Regioselective allylation

All that remains before the final destination is reached is the introduction of the C-l3 oxygen and attachment of the side chain. A simple oxidation of compound 4 with pyridinium chlorochro-mate (PCC) provides the desired A-ring enone in 75 % yield via a regioselective allylic oxidation. Sodium borohydride reduction of the latter compound then leads to the desired 13a-hydroxy compound 2 (83% yield). Sequential treatment of 2 with sodium bis(trimethylsilyl)amide and /(-lactam 3 according to the Ojima-Holton method36 provides taxol bis(triethylsilyl ether) (86 % yield, based on 89% conversion) from which taxol (1) can be liberated, in 80 % yield, by exposure to HF pyridine in THF at room temperature. Thus the total synthesis of (-)-taxol (1) was accomplished. 

Isoquinoline alkaloids. The regioselective allylation of N-acyl heterocycles (13, 10) can be used for synthesis of isoquinoline alkaloids. Thus simultaneous reaction of the dihydroisoquinoline (1) with a diunsaturated acyl chloride (2) and allyltributyltin affords the 1,2-adduct (3), which undergoes a Diels-Alder cyclization... 

Scheme 6.36. Regioselective allylic substitution of Z carbamate 170 with a silylcuprate reagent.

Calb et al. have thoroughly investigated the use of allylic electrophiles containing heterocyclic leaving groups in regioselective allylic substitution (Scheme 8.7) [22]. 

A study of substituent influences on reaction of carbonyl compound (102) with trialkylallyltin (103) has established that high yield of allylation product (104) is obtained only when X = NO2 and R = OH (where R = Bu or Me and R = H or Me). The role of the intramolecular hydrogen bond in promoting rate enhancement and regioselective allylation (and reduction by HSnBus) of carbonyl compounds has been discussed. 

Concerning the lithium-zinc transmetallation from intermediate 331, it has been used (THF, room temperature) to perform a palladium-catalyzed Negishi cross-coupliug reaction with aryl bromides - and a 8 2 regioselective allylation in the presence of copper cyanide. 346 and 347 were, respectively, obtained (Scheme 101). 

Risk labels, lATA/ICAO, 751-3 Risk phrases, 621, 748, 749 River water, peroxide determination, 642 RNA, ozone disinfection, 616 ROS see Reactive oxygen species Rose Bengal sensitized photooxidation, 890 Rotational barriers, regioselective allylic hydroperoxide formation, 836, 847-9 Rotational isomers, peroxynitrous add, 8-9 Rotational spectra, ozonides, 721, 722-3 RP-HPLC, hydrogen peroxide determination, 627... 

A combination of rhodium complexes and phosphates promotes a highly regioselective allylic alkylation of unsym-metric allylic esters, where alkylation occurs at the more substituted allylic terminus of the esters (Equation (46)). As Evans and his co-workers reported, both the regio- and stereochemistry of the starting allylic esters are maintained in the allylic alkylated products (Equation (47)). Thus, the rhodium-catalyzed allylic alkylation takes place at the carbon substituted by a leaving group with net retention of configuration. A variety of carbon-centered... 

SCHEME 74. Regioselective allylations with zinc-copper reagents... 

Simple, direct and regioselective allylation of 1,4-benzoquinones is an important objective in the synthesis of many isoprenoid quinones such as plastoquinone-1 and vitamin Ki, which play important roles in cellular metabolism and photosynthesis. In a new procedure where the use of a strongly coordinating solvent was found to play an important role, it was shown that reaction of 3 equivalents of (3-methyl-2-butenyl)trifluorosilane with one equivalent of 2,3-dimethyl-l,4-benzoquinone in formamide in the presence of 5 equivalents of FeCl3.6H20 for 23 hours at 40°C gave plastoquinone-1 1, in 90% yield. 

Yadav, J. S. Anjaneyulu, S. Ahmed, M. M. Reddy, B. V. S. Indium-mediated regioselective allylation of terminal epoxides a facile synthesis of bishomoallyl alcohols. Tetrahedron Lett. 2001, 42, 2557-2559. 

Yanagisawa, A. Habaue, S. Yamamoto, H. Regioselective allylation and propargylation using acylsilanes facile synthesis of PGE3 and F3a methyl ester. Tetrahedron 1992, 48,1969-1980. 

Marino, J. P. Nguyen, H. N. Copper-mediated regioselective allylation and propargylation of 2-(alkylthio)oxazoles. Tetrahedron Lett. 2003,... 

An enantioselective rhodium(II)-catalysed intramolecular cyclopropanation, follow- (g) ed by a regioselective allylic alkylation and a diastereoselective rhodium(I)-catalysed 5 + 2-cycloaddition has been reported.102 ... 

The bicyclic isoxazol-5-ones 45 and 47 (Scheme 12) were regioselectively allylated at the nitrogen atom to afford compounds 46 and 48 with three different allylic systems featuring primary and secondary electrophilic cen-... 

Similar results tQ those reported for 107 and 110 were found for benzotriazoles 113 and 116, which were efficiently, but not regioselectively, allylated. Many experimental conditions were tested (95JHC1325). However, mixtures of compounds 114 and 115 in one case and of 117-119 in the other were always formed (Scheme 24). Allylbenzotriazoles 114 and 115 do not equilibrate under Pd(0) catalysis. 

One important study on the regio- and stereoselectivity of the Pd(0)-catalyzed allylations of adenine 238 and the related 6-substituted 2-aminopurine derivatives 241 and 242 (Scheme 49) is that of Benneche and Undheim (92ACS761, 92TL1085). This group reported that adenine 238 is regioselectively allylated at N-9 both by allyl acetate and by rac-160 to afford 239 and rac-240 as the only isolated reaction products. However, when treated with allyl acetate, 241 and 242 afforded mixtures of N-allylation products at N-9 (243 and 244) and at N-7 (245 and 246). The ratios 243/245 and 244/246 indicate that the difference in reactivity between both positions is low. When both 241 and 242 were allylated with the more sterically demanding acetate rac-160, the ratio of N-9/N-7 allylation... 

AUylic acetoxylation.1 A new method for regioselective allylic oxidation of ger-anylacetone (1) is based on oxidation of the 7r-allylpalladium complex known to be formed selectively with the terminal allylic methyl group. The most satisfactory results are obtained... 

Copper salts such as CuCN, CuBr-SMc2 and CuCN-2LiCl were demonstrated to mediate the regioselective allylation, alkylation, and propargylation of the lithium anion of n-butylthiooxazole 128 providing 2,5-disubstituted systems 129. Desulfurization with deactivated W2-Raney nikel produced 5-substituted derivatives <03TL7395>. 

Kresze et al. found that the ene product 117, after diazomethane treatment, formed a mixture of the 0- and N-methylated compounds 118 and 119, respectively [83] (Scheme 32). Only a year later Delerit et al. used this possibihty to 0-silylate S-allylsulfinamides derived from ene reactions between olefins and N-sulfinyl sulfonamides to develop a regioselective allylic amination sequence (Scheme 33) [84]. After the ene reaction had occurred the resulting sulfi-... 

Recently, a new ruthenium catalyst that also provides regioselective allylic alkylation has been reported. In DMF, a highly branched alkylation product of cinnamyl carbonate vsdth malonate anion was obtained in quantitative yield (branched/line-ar = 14/1) within 30 min in the presence of 1 mol% of [(C5Me5)Ru(MeCN)3]PF6 catalyst (Eq. 5.30) [46]. 

A highly regioselective allylic substitution of allyl phosphates, with retention of the regiochemical integrity of the staring material (i.e., SN2-type alkylation, rather than Sn2 ) was reported [Eq. (Ill) 121]. This mode of substitution is in stark contrast to that of the copper-catalyzed one described in the foregoing section [see Eq. (54)]. 

Example of regioselective allylation from copper complexes... 

A noticeable effect of Cu+ on the reaction of 3-pyridyl borates 84 with allylic halides can be found regioselective allylation at the 3-position of pyridines 85 results (85H117). This is a common feature with benzo-fused pyridyl borates (87JHC377) (Scheme 33). 

The regioselective allylation of selected aldehydes as well as asymmetric carbon-carbon bond-forming reactions using the same carbonyl species are featured in Chapter 8. 

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