Alkenes isoprenoidal

Belt, S.T. Allard, W.G. Masse, G. Robert, J.-M. Owland, S.J. (2001) Structural charactaisation of Cjo highly branched isoprenoid alkenes (rhizenes) in the marine diatom Rhizosolenia setigera. Tarahedron Lett., 42, 5583-5. 

Kerogens isolated from the Fig Tree cherts produced very complex mixtures of pyrolysis products, dominated by a series of methyl branched alkenes with each member of the series having 3 carbon atoms more than the previous member. At each carbon number a highly complex mixture of branched alkanes and alkenes plus various substituted aromatic compounds was found. The highly branched structures may have actually incorporated isoprenoids originally present in the Precambrian microorganisms (Philp Van DeMent, 1983)6>. 

Reaction of (180, X = Cl) with methyl lithium in the presence of alkenes at ambient temperature leads to apparent carbene adducts (183), in this case derived from ring opening of (181) to the highly functionalised isoprenoid carbene (182). Surprisingly, the bromide (180, X = Br) reacts by a different course, leading to (184), apparently through initial lithium-bromine exchange followed by 1,3- rather than 1,2-elimination of LiCl 130>. 

White peach scale. Several scale sex pheromones have now been elucidated each of them possesses an asymmetric center and usually a trisubstituted alkene link within an isoprenoid framework (43). The structure of the white peach scale pheromone, R,Zb-II (Figure 8), lent itself to synthesis with another chiral starting material, namely limonene (44). Selective ozonlysis followed by workup with dimethyl sulfide-methanol provided a ketoacetal, III. Wittig methylenation followed by hydrolytic cleavage of the acetal gave a dienaldehyde, IV. Conversion of the aldehyde via the acid to an amide (45) with enantiomerically pure ot-methylbenzylamine permitted chromatographic assessment of the purity of the diene aldehyde (and the limonene). The required R-isomer of the diene aldehyde was >48% ee. 

Fig. 5a. A surface rinse of Murchison (benzene-methanol and -hexane at room temperature) contains 4 isoprenoid hydrocarbons (capitalized). For peak identifications, see Studier et al. (1972). b Benzene-methanol extract of the same sample after grinding still yields a variety of alkanes and alkenes, but no isoprenoids. Peak 218 preceding n-Ci7 is not pristane but an alkene. Apparently the heavier isoprenoids are surficial contaminants of terrestrial origin...

As an electrochemical reaction, Torii and co-workers demonstrated that the facile transformation of alkenes into allylic alcohols and ethers proceeded in the presence of a catalytic amount (10 mol%) of diphenyl diselenide (Scheme 15) [18]. Most of terminal co double bonds of isoprenoids undergo regioselective oxyselenenylation-deselenenylation to give frans-allylic alcohols in aqueous acetonitrile and methyl ethers in methanol. The addition of SOI salts improves chemical yields since SOI salts prevent the conversion of phenylselenenic acid (PhSeOH) into the inert phenylseleninic acid (PhSe02H) by both disproportionation and electro-oxidation. This method was also applied to intramolecular reaction to form -lactone in high yield. 

An alternative approach to the commonly utilized cleavage of allylic isoprenoid compounds (Scheme 16) using attack of +2e in place of H at a Pd° ir-allyl complex also requires a catalyst addition.Electrolysis of allylic acetates with Pb cathode and Pt anode in the presence of Pd(PPh3)4 in MeCN, gave moderate yields of the inner alkenes from both terminal and inner acetates (equations 92 and 93). Control of the regioselectivity was clearly dictated by the attack of H+ from the less-hindered side of the allylic carbanions. 

The natiue of the higlier-molecular-weight aliphatic hydrocarbons from the Murchison has been controversial. Kvenvolden et al. [33] detected a wide variety of coeluting isomers dominated by polycyclic aliphatic structures. Oro el al. [41] reported Cg to Cis metliyl and dimetliyl alkanes, alkenes, and cycloalkanes. Studier et al. [42] detected straight-chain alkanes with some isoprenoidal hydrocarbons. It is now widely believed that straight-chain alkanes were the dominant components of the ahphatic fraction in the solar nebula. However, Cronin and Pizzarello [43] analyzed organic materials from the Murchison under less environmental contamination and concluded lliat tire n-alkanes, methyl alkanes, and isoprenoid alkanes reported in the Murchison were terrestrial contaminants. Tliese authors identified Cis to C30 cyclic alkanes as the major indigenous alipliatic components. 

Highly branched C20, C25, C30 and C35 isoprenoidal alkanes and their alkene counterparts with 1—4 double bonds (Fig. 2.18) occur widely in Recent marine and... 

Belt S.T., MasseG., Allard W.G., Robert J.-M., Rowland S.J. (2001) c25 highly branched isoprenoid alkenes in planktonic diatoms of the Pleurosigma genus. Org. Geochem. 32, 1271-5. 

Huizinga B.J.,Tannenbaum E., Kaplan I.R. (1987) The role of minerals in the thermal alteration of organic matter—IV. Generation of re-alkanes, acyclic isoprenoids and alkenes in laboratory experiments. Geochim. Cosmochim. Acta 51, 1083-97. 

VolkmanJ.K., BarrettS.M., Dunstan G.A.(1994) C25andC30 highly branched isoprenoid alkenes in laboratory cultures of two marine diatoms. Org. Geochem. 21, 407-13. 

Many plant products, familiar to us because of their distinctive aromas, are isoprenoids, w/hich are alkenes having several double bonds. 

Alkenes and polyenes (alkenes with several carbon-carbon double bonds) are common in nature. Ethene, the simplest alkene, is a plant growth substance involved in fruit ripening, senescence and leaf fall, and responses to environmental stresses. Isoprenoids, or terpenes, are polyenes built from one or more isoprene units. Isoprenoids include steroids, chlorophyll and other photos)mthetic pigments, and vitamins A, D, and K. 

Among the alkenes listed as tobacco smoke components are several series of isomeric isoprenoid compounds, including the phytadienes (3247), the solanesenes (3297), and the squalenes (2175, 3297, 4033), plus several homologous series of monoalkenes (1144). 

Branched alkenes from bacteria The highly unsaturated isoprenoid squalene is the major constituent of the branched and cyclic fraction in the photosynthetic bacteria Rhodospirillum rubrum and Rhodomicrobium vannielii (Han and Calvin, 1969). 

In attempts to circumvent these difficulties, a related but conceptually discrete tail-to-head construction protocol has recently been developed. For discussions of some intricate aspects, it is useful to distinguish two modes of oligoisoprenoid construction, that is, head-to-tail (H-to-T hereafter) and tail-to-head (T-to-H hereafter) modes, where the head and tail of the isoprene unit are defined as the disubstituted (head) and monosubstituted (tail) ends of a trisubstituted alkene moiety, respectively. Thus, the protocol developed in 1980 represents the H-to-T mode of isoprenoid construction. The recently developed T-to-H construction protocol entails one-pot homologation cycles using (E)- and (Z)-l,4-diiodo-2-methyl-l-butenes (1 and 2, respectively, in Scheme 3). More specifically, it involves (i) Pd-catalyzed... 

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