Michael-allylic substitution reaction

Scheme 3.9 Tandem Michael-allylic substitution reaction catalysed by a combination of chiral copper catalysis and palladium catalysis.

As shown in Scheme 2.20, selective lithiation of substrate 2-87 by treatment with LDA in THF at -78 °C triggers an intramolecular Michael/intermolecular aldol addition process with benzaldehyde to give a mixture of diastereomers 2-90 and 2-91. 2-91 was afterwards transformed into 2-92, which is used as a chiral ligand for Pd-catalyzed asymmetric allylic substitution reactions [29]. 

Baylis-Hillman carbonate is a good substrate for asymmetric allylic substitution reaction, and various nucleophiles have been involved in this transformation. As shown in Scheme 9.36, the intermediate 72 (mechanistically formed by Michael... 

The reaction of vinylic phenyliodium salts (57) with cyanide anions could be mistaken for a simple substitution reaction.59 However, the presence of both allylic (58) and vinylic (59) nitrile products suggests a more complex picture. Deuterium labelling experiments show that the allylic product is formed via the Michael addition of cyanide to the vinylic iodonium salt, followed by elimination of iodobenzene and a 1,2-hydrogen shift in the 2-cyanocycloalkylidene intermediate (60). H-shift occurs from the methylene carbon in preference to the methine carbon. The effects of substitution and different nucleophiles were examined. 

Furthermore, following an analogous methodology, combining the Morita-Baylis-Hillman reaction and the Trost-Tsuji reaction, Krische and co-workers have obtained allyl-substituted cyclopentenones 94 [84], Reaction was initiated by Michael addition of tributyl phosphine to an enone moiety 92, generating a latent enolate 93 which reacts intramolecularly with a jr-allylPd complex as the electrophile partner. A final -elimination step of trib-utylphosphine, favored by the presence of the methoxide ion, delivered the substituted cyclopentenones 94 (Scheme 36). 

Allylic alcohols can serve as 7t-allyl cation precursors to act as electrophiles in Sn reactions with a tethered O-nucleophile giving rise to the formation of spiroannulated tetrahydrofurans <2000TL3411>. Michael acceptors are also suitable electrophiles for the cyclization to tetrahydrofuran rings <2003T1613>. The Tsuji-Trost allylation has found widespread application in the synthesis of carbo- and heterocyclic compounds. Allylic substitution has been employed in the stereoselective synthesis of 2-vinyl-5-substituted tetrahydrofurans <2001H(54)419>. A formal total synthesis of uvaricin makes twofold use of the Tsuji-Trost reaction in a double cyclization to bis-tetrahydrofurans (Equation 73) <20010L1953>. 

The lithium cuprates 39, prepared from a- and P-2-deoxy-D-glucopyranosyl-stannanes a- and P-38 are configurationally stable and provide the corresponding Michael addition products 40 on reaction with methyl vinyl ketone [Eq. (14)] [27]. The cuprates a-39 [28] and 41 [29] have been used by Kocienski et al. for allylic substitution at q -molybdenum complexes. 

The method is quite useful for particularly active alkyl halides such as allylic, benzylic, and propargylic halides, and for a-halo ethers and esters, but is not very serviceable for ordinary primary and secondary halides. Tertiary halides do not give the reaction at all since, with respect to the halide, this is nucleophilic substitution and elimination predominates. The reaction can also be applied to activated aryl halides (such as 2,4-dinitrochlorobenzene see Chapter 13), to epoxides, " and to activated alkenes such as acrylonitrile. The latter is a Michael type reaction (p. 976) with respect to the alkene. 

ISOC reaction was employed to synthesize substituted tetrahydrofurans 172 fused to isoxazolines (Scheme 21) [44b]. The silyl nitronates 170 resulted via the nitro ethers 169 from base-mediated Michael addition of allyl alcohols 168 to nitro olefins 167. Cycloaddition of 170 followed by elimination of silanol provided 172. Reactions were conducted in stepwise and one-pot tandem fashion (see Table 16). A terminal olefinic Me substituent increased the rate of cycloaddition (Entry 3), while an internal olefinic Me substituent decreased it (Entry 4). 

A typical second step after the insertion of CO into aryl or alkenyl-Pd(II) compounds is the addition to alkenes [148]. However, allenes can also be used (as shown in the following examples) where a it-allyl-r 3-Pd-complex is formed as an intermediate which undergoes a nucleophilic substitution. Thus, Alper and coworkers [148], as well as Grigg and coworkers [149], described a Pd-catalyzed transformation of o-iodophenols and o-iodoanilines with allenes in the presence of CO. Reaction of 6/1-310 or 6/1-311 with 6/1-312 in the presence of Pd° under a CO atmosphere (1 atm) led to the chromanones 6/1-314 and quinolones 6/1-315, respectively, via the Jt-allyl-r 3-Pd-complex 6/1-313 (Scheme 6/1.82). The enones obtained can be transformed by a Michael addition with amines, followed by reduction to give y-amino alcohols. Quinolones and chromanones are of interest due to their pronounced biological activity as antibacterials [150], antifungals [151] and neurotrophic factors [152]. 

The retrosynthesis involves the following transformations i) isomerisation of the endocyclic doble bond to the exo position ii) substitution of the terminal methylene group by a more stable carbonyl group (retro-Wittig reaction) iii) nucleophilic retro-Michael addition iv) reductive allylic rearrangement v) dealkylation of tertiary alcohol vi) homolytic cleavage and functionalisation vii) dehydroiodination viii) conversion of ethynyl ketone to carboxylic acid derivative ix) homolytic cleavage and functionalisation x) 3-bromo-debutylation xi) conversion of vinyl trimethylstannane to methyl 2-oxocyclopentanecarboxylate (67). 

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