Homoallyl acetates

Scheme 11.10. INAS reaction of homoallyl acetates of type 34. For details, see Table 11.6.

Yadav JS, Reddy BVS, Madhuri C, Sabitha G (2001) Indium(III) chloride catalyzed allylation of gem-diacetates a facile synthesis of homoallyl acetates. Chem Lett 18... 

Anzalone PW, Baru AR, Danielson EM, Hayes PD, Nguyen MP, Panico AF, Smith RC, Mohan RS (2005) Bismuth compounds in organic synthesis a one-pot synthesis of homoallyl ethers and homoallyl acetates from aldehydes catalyzed by bismuth triflate. J Org Chem 70 2091-2096... 

In conjunction with this, Jeong reported the cycloadditions of bis(allyl) and bis(homoallyl) acetals of alkynals leading to bicyclic lactols. Smaltz extended its utility to the synthesis of carbocyclic nucleoside by coupling with nucleophilic substitution of a 7r-allylic palladium complex (Equation (46)). ... 

The 2-aryl substituted cyclopropylcarbinyl cations have partial homoallylic character, whose contribution to the resonance hybrid increases when strong electron-withdrawing substituents (e.g. phenyl) are attached at the C2. Thus, 3-arylcyclobutyl tosylates on acetolysis give the homoallylic acetates predominantly, through the intermediate formation of the 2-arylcyclopropylcarbinyl cations (equation 21). 

Oxidation of allylic andhomallylic acetates (cf. 10,175-176).1 This system is an efficient catalyst for oxygenation of terminal alkenes to methyl ketones (Wacker process). Similar oxidation of internal olefins is not useful because it is not regioselective. However, this catalyst effects oxygenation of allylic ethers and acetates regioselectively to give the corresponding /i-alkoxy ketones in 40-75% yield. Under the same conditions, homoallylic acetates are oxidized to y-acetoxy ketones as the major products. 

A stereocontrolled synthesis of 2,4,5-trisubstituted tetrahydropyrans 331 can be achieved via a Lewis-acid-catalyzed intramolecular Prins cyclization of homoallylic acetals 332. Incorporation of a variety of substituents at C-4 of the resulting tetrahydropyrans is possible by simple variation of the reaction conditions (Equation 141, Table 13) <2001CC835>. 

Homoallyl acetates were oxidized to form the corresponding y-acetoxy ketones with high regio-selectivity. The results are shown in Table 4. In this oxidation, small amoimts of B-acetoxy ketones were sometimes formed (<10%). 

P-Alkoxy kettmes and y-acetoxy ketones prepared by the oxidation of allyl ethers and homoallyl acetates, respectively, are synthetically useful intermediates. The reaction of (K) in the presence of excess sodium methoxide with 2-methylcyclohexanone afforded methyloctalone (94) in 42% yield (equation... 

A different type of 1,3-acyl migration occnrs from homoallyl acetates (eqnation 88). In this case, npon coordination of the aUcyne with An(III) and migration of the acetate, a zwitterionic species is probably formed and collapses to afford the final prodnct. 

This rearrangement was a key step in a synthesis of the homoallylic acetate 2, a sex pheromone of the California red scale. ... 

The oxidation of cyclohexene has been the subject of considerable discussion, and it is now apparent that it behaves differently from the straight-chain olefins. Cyclohexene was originally reported to yield both cyclohex-2-en-l-yl acetate, structure (VII), and cyclohex-3-en-l-yl acetate, structure (VIII), in chloride-containing acetic acid (76) and only the allylic isomer with Pd(OAc)a in chloride-free acetic acid (6). However, it has now been demonstrated that if no oxidants are present to regenerate the Pd(0) to Pd(II) in neutral or basic HOAc, the Pd(0) formed will disproportionate the cyclohexene to give benzene (22, 295). In acetic acid containing perchloric acid, cyclohexanone (structure VIII) and cyclohex-1-en-l-yl acetate are formed (22). If Pd(0) is prevented from precipitating by use of oxidants in neutral or basic acetic acid, the allylic and homoallylic acetates are formed. 

Studies of the oxidation of 3,3,6,6-d4-cyclohexene (127, 295) indicated the allylic product arose from a 7r-allyl intermediate, whereas the homoallylic acetate arose from the mechanism suggested for oxidation of straight-chain olefins. Thus the allylic product was a 50 50 mixture of the two deuterium-labeled isomers, structures (IX) and (X), which would be expected from a symmetrical intermediate. An acetoxypallada-tion mechanism would have predicted only X. It has also been demon-... 

The homoallylic acetate contained all the deuteriums initially present in the deuterated cyclohexene. One of the deuteriums had been stereo-specifically transferred to an adjacent carbon. The stereochemistry of the product is most consistent with trans acetoxypalladation and cis Pd(II)-H(D) eliminations and additions (J27, 295) ... 

A recent study of the product distribution from cyclohexene in the presence of CuCl2 sheds some light of the stereochemical aspects of the reaction (114, 127). The products consisted of 1,2,- 1,3,- and 1,4-chloro- and diacetates plus the allylic and homoallylic acetates found in the absence of CuCl2. No enol acetate was detected. Only certain isomers were found. The diacetates were always the cis isomer, whereas the 1,3- and 1,4-chloroacetates were almost exclusively trans. The 1,2-chloroacetates were about in equal mixtures of cis and trans isomers. Also, deuterium-labeling studies indicated that acetoxypalladation was a trans process. Thus, the chloroacetates must be formed mainly by cis displacement of Pd(II) and diacetates by trans attack on the Pd(II)— carbon bond. For example, the scheme for formation of 1,2 and 1,3 products is shown in Eq. (99). 

As mentioned earlier, Heck found that reaction occurred with retention of configuration, which is consistent with phenylpalladation and hydride elimination having the same stereochemistry. Since free hydride or phenyl radical would not exist free in hydroxylic solvents such as acetic acid, it seems reasonable to assume that both are cis. Support for this view comes from studies with 3,3,6,6,-tetradeuterated cyclohexene (128). The main product was the homoallylic acetate which contained all four deuteriums originally present. Moreover, one of the deuteriums had... 

Cyanohydrins and homoallylic acetals. Reaction of MCjSiCN with aldehydes and allylsilanes with acetals are catalyzed by BiBrj at room temperature, whereas SbClj is completely ineffective. 

Using an alternative method, homoallylic iodides are efficiently transformed into cyclopropylmethyl acetates using silver acetate in anhydrous benzene (Table 1). y-Disubstituted or conjugated homoallylic iodides are particularly reactive and rearrange quantitatively. y-Monosub-stituted iodides afford mixtures of cyclopropylmethyl and homoallyl acetates in a 1 1 ratio, whereas the absence of a y-alkyl or aryl group in the homoallylic iodides leads to elimination, only. 

Indium(III) chloride catalyzes the allylation of gem-diacetates with allyltrimethylsi-lane to afford the corresponding homoallylic acetates (Scheme 8.131) [174]. Gly-cals react with silyl nucleophiles, e.g. allyltrimethylsilane, cyanotrirnethylsilane, and azidotrimethylsilane, in the presence of a catalytic amount of InBr, to give the corresponding 2,3-unsaturated allyl-, cyano-, and azidoglycosides, respectively (Scheme 8.132) [175]. 

The homoallylic acetal radical cyclization product reported by Stork [55] was later used in the construction of a significant portion of the gelsemine structure [76]. Further examples for homoallylic acetal cyclization exhibiting useful stereoselectivity include the (-)-protoemetinol synthesis by Fukumoto [77] and the rhizoxin partial syntheses by Rama Rao [78] and White [79]. In the synthesis of (+)-12b-epidevinylantirhine (117) [80] (Scheme 40), Ihara adopted low-temperature conditions for radical cyclization of the chiral unsaturated ester 114 in the presence of MAD. The lactone 116 in high diastereomeric excess was obtained from the cycli-... 

Carbonyl compounds in certain protected forms such as gcm-diacetates" and hemiac-etals (e.g., trifluoroacetaldehyde ethyl hemiacetal ) also undergo allylation, giving in these cases homoallylic acetates and homoallyl alcohols, respectively. 

Aziridination. Ethoxycarbonylnitrene generated in the presence of solid K2CO3 or CaO adds to alkenes. The rapid reaction, requiring no catalyst, is useful for the aziridination of allylic and homoallylic acetals, which are quite unreactive toward other conditions. The reaction probably occurs at the solid-solid interface. 

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