Prenyl acetate, preparation

Isoprene epoxidized on the vinyl group (l,2-epoxy-3-methyl-3-butene, 14) would also be an attractive synthon for monoterpenoids if it were more readily accessible. Among the possible preparative methods (Scheme 2), acid-catalyzed isomerization of 2,3-epoxy-l-halo-3-methylbutanes (epoxidized prenyl halides) and dehydrohalogenation of the resulting isomers seemed the best route until chlorination of prenyl acetate (15, R = OAc) with hypochlorous acid was published. Dehydration of the mixture of products gave a maximum of the chloroacetate 16, which yielded the epoxide 14 with sodium hydroxide in methanol. ... 

A small number of hemiterpenes are used in perfumery, the most important of which are prenyl acetate and benzoate. Thioesters, such as those shown in Scheme 4.11, have extremely intense green odours reminiscent of galbanum, in which they occur naturally. The esters are usually prepared from prenyl chloride and the thioesters from the corresponding thiol and acid chloride. 

Typical of the methods available for the preparation of 7t-allylpalladium complexes is the preparation of the crystalline compound 70 by heating prenyl acetate 71 in acetic acid with PdCl2 in the presence of copper(II) chloride, followed by chromatographic purification. Alkylation of 70 with the anion derived from the Ci5-sulphone 72 is then carried out in DMF in the presence of at least four equivalents of triphenylphosphine (two per Pd) and gives the crystalline C2o-sulphone 73 from which vitamin A may be obtained by ethoxide-catalysed elimination of phenylsulphinic acid [40] (Scheme 16). Despite the moderate yield (52%) in the alkylation step and the use of stoichiometric amounts of palladium, this synthesis of vitamin A (7) avoids the lengthy functionalization process that is often necessary with more conventional methods of carbon-carbon bond formation. 

In addition to l-chloro-2-methyl-4-acetoxybutene-2 (130), other C5 building blocks have also been tested with the C15 sulfone (13) to synthesize retinyl acetate (9). When this concept is realized industrially, cost-efficiency calculations may indicate that another C5 building block has to be used, so that the equation given may change slightly. It was also possible to alkylate the anion of the C 5 sulfone (13) with the dimeric allylpalladium tt complex (135) in the presence of a large excess of triphenylphosphine, the complex (135) being prepared from prenyl acetate and palladium(II) chloride (Manchand et al., 1978). 

A synthesis of 95% pure all-rran5 vitamin A has been achieved by the ir-allylpalladium complex coupling of appropriate Cs + Cis units in 41% yield. The reaction is accomplished when a stoicheiometric amount of the crystalline Pd chloride complex of prenyl acetate is coupled to the sulphinate of vinyl-/8-ionol. All five possible nor-analogues of all-rran5-geranylgeraniol have now been prepared by conventional procedures. The n.m.r. data are usefully... 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

Azomethine ylides derived from (55,6/ )-2,3,5,6-tetrahydro-5,6-diphenyl-1,4-oxazin-2-one (53) and various aldehydes have been prepared by Williams and co-workers (87,88) (Scheme 12.19). In a recent communication they reported the application of the azomethine ylide 54 in the asymmetric total synthesis of spirotryprostatin B 56 (88). The azomethine ylide 54 is preferentially formed with ( )-geometry due to the buLkiness of the aldehyde substituent. The in situ formed azomethine ylide 54 reacted with ethyl oxindolylidene acetate to give the 1,3-dipolar cycloaddition adduct 55 in 82% yield as the sole isomer. This reaction, which sets four contiguous stereogenic centers, constmcts the entire prenylated tryprophyl moiety of spirotryprostatin B (56), in a single step. 

Isoprene reacts with dimethylallyl chloride in the presence of CuC1-A1203 to yield geranyl chloride, neryl chloride, lavandulyl chloride, and myrcene(12) 111 reaction of (39 X = C1) with antimony trichloride gives (E)- and (Z)-2,6-dimethylocta-2,5,7-trien-4-one.111 (E)- and (Z)-3,7-Dimethylocta-3,6-dien-l-ols, geraniol (8), nerol (18), and the dienol (40) are prepared from isopentenyl acetate and prenyl chloride... 

Ethyl geranate has been prepared stereoselectively under phase-transfer conditions by prenylation of ethyl -3-methyl-4-benzenesulphonylbut-2-enoate and desulphurization. Biomimetic synthesis of (39) from dimethylallyl acetate and isopentenyl acetate (1 3) catalysed by LiC104-Ac0H has been examined the corresponding alcohol (40) is obtained efficiently from isopentenyl acetate by... 

For the synthesis of sarcinaxanthin (441) in racemic form a biomimetic, acid-promoted prenylation reaction has been described. The key step was the alkylation-cyclization of geranyl acetate (214) with the isoprene epoxide 215 to give, in a mixture, 216 with the desired 2,6-cw-stereochemistry. The primary alcohol group was transformed to the corresponding mesylate which was dehydrated and subsequent hydrolysis gave 217. Conversion of 217 into the sulphone 218 was achieved by preparation of the corresponding mesylate, followed by... 

The carbon flow from 3-phosphoglycerate, phosphoenolpyruvate, pyruvate and acetyl-CoA. Even if the synthesis of aromatic amino acids by shikimate pathway /28,29,30,31/ and also prenyl-PP synthesis via mevalonate /32,33,34/ has been established in chloroplasts by identification of respective plastidic enzymes, it is still a matter of discussion from where PEP origins to supply DAHP synthesis of the shikimate pathway and from where pyruvate is delivered to supply the plastidic pyruvate dehydrogenase complex (for isolation see Treede and Heise, this Conference). Because phosphoglycerate mutase (PGM) to form 2-PGA from 3-PGA could not be detected in chloroplasts /35/ and acetyl-CoA is preferably synthesized from added acetate by the actetyl-CoA synthetase /36/, particularly in spinach chloroplasts, it was argued that chloroplasts are dependent on import of these substrates from the external site. Evidence for PEP formation from 3-PGA within the chloroplast could be obtained by three different approaches (D. Schulze-Siebert, A. Heintze and G. Schultz, in preparation D. Schulze-Siebert and G. Schultz, in preparation, for plastidic isoenzyme of PGM in Ricinus see /37/ and in Brassica /38/). 

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