Acyl-allenyl

In the Pd-catalyzed cross-coupling reactions of acylzirconocene chlorides with allylic halides and/or acetates (Section 5.4.4.4), the isolation of the expected p,y-unsaturated ketone is hampered by the formation of the a, P-un saturated ketone, which arises from isomerization of the p,y-double bond. This undesirable formation of the unsaturated ketone can be avoided by the use of a Cu(I) catalyst instead of a Pd catalyst [35], Most Cu(I) salts, with the exception of CuBr - SMe2, can be used as efficient catalysts Thus the reactions of acylzirconocene chlorides with allyl compounds (Table 5 8 and Scheme 5 30) or propargyl halides (Table 5.9) in the presence of a catalytic amount (10 mol%) of Cu(I) in DMF or THF are completed within 1 h at 0°C to give ffie acyl--allyl or acyl-allenyl coupled products, respectively, in good yields. ill... 

Acylzirconocene chlorides 78, which are easily available through the hydrozirco-nation of alkenes or alkynes with Cp2Zr(H)Cl and subsequent CO insertion, can be used as acyl anion equivalents Cu(I)-catalyzed reactions with propargyl compounds 77 afford allenyl ketones 79 (Scheme 3.40) [86]. The use of an excess of 77 (2 equiv. to 78) is important for the selective preparation of 79, which prevents an undesirable side reaction of the allenic products 79 with 78. 

The AlCl3-mediated acylation of but-l-en-3-ynes such as 106 to give mixtures of allenyl ketones of type 107 and 1,2-addition products such as 108 have been published several times (Scheme 7.16) [144, 145]. In the case of other substitution patterns of the starting enynes, the stereochemistry of the resulting allenyl ketones was investigated [146]. 

Lewis acid-catalyzed ene reactions proceed between allenyl sulfides, e.g. 330, and aldehydes 329 to afford cis-trans mixtures of 1,3-butadienes 331 (Scheme 8.90) [168, 175b], Similar ene reactions observed with imines such as 332 provide the corresponding allylamines [168,177]. It was also found that the ene reaction of 1-silylated allenyl sulfide 333 with various aldehydes (or acetals) furnishes a,/l-unsaturatcd acyl compounds such as 334 and 335 under BF3-etherate catalysis [175b]. 

An a-allenic sulfonamide undergoes Pd-catalyzed carbonylative cyclization with iodobenzene, affording a mixture of isomeric heterocycles (Scheme 16.12) [17]. The coupling reaction of an allene with a PhCOPdl species takes place at the allenyl central catrbon to form a 2-acyl-Jt-allylpalladium complex, which is attacked by an internal sulfonamide group in an endo mode, affording a mixture of isomeric heterocycles (Scheme 16.13). 

Other magnesium allenyl enolates, such as 22, obtained by transmetallation of the lithium species have been used successfully in the preparation of a,-unsaturated acyl silanes (equation 28). ... 

The most versatile synthesis of w./i-unsaUi rated acyl silanes involves the use of allene methodology, developed by a number of groups14,22. Deprotonation and silylation of allenyl ethers followed by hydrolysis gives rise directly to ,/3-unsaturated acyl silanes via their enol ethers, 1-alkoxy-l-trimethylsilylallenes (Scheme 43). Indeed, the first example of an a,/l-unsaturated acyl silane was prepared by such a route223, as was the first example of an allenic acyl silane (from a l-trimethylsilyl-l-trimethylsilyloxy-l,2,3-alkatriene)22b. 

In a further development of this approach, the synthesis of cr,/J-acetylenic acyl silanes has been achieved as shown in Scheme 4514. Oxidation of the 3-selenenyl allenyl ethers (16) with m-chloroperbenzoic acid at —78 °C gave the corresponding unstable selenoxides, which underwent in situ [2,3] sigmatropic shift producing acetals (17). Loss of selenenyl ester on work-up gave the cr,/J-acetylenic acyl silanes in ca 50% yields. 

The related Brook rearrangement of a-halo-a,jS-unsaturated acyl silanes produces silyloxy allenes (63), from which several sesquiterpenes have been synthesized (Scheme 106)21. Silyloxy allenes may also be prepared by the alkylation of silyloxy allenyl lithium reagents the acyl silane route is, however, less sensitive to solvent effects and other experimental parameters. An outline of the synthesis of dehydrofukinone (64), which elegantly exemplifies this methodology, appears in Scheme 107. 

Mononuclear osmium half-sandwiches, with rf-cyclopentadienyls and 7]5-indenyls alkenyls and allenyls with t/-M-C bonds, 6, 558 alkenyl vinylidenes, 6, 593 alkyl, aryl, acyl complexes, 6, 552 with alkylidyne complexes, 6, 599 alkynyl and enynyl complexes, 6, 567 allenylidene and cumulenylidene complexes,... 

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... 

Allenyl trialkylsilyl ethers 771 can be a-deprotonated with f-BuLi in THF at — 78 °C to give the allenyllithiums 77210901091. They underwent reverse Brook rearrangement to afford the silaacrolein enolates 773, which react with aldehydes and ketones to yield the a,/9-unsaturated acyl silanes 774 (Scheme 200). For enolizable aldehydes transmetallation with ZnCl2-TMEDA, and MgBr2 for ketones, provided better yields. 

Ethynyl carbinols (propargylic alcohols) such as 134 (Scheme 2.58) represent another important group of oxidation level 3 compounds. Their preparation involves nucleophilic addition of acetylides to the carbonyl group, a reaction that is nearly universal in its scope. Elimination of water from 134 followed by hydration of the triple bond is used as a convenient protocol for the preparation of various conjugated enones 135. Easily prepared O-acylated derivatives are extremely useful electrophiles in reactions with organocuprates, which proceed with propargyl-allenyl rearrangements to furnish allene derivatives 136. 

Allenyl-ketones pre-synthesised, or prodnced in situ from palladium(O)- or copper(I)-catalysed isomerisation of conjugated or non-conjugated ° alkynyl-ketones, can be cyclised to fnrans. The ring closure can be effected with palladium " or gold " catalysis. Acylation of silylallenes leads to a fnran directly. ... 

Diastereoselective additions. The y-substituents of allenyl carbonyl compounds exert stereodirecting effects on the Grignard reaction. On the other hand, the diastereoselectivity observed in the reactions of the aldehydes derived from a serine orthoester and 2-acyl-1,3-dithiane 1-oxides can be attributed to chelation control. 

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