Isopropenyl side chain

The isopropenyl side chain may derive by elimination of a tertiary alcohol or ether as in 202. Such a masking of the olefin avoids a possible competing vinylcyclopropane rearrangement. The correspondence of the cyclopentene of 202 with the vinylcyclopropane in 203 now becomes obvious. The presence of the dimethylcarbinol side chain now also offers the opportunity for its introduction by addition of a cyclopropyl anion to acetone. The feasibility of creating such an anion by fluoride initiated desilylation... 

The catalyzed ene reaction has been used for isoprenylation of monoterpenes with an isopropenyl side chain to form sesquiterpenes of the bisabolane group. An example is the three-step conversion of (+)-limonene (1) into / -bisabolene (2). ... 

Potent excitatory effects, parent of a family of toxic natural kainoids present in fungi. Domoic acid, which has trans, traws-CHMe-CH=CH-CH=CH—CHMeC02H in place of the isopropenyl side-chain of kainic acid, is extraordinarily toxic, with fatalities ensuing through eating contaminated shellfish (Baldwin et at, 1990). 

It is relevant to note that 2- C-mevalonate is incorporated specifically into the methyl group of the isopropenyl side chain of the lupane triterpene betulinic acid (348, R = C02H) (447). Hence, in the biosynthesis the two methyl groups in postulated intermediate (347) are not randomized. A related chemical observation is the acid-catalyzed equilibration of two C20 epimeric dammarane diol derivatives (12p-hydroxy-24,25-dihydro version of 320 and 345) (448). 

Franssen et al. [24] pointed out an alternative method of production of nootkatone from valencene catalysed by (-i-)-germacrene A hydroxylase, an enzyme of the cytochrome P450 monooxygenase type that was isolated from chicory roots. In general, this enzyme appeared to accept a broad range of sesquiterpenes and hydroxylates exclusively at the side-chain s isopropenyl group. Valencene is an exception it was not hydroxylated at the side chain, but -nootkatol was formed in the first step (Scheme 22.5) it is not yet clear if the second step is enzyme-catalysed. 

Some initiator may also remain unreacted. The side products formed, butyl isopropenyl ketone and methanolatemagnesium chloride, are not inactive under these conditions. They contribute to the formation of oligomers (by incorporation of dormant butyl isopropenyl ketone chain ends) and to the formation of syndiotactic polymer (by coordination of CHaOMgCl with propagating chain ends). 

Since polymers with tetrasubsltuted centers are difficult to synthesize, two main types of positive resists have been dlsclosed. Polymethylmethacrylate r polymethyl Isopropenyl ketone hich undergo side chain elimination and subsequent main chain fracture have been used as positive electron beam resists (cf. 

Plukenetione A (76) was the first PPB isolated with a adamantyl skeleton and an methylpropenyl group [65], Sampsonione D (118) and 1 (124) each have one isopentenyl side chain replaced by an isopropenyl moiety [11,13,14],... 

Only one major compound (SF-X) was present in the estrogenically active fractions, and this was isolated using semipreparative HPLC. The principles of separation are the same as for qualitative HPLC (see Section 25.4.1.2), with the difference that higher amounts of material can be loaded onto the ODS-column (Alltech, Econosil, Cig lOp, 250x22mm). For the identification of SF-x, a combination of spectroscopic techniques was used. Electrospray ionization in the mass spectrometer (HPllOO LC/MSD, Hewlett-Packard) with positive ionization mode gave a pseudo-molecular ion with m/z=439. H-NMR, C-NMR, DEPT, HMQC, and COSY spectra were recorded on a Varian-300 (300MHz) spectrometer. Analysis of the COSY spectram showed the presence of a lavandulyl (5-methyl-2-isopropenyl-hex-4-enyl) side chain. From the HMQC spectrum and a DEPT experiment, it appeared that SF-x possesses a disubstituted flavanone skeleton. 

Substitution of an alkyl side chain onto the isopropenyl groups of 1,4-diiso-propenylbenzene leads to substituted polyindanes with Tg=26°C and a thermal stability... 

Coupling of diazotized p-chloroaniline with 4-methyl- and with 4,6-dimethyl-pyrimidine occurs at the 4-methyl group, and not, as previously reported, at the 5-position. Pyrimidines bearing unsaturated carbon side-chains are uncommon species. Timely, therefore, are reports on the synthesis of the hitherto undescribed 2-isopropenyl-pyrimidines by dehydrobromination of 2-(2 -bromoprop-2 -yl)-pyrimidines and of 2-alkynyl-pyrimidines by coupling of 2-iodo-pyrimidines with monosubstituted alkynes in the presence of bis(tri-phenylphosphine)palladium(ii) chIoride[Pd(PPh3)Cl2] cuprous iodide, and tri-ethylamine. Nitration of some pyrimidine bases and nucleotides has been... 

Note Dry reaction conditions maximize the rate of the hydrolase-catalyzed reaction and minimize the competing hydrolysis of vinyl or isopropenyl acetate, which generates acetic add. Serine hydrolases are active only above pH 6, where the side chain imidazole of the active site histidine is in the uncharged form. Triethylamine prevents addification of the reaction mixture due to hydrolysis and may have other effects [34-36]. 

CM with the Hindered Olefin 1,1-Disubstituted and trisub stituted olefins do not usually dimerize even in the presence of [Ru]-II, and these olefins are categorized as type Iff or type fV. The sterically crowded olefin such as 2-methyl-2-butene, however, was found to be available as a CM partner by Grubbs and co-workers. Porco and Qi employed this CM partner for the synthesis of clusianone 105, which is a potential anti-HIV therapeutic agent isolated from the floral resin of clusia species (Scheme 24.25). The isopropenyl moiety was smoothly incorporated into the side chain of clusianone precursor 104 by CM reaction of 102 with 103 as a solvent. 

This cyclization proceeds with remarkable stereospecificity. The isopropenyl group seems to direct the approach of the end of the side chain to the unhindered side of the cyclopentene ring. 

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