Allyl acetates acetylation

Schemes 28 and 29 illustrate Curran s synthesis of ( )-hirsutene [( )-1]. Luche reduction58 of 2-methylcyclopentenone (137), followed by acetylation of the resulting allylic alcohol, furnishes allylic acetate 138. Although only one allylic acetate stereoisomer is illustrated in Scheme 28, compound 138 is, of course, produced in racemic form. By way of the powerful Ireland ester enolate Clai-sen rearrangement,59 compound 138 can be transformed to y,S-unsaturated tm-butyldimethylsilyl ester 140 via the silyl ketene acetal intermediate 139. In 140, the silyl ester function and the methyl-substituted ring double bond occupy neighboring regions of space, a circumstance that favors a phenylselenolactonization reac-...

Palladium-catalyzed coupling of allylic acetate 113 (Scheme 23) with diethyl malonate, or with acetylated diethyl tartronate, yielded the Boc-protected allylic amines 114 in 99 1 E stereoselectivity.1 "1 Saponification of 114 (R1 = H) and decarboxylation gave the Tyr- Ala alkene isostere 116 in quantitative yield, whereas 114 (R = OAc) was converted into the Tyr-Gly alkene isostere 115 in a two-step procedure in 62% overall yield. Considering the number of steps and the overall yield, this procedure is among the most efficient to prepare Xaatp, CH=CH]Gly dipeptide isosteres. 

Then, pseudo-p-DL-gulopyranose (14) was synthesized by hydroxylation of 2,5-di-hydroxy-3-cyclohexene-l-methanol triacetate (12), which was prepared by Diels-Alder cycloaddition of 1,4-diacetoxy- 1,3-butadiene (10) and allyl acetate (11), with osmium tetroxide and hydrogen peroxide and successive acetylation as the pentaacetate (13). Analogous hydrolysis of 13 in ethanolic hydrochloric acid afforded the free pseudosugar 14 in 33% yield from 12 [2] (Scheme 7). 

By changing the relative reactivity of the allylic leaving groups, namely acetate and the more reactive carbonate, either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. The Pd-catalysed reaction of the allylic acetate moiety of 150 with malonate affords 151. Acetylation of 151 and Pd-catalysed 1,4-elimination of... 

The best known example of this kind is the rearrangement of allyl acetates, a synthetically very useful reaction, especially in the case of tertiary systems (equation 10). These undergo isomerization sometimes even under the conditions of acetylation. OAerwise, Lewis acid catalysis, preferably with PdCl2(MeCN)2, will bring about the isomerization smoothly (equation 11). ... 

Thallium(i) azide and iodine converted 5a-cholest-2-ene into a mixture of three isomeric iodo-azides. Reaction of either a 17-methyleneandrostane (77) or a 17-methylandrost-16-ene (78) with thallium triacetate gave mixtures of the allylic acetates (79), (80), and (81). Oxymercuration-demercuration was more selective in giving, after acetylation, the 16/8-acetoxy-17-methylene compound (80) as the main product. 

Lindlar semi-hydrogenation to C1S vinyl alcohol 23, subsequent acetylation, and Pd-mediated rearrangement of the tertiary allylic acetate [64] provided access to 24 as a ca. 85 15 (E/Z)-mixture. Notably, full conversion in the preceding hydrogenation reaction is important, since the acetylenic acetate is a strong catalyst poison for the allylic rearrangement. 

S Synthesis from n-ribonolactone D-Ribonolactone has been converted to tre-hazolamine derivatives via the allylic alcohol 99, whose condensation with p-methoxybenzylisothiocyanate followed by anti-Markovnikov iodo cyclization with iodine afforded the iodo oxazolidinone 100 (82%) (Scheme 14). The latter was treated with a mixture of acetic anhydride and snlfnric acid followed by activated zinc to furnish the allylic acetate 101 (90%), which nnderwent inversion at C-2 nnder Mitsnnobu conditions and the resnlting alcohol was epoxidized to produce 102. Hydrolysis of the epoxide 102 followed by acetylation of the resulting triol 103 afforded 104, which was treated with CAN to furnish the triacetate 105. Finally, 105 was converted into hexaacetate 86 in three steps. 

The gradated reactivities of allylic acetates and carbonates have been exploited in synthesis. Thus chiral c/s-4-acetoxycyclohex-2-enol is converted into either one of the enantiomers of 2,4-cyclohexadien-l-ylacetic acid by direct displacement with sodiomalonate followed by acetylation, elimination, and saponification, or by displacement of the derived carbonate followed by elimination of the unreacted acetate. 

The reactions of allyl alcohol are in accord with the structure assigned to it. The presence of a hydroxyl group is shown by the fact that the substance reacts with sodium with the evolution of hydrogen, and by the fact that allyl acetate is formed when it is treated with acetyl chloride. The presence of a double bond in allyl alcohol is shown by the fact that it unites readily with two atoms of chlorine, bromine, or iodine. It shows, in general, the reactions which are characteristic of ethylene and its homologues. The structure of the alcohol follows from the reactions which have been mentioned, and from the fact that by careful oxidation it can be converted into an aldehyde and an acid which contains the same number of carbon atoms as the alcohol. It contains, therefore, a primary alcohol group. 

Chiral allylic acetates 426 can be prepared using a similar j5-ketophosphonate (425), also derived from lactic acid. The desired 425 is formed via reaction of lithiated diphenylphos-phonate with 401. Reduction of the ketone gives an intermediate alcohol which, upon treatment with base, forms the ( )-Wittig olefin. Removal of the silyl protecting group followed by acetylation gives the product 426 (> 98% ee) [133]. 

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