Homoallylic and

A chemoenzymatic methodology has been developed using indium-mediated allylation (and propargylation) of heterocyclic aldehydes under aqueous conditions followed by Pseudomonas cepacia lipase-catalyzed enantioselective acylation of racemic homoallylic and homo-propargylic alcohols in organic media.192... 

Table 11.6. 1,2-Disubstituted cyclopropanols of type 35 from homoallyl and 3-butenyl carboxylates oftype...

Because dianion formation appears to be more important when lithium rather than potassium is used, many of the Birch reductions and reduction-alkylations of I and II that have been developed utilize potassium as the reducing metal. Piperylene is added prior to the alkylation reagent to consume any remaining metal and thereby prevent reduction of the alkylation reagent. In the event that the alkylation reagent is unstable to strong bases (e.g. homoallylic and arylethyl halides) LiBr is added to reduce the basicity of the reaction medium. 

On the basis of the mechanism proposed for the addition of the same TeOj/LiBr/HOAc system to single olefins, the addition to conjugated dienes probably involves the acetoly-sis of a homoallylic and an allylic telluroacetate (or teUiirohaUde) generated by the initial addition of a Te(IV) species and an acetate (or halide) anion to the conjugated system. 

The effect of the BF3 activation on the regioselectivity of the ring opening of vinylic and acetylenic oxiranes is dramatic, as in these conditions the reaction occurs exclusively on the allylic " or the propargylic " position, and still with inversion (Scheme 40). This excellent regioselectivity allows the direct stereospecific preparation of homoallylic and homopropargylic alcohols in excellent yields. In the case of acetylenic oxiranes, a remarkable difference in the reactivity of cis and trans oxiranes has been evidenced, the former being more reactive. 

The product mixture is more complicated when substituents are present on the allenes. When C4 and C5 of the penta-l,2-dien-5-yl system carry different substituents, an apparent exchange of the positions of these substituents occurs during cyclization, owing to competition between homoallyl and 1234 1243 rearrangement (see Section 2.3.).12 Moreover, the cis- and tow. -isomers are formed. This is exemplified in the product distribution obtained from the hydrolysis and acetolysis of 3,4-dimethyl-5-tosyloxypenta-1,2-diene (7, R1 — Me R2 = H) and 3-methyl-5-tosyloxyhexa-1,2-diene (7, R1 = H R2 = Me).12... 

Solvolysis of optically active 5-tosyloxypenta-l, 2-dienes leads to methylenecyclobutanols with inversion of configuration at the carbon bearing the leaving group, without loss of optical activity.13 Once again there is competition between the homoallyl and 1234-1243 rearrangement. 

The equilibrium between homoallyl and cyclopropylmethyl radicals favors the former (i.e. kf kQ) in the parent as well as in simple, alkylated systems (equation 38). Hence, to... 

Allylic and cis-homoallylic alcohols are epoxidized readily, but frans-homoallylic and bishomoallylic alcohols react slowly, if at all. The stereoselectivity in the epoxidation of acyclic allylic alcohols is the same as and is comparable to that observed with r-BuOOH/VO(acac)2. The stereoselectivity in epoxidation of acyclic homoallylic alcohols is also the same but lower than that obtained with t-BuOOH/ VO(acac)2. Epoxidation of cyclic allylic alcohols proceeds more slowly and in lower yield than that of acyclic allylic alcohols. 

This reaction is applicable to both homoallyl and diallyl ethers and results in multicyclic furanes. These products can be oxidized to novel butyrolactones.2 Examples ... 

Han and Widenhoefer have developed a Pd-catalyzed alkoxidation protocol to furnish 2,3,5-trisubstituted furans from easily accessible 2-allyl-l,3-diketones (Equation 24). Electron-rich and electron-poor aromatic groups as well as heteroaryl substituents give comparable good results. Symmetric 1,3-diketones with two identical alkyl or aryl substituents performed equally well. The reactions of homoallyl and 4-hexenyl-substituted 1,3-diketones also gave rise to furans, albeit in moderate yields <2004JOC1738>. 

Cahiez, Knochel, and co-workers have developed a mixed catalytic system consisting of MnBr2/GuGl and diethyl-zinc in iV,A -dimethylpropyleneurea (DMPU), which can be used for the stereocontrolled formation of tetrahydro-furan organozinc compounds from readily available unsaturated bromoacetals. The organozinc compounds are readily transmetalated with GuGN-2LiGl, and upon treatment with ethyl (o -bromomethyl)acrylate or ethyl propiolate homoallyl- and allyl-substituted bicyclic tetrahydrofurans are obtained in 71% and 63% yield (Scheme 70). 

HomoaUylic, Bis(homoallylic) and Tris(homo(dlylic) Alcohols 419... 

Maiti and Roy reported a selective method for deprotection of primary allylic, benzylic, homoallylic and aryl TBS ethers using aqueous DMSO at 90° C. All other TBS-protected groups, benzyl ethers, THP ethers as well as methyl ethers remain unaffected. 

Reasonable selectivities are also observed with some homoallylic and bishomoal-lylic alcohols. The most common competing reaction is observed with cyclohexenols with a quasiequatorial group and a hindered double bond. 

Further selected examples of the decarboxylative acetoxylation and methoxylations are listed in Table 7 [11-13,17,30,142-147]. The electrogenerated stable carbenium ions, such as benzylic, allylic, homoallylic, and tertiary carbocation intermediates, are likely to... 

The related (Z)-lithium dialkenylcuprates (147) derived from acetylene itself also react well with epoxides to provide a useful route to (2)-homoallylic alcohols the lack of reactivity with esters allows an easy access to lactones (148) by condensations between epoxy esters and this type of cuprate (Scheme 29). Likewise, the lower homologs (149) and (151), both of which are relatively easy to prepare in optically active forms, can be readily converted into homoallylic and bishomoallylic alcohols (150) and (152) respectively. An ester unit can also be incorporated into the cuprate functions thus, addition of a mixed lithium cuprate, RCuYLi , to ethyl propiolate gives the cuprates (153), which add to epoxides to give unexpectedly the (Z)-crotonates (154). Such isomerization is not uncommon with vinyl carbanions in general, and is obviously a limitation when isomeric mixtures are produced. 

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