Allenol

Allenols can be converted into terminal vinylepoxides by a Pd(0)-catalyzed insertion of aryl or vinyl halides [81]. The reactions take place with high trans-selectivity and in good yields (Scheme 9.17a). Chiral 2,3-allenols, which can be easily... 

To support the 2,3-allenol as an intermediate in this reaction, 2,3-allenols were employed in test experiments as starting materials (Scheme 10, route F). The crossover aldol product was obtained as the sole product, when the reaction was run in the presence of the suitable aldehyde. In the absence of an aldehyde the corresponding ( )-a,p-unsaturated ketone was obtained (Scheme 24) [37]. 

Scheme 24 Coupling of 2,3-allenols and aldehydes promoted by iron(III) halides...

The electrophilic addition of I2 to 2,3-allenols 340 in Et20 was highly regioselec-tive with respect to the terminal C=C bond, leading to the diiodination products 341 with a preponderance of the Z-isomer. The diiodide 341 may be further converted to trans/cis vinylic epoxide 342upon the treatment with a base (Scheme 10.137) [159]. 

Friesen et al. reported that 2,3-allenols 340 can be converted to the corresponding carbamate derivatives 343, in which the terminal C=C bond of the allene moiety can be iodinated to afford diiodides 344. Under the catalysis by an Ag+ salt, compounds 344 react to give iminocarbonates 345 and oxazolindinones 346, leading to diols 347 and amino alcohols 348, respectively, after hydrolysis (Scheme 10.138) [160-162], A similar reaction was observed with trichloroacetimidates 350 [163],... 

Under the catalysis of 1 mol% of HgCl2, 4-methoxy-2-thiophenyl-2,3-allenols 365 and 367 can undergo cyclization to afford 3-phenylthiofurans 366 and 368 (Scheme 10.144) [169]. In this cycloelimination reaction, the methoxy or phenylthio substituent served as the leaving group. 

With hexylamine at room temperature, 4,4-difluorobuta-2,3-dienols also cyclize to 2,5-dihydrofuran 390 [176], A similar cydization was also observed with 2-benzotri-azole-substituted-2,3-allenol 391 [177]. 

The reaction of 2,3-allenols 398 with Ph2PCl afforded dienyl phosphine oxide 400 via the intramolecular nucleophilic attack of the phosphorus atom in 399 [179]. 

Recently, Trost et al. reported the vanadium-catalyzed addition reaction of 2,3-allenols [180], Here the oxygen in 401 served as an intramolecular nucleophile to attack the center carbon atom of allene to form a vanadium enolate 402. Aldol condensation of 402 with an aldehyde afforded (2-hydroxy)alkyl vinylic ketones 403. 

With NIS and aqueous NaHC03, the carbamates of tertiary 2,3-allenols 404 reacted to give 4,4-dialkyl-l-iodo-3-buten-2-ones 405 via the sequential intramolecular attack of the carbonyl oxygen or intermolecular attack of OH in 406 and iodina-tion [181]. 

Ma and Zhao reported a highly regio- and diastereoselective synthetic method for 2-amino-3-alken-l-ols and 4-amino-2-( )-alken-l-ols by the palladium-catalyzed reaction of 2,3-allenols, aryl iodides and amines (Scheme 16.24) [29]. Carbopalladation of PhPdl to the allene probably generates a thermodynamically more stable anti-Jt-allylpalladium species for steric reasons. Regioselectivity of the amine attack depends largely on the stereoelectronic effect on the a-substituents. 

In 2003, FUrstner and Mdndez reported an elegant reaction between optically active propargyhc epoxides and organomagnesium compounds in the presence of iron(III) acetylacetonate (Scheme 50). Interestingly, the chirality of the starting product is transferred to the final 2,3-allenols which are obtained with a good enantiomeric purity. 

Although the iron-catalyzed synthesis of allenes from propargylic halides was reported by Pasto and coworkers in 1978 [67], little progress was achieved in this field until recently [68]. In 2003, Fiirstner and coworkers discovered propargylic epoxides as valuable substrates for the reaction with Grignard reagents in presence of catalytic amounts of Fe(acac)3 to generate 2,3-allenol derivatives (Scheme 5.24) [69]. 

Scheme 5.24 Synthesis of 2,3-allenol derivatives developed by Furstner and coworkers.

Allenes, another class of compounds having interesting properties, have been also resolved by lipases, the kinetic resolution of a variety of racemic 1 -ethenyl and ethynyl-substituted 2,3-allenols was reported using a lipase from Candida antarctica type B (CAL-B) with vinyl acetate as acyl donors in organic solvent. The biocatalytic resolution afforded (S)-2,3-allenols (S)-98 and (i )-2,3-allenyl acetates (R)-99 in chemical yields up to 55% and an enantiomeric excess ee up to 99% for both enantiomers depending on the substituents.91... 

Figure 51 Lipase-catalyzed kinetic resolution of a variety of racemic 1-ethenyl and ethynyl-substituted 2,3-allenols 105.91...

Xu, D. Li, Z. Ma, S. Novozym-435-catalyzed efficient preparation of (LS )-ethenyl and ethynyl 2,3-allenols and (lR)-ethenyl and ethynyl 2,3-allenyl acetates with high enantiomeric excess. Tetrahedron Asymmetry 2003, 14, 3657-3666. 

Fiirstner, A. Mendez, M. Iron-catalyzed crosscoupling reactions efficient synthesis of 2,3-allenol derivatives. Angew. Chem. Int. Ed. 2003, 42, 5355-5357. 

In 2003, Fiirstner s group reported applications of this methodology to total syntheses of some natural products, and to the stereospecific Sn2 reaction of alkyl and aryl Grignards with optically enriched propargyl epoxides to yield alkyl- and aryl-substituted 2,3-allenols with high stereochemical fidelity (equation 29). ... 

Allenols undergo a palladium(0)-catalyzed cyclization process to yield highly functionalized vinyl oxiranes in a diastereoselective manner (Equation 77) <1999JA7943>. Initial carbon palladation forms a palladium allyl intermediate that suffers subsequent attack by the internal oxygen nucleophile to generate the oxirane fragment. 

Reactions of 2,3-allenols and their derivatives, particularly those leading to O-heterocycles 06CJO1468. 

A cyclization of 1 -(indole-2-yl)-2,3-allenols in the presence of platinum(II) chloride to give polysubstituted carbazoles via 1,2-methyl or an 1,2-aryl migration has been reported to proceed with the intermediacy of a metal carbene intermediate which induces the carbon-carbon bond cleavage via 1,2-migration to give the products. ... 

Since 2002, Ma et al. have demonstrated the cross-coupling/cyclization of 2,3-allenoic acids 346 in the presence of other functionalized allenes, such as 1,2-allenyl ketones 368 [121] or 2,3-allenols 370 [122], affording differently substituted 2(5/f)-furanones 369 and 371. They also studied the cascade cycUzation/cross-coupling reaction of 2,3-allenoic acids in the presence of simple monosubstituted allenes, which yielded the stereodefined 4-(bromo-2( )-alken-2-yl)-2(5 0furanones Z-372 [123] (Scheme 6.94). 

Various propargylic dithioacetals react with organomagnesium compounds to yield substituted allenes. Alkynyl oxiranes like 137 lead to 2,3-allenols of type 138 under iron catalysis with good chirality transfer (Scheme 2-50). ... 

The nucleophilic attack of Grignard reagents to alkynyl oxiranes can be catalyzed by tris(acetylacetonato)iron(lIl) to give 2,3-allenols (Scheme 4-205). The stereochemical... 

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