Allenecarboxylates

One recent example of preferential [2+2] cycloaddition of dienes is the reaction of 2-siloxybutadienes with allenecarboxylates to afford cyclobutanes used for the preparation of very hindered cyclohexene systems [22]. 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

Scheme 4.59 Synthesis of chiral allenecarboxylates 233 using chiral diferrocenyl diselenides 231.

A variety of optically active 4,4-disubstituted allenecarboxylates 245 were provided by HWE reaction of intermediate disubstituted ketene acetates 244 with homochiral HWE reagents 246 developed by Tanaka and co-workers (Scheme 4.63) [99]. a,a-Di-substituted phenyl or 2,6-di-tert-butyl-4-methylphenyl (BHT) acetates 243 were used for the formation of 245 [100]. Addition of ZnCl2 to a solution of the lithiated phos-phonate may cause binding of the rigidly chelated phosphonate anion by Zn2+, where the axially chiral binaphthyl group dictates the orientation of the approach to the electrophile from the less hindered si phase of the reagent. Similarly, the aryl phosphorus methylphosphonium salt 248 was converted to a titanium ylide, which was condensed with aromatic aldehydes to provide allenes 249 with poor ee (Scheme 4.64) [101]. 

H. Tanaka, S. Sumida, K. Sorajo et al., Ni/Pb Bimetal-redox mediated reductive addi-tion/cyclization of allenecarboxylate with allyl bromide in a electrolysis media in Novel Trends in Electroorganic Synthesis (Ed. S. Torii), Kodansha, Tokyo, 1995, pp. 193, 194. 

A new electrolysis system comprising two metal redox couples, Bi(0)/Bi(III) and A1(0)/A1(III), has been shown to be effective for electroreductive Barbier-type allylation of imines [533]. The electrode surface structure has been correlated with the activity towards the electroreduction of hydrogen peroxide for Bi monolayers on Au(III) [578], Electroreductive cycliza-tion of the 4-(phenylsulfonylthio)azetidin-2-one derivative (502) as well as the allenecarboxylate (505) leading to the corresponding cycKzed compounds (504) and (506) has been achieved with the aid of bimetallic metal salt/metal redox systems, for example, BiCh/Sn and BiCh /Zn (Scheme 175) [579]. The electrolysis of (502) is carried out in a DMF-BiCh/Py-(Sn/Sn) system in an undivided cell by changing the current direction every 30 s, giving the product (504)in 67% yield. 

The synthetic methodology based on the reactions of these highly functionalized methylenecyclopropanes, which can be regarded as allenecarboxylate homologues, has steadily been growing over the past ten years [5] it thus appeared timely to report the considerable progress in this field. 

Allenecarboxylic acid esters reacted with 2 equiv of an aromatic aldehyde at the terminal double bond. The reaction was mediated by trimethylphosphine <20050L1387>. 4-Alkylidene-m-2,6-diaryl-l,3-dioxanes have been obtained in good to excellent yields (Equation 79). 

Somewhat better results are obtained by employing a chiral phosphonate in reactions with alkyl(phenyl)ketenes, which results in destruction of the chiral phosphonate and in the enantioselective formation of allenecarboxylic esters with 15-20% optical purity114-115. 

A related synthesis is the reaction of a racemic chiral phosphonium ylide with enantiomerically enriched (—)-(/t)-2-phenylpropanovl or -butanoyl chloride in a 2 1 molar ratio, which results in partially resolved (excess) phosphonium ylide and in the enantioselective formation of (Af)-allenecarboxylic esters116. 

Similar reactions were also achieved by the formation of diastereomeric optically active selenoxides as intermediates in the elimination reaction. Optically active ferrocenyl diselenide 19 was used in selenenylations of alkynes generating vinyl selenides of type 164. Oxidation of the selenides was performed with mCPBA under various reaction conditions which afforded the corresponding chiral selenoxides, which, after elimination, afforded axial chiral allenecarboxylic ester derivatives 165 in high enantioselectivities (R = Me 89% ee, R=Et 82% ee, R = C3H7 85% ee) (Scheme 47)>85 87... 

The silver-catalyzed cycloisomerization of allenic carboxylic acids to butenolides was also reported by Marshall and co-workers.329 Starting from the enantiomerically enriched propargyl mesylate 390, palladium-catalyzed hydroxycarbonylation led to the chiral allenecarboxylates 391 which afforded the butenolides 392 upon treatment with silver nitrate (Scheme 114). Unfortunately, partial racemization could not be avoided in this two-step sequence. In a related study, Ma and Shi330 have shown that the combination of Pd(PPh3)4 and Ag2C03 promotes the cyclization of allenecarboxylates to the corresponding butenolides, accompanied by the introduction of aryl or alkenyl groups at C3. 

Diastereoselective Selenoxide Elimination Producing Optically Active Allenecarboxylic Esters... 

The results of the following experiments give important clues for mechanistic considerations. Isolation of a trienecarboxylate ester as a hypothetical intermediate is not possible in most cases. As an exception, the trienecarboxylate 127 was isolated (64%) as a mixture with the cyclized product 128 (22%) after 4h reaction of 126. Compound 127 was converted to the cyclized product 128 on further exposure to the carbonylation conditions, showing that the allenecarboxylate 127 is a precursor of the cyclized product 128 (Scheme 11-36). Also, the carbonylation of the 1,4-enyne 129 gave only the allene carboxylate 130 (Scheme 11-37). No further carbonylation to give the cyclic ketone 131 or 132 was observed. In addition, the 1,3-diene ester 133 was recovered intact under the same carbonylation conditions without giving the cyclized product 134 (Scheme 11-38). 

Ethyl phosphonoacetate reacts with 3-keto-substituted thiophenes to give 81 which are precursors to bridged dithienylethylenes such as 82. The synthesis of a-ylidene-y-amidobutyronitriles, RCONH(CH2)2C(CN) = CR R" (R = NPh2, r2 = R" rz Me R = R = Ph, R" = H, Me, Ph), has been achieved by the reaction of the phosphonates RC0NH(CH2)2CHCNP(0)(0Et)2 (R = NPh2, Ph) with ketones. The enantioselective synthesis of allenecarboxylates is accomplished by asymmetric Homer-Wadsworth-Emmons reaction of chiral phosphonoacetate-... 

Under the same conditions, dichlorocarbene underwent addition to the more nucleophilic double bond of the ester of allenecarboxylic acid 11 to give cyclopropane... 

Two-bond proton-nitrogen-15 coupling constants in diazocompounds may be assumed to follow qualitatively trends observed for VC Ca-Ha) in allenes. Such an example is found for C NH) in diazomethane (42) (158) and ethyl diazoacetate (254) (94b). In 254 V( NH) is more positive than in 42, comparable to the situation in allene and allenecarboxylic acid ester. 

Penta-2,3-dienoic acid 3-methyl-l -allenecarboxylic acid) (70) Total electron density 367,368 ... 

The first examples of a Pd-catalyzed addition of terminal alkynes to allenes have been reported by Trost and Kottirsch (Scheme They observed that different palladium complexes efficiently catalyze the addition of terminal alkynes I to 3-substituted allenecarboxylates 23 to produce a mixture of the a,)8-unsaturated ester 24 and the two isomeric /S.-y-unsaturated esters (Z)- and (E)-25 (Scheme 9). Remarkably, the type of Pd catalyst used played a crucial role in determining the regiochemistry of addition to the 1,3-disubstituted allenes 23. Thus, with the Pd(OAc)2 TDMPP system (cat. A) a 76 24 ratio of 24 and E/Z)-25 was produced, whereas with a combination of tetrakis(carbo-methoxy)palladacyclopentadiene (TCPC) and TDMPP (cat. B) these products were obtained in a ratio of 9 91 (Scheme 9). ° ... 

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