been less utilized in stereoselective reactions. They are prepared by transmetallation of the... 

The first examples of a Michael-Stork enamine addition to allenyl esters and ketones R1CH=C=C(R2)COX (X = alkyl, alkoxyl) has been reported. Mechanistic investigation revealed that 2 equiv. of enamine are required for optimum yields. In the case of an allenyl methyl ketone, cyclopentyl enamine addition afforded 8-oxobicyclo[3.2.1]octane, providing evidence for the in situ formation of an enamine intermediate following the initial Michael-Stork reaction.187... 

Both simple and fused poly substituted pyran-2-ones can be obtained from the reaction between substituted allenyl esters and ketones activated by an electron-withdrawing group. A base-catalysed nucleophilic conjugate addition affords a homoallylic ester and lactonisation completes the one-pot sequence (Scheme 19) <06S2731>. 

The allenyl esters formed by the carbonylation undergo an intramolecular ene reaction when an olefinic bond exists in a suitable position [12]. A particularly facile ene reaction is observed with these propargyl carbonates having an ester group attached to the triple bond. 

Under basic conditions, Pd(II) catalyses the bisfunctionalisation of allenyl esters by boronic acids and aldehydes in a one-pot reaction which yields 4,6-disubstituted 5,6-dihydropyran-2-ones 20. The tandem process involves attack by nucleophiles at the 3-position and by electrophiles at C-4 some uncyclised 5-hydroxypent-2-enoic acid derivatives can also be recovered <05JOC6848>. The aerobic cyclisation of O-alkenyl P-ketoamides affords 2,6-dihydropyran-3-ones (Scheme 7) <05OL5717>. 

Reaction of 1,2-allenyl esters and 1, catalyzed by potassium carbonate, gave 456a through a tandem nucleophilic addition-lactoniza-tion process (06S2731). The (chlorocarbonyl)phenyl ketene was condensed with 1 to give 456 (R = Ph) (04T5931). 

While rearrangement of propargylic esters to allenyl esters is facile, denouement of such intermediates is highly dependent on the presence of other multiple CC bonds in juxtaposi-tion. In any event, the generation of polycyclic compounds in one synthetic operation deserves serious consideration of the method for exploitation in the construction of significant and complex target molecules. 

Addition to alk-4-yn-2-enoic esters gives deconjugated allenyl esters. The extended enolate ions can be methylated at C-2. Interestingly, allylation occurs at C-4, and the products contain a conjugate diene unit. 

Palladium-catalyzed carbonylation of allylic and propargylic compounds offers a potential tool of one-carbon homologation. Particularly, pd-catalyzed carbonylation of propargylic compounds further provides synthetically valuable transformations because of the high reactivity of the intermediary allenyl esters. 

Liang et al. reported a PtCla-catalyzed transformation of 3-(2-alkyl)phenyl-propynyl acetate 130 to prepare naphthalenyl acetate 131 (Scheme 50) [45]. The electrophihc Pt-allene complexes formed in situ worked as hydride acceptors. Mechanistically, the Pt(Il)-promoted [l,3]-OAc shift leads to the formation of platinum-activated allenyl ester 1, which undergoes a [l,5]-hydride shift to form 1,3,5-hexatriene II. A subsequent bir-electrocyclic ring closure affords intermediate... 

Zirconocene-derived allenyl enolate (6) was used for the synthesis of allenyl alcohol (7) (Equation 3) [4], Compared with the corresponding titanocene or lithium enolates derived from allenyl esters (5), zirconium enolates (6) gave alcohol (7) in better yields after oxidation with dimethyldioxolane. 

Synthesis of S yn-jfi-hydroxy-a-vinyl hydroxyl esters in 68-91% yields with excellent diastereoselectivities >40 1 and with 73-89% ee has been achieved via aldol reactions of aldehydes with [(lO l-O-KlAl-l-ethoxybuta-TS-dienoxy]-9-borabicyclo[3.3.2]decan-10-yl]-trimethyl-silane that is generated in situ by kinetically controlled 1,4-hydroboration of allenyl ester with lO-trimethylsilyl-9-borabicyclo [3.3.2]decane." ... 

The conjugate addition reactions of a-aminoalkylcuprates (derived from r-butoxy-carbonyl protected amines) with Q, /3-/3,y-allenyl esters occurs with preferred approach from the side opposite the substituent on the 4-position and results in trans alignment of the respective groups about the unconjugated double bond on the 1,4-adduct (Scheme 11). ... 

Basic conditions and a nucleophilic base such as PPh3 or DABCO were required to perform the sequence SN2/[2 -I- 2]-cycloaddition displayed in Seheme 5.52 [135]. The allenyl ester 149 reacts with allylamine 150, leading to diastereomerically pure bicycles 151. 

Propargylic esters 67 have been studied extensively in ttansition-metal catalysis [35]. Recently it has been found that these compounds were effectively converted to useful molecular skeletons by interaction with -ir-acidic metal catalysts. These reactions proceed via initial 3,3-rearrangement, leading to allenyl ester intermediate 68 or 1,2-acyloxy rearrangement to metal carbenoid intermediate 69 (Scheme 27.25). These rearrangement reactions have been extended to the corresponding phosphates and sulfonates. 

Numerous examples include the rearrangement of propargyl esters into allenyl esters (see also Silver(I)... 

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