Monoenes

For the synthesis of monoenes, three general methods may be employed involving. 

In earlier syntheses of the methyl ethers of (15 1)-cardanol (ref. 136) and urushiol (ref. 137) it is doubtful if the products were stereochemically pure. Nonetheless this work represents the first progress towards obtaining unsaturated methyl ethers of the phenolic lipids. Early difficulties centred on the lack of specificity in the reduction of the triple bond, in the removal of the protective group on the phenolic OH without affecting the double bond and, before kinetic and thermodynamic control was used, on lack of steric control in the Wittig reaction. 

In the cardanol series, 3-benzyloxybenzaldehyde was used leading by way of a lithium or magnesium-based Grignard to a diol, selective hydrogenolysis of which gave 3-(7-hydroxyheptyl)phenol. The bromide of this was found to react satisfactorily 

Reagents Mg or Li, CI CH2)60Thp, THF 4-TSA.MeOH H Ciii) PBra or I-Pd-BaS04, quin. 

In a similar way, 2,3-dimethoxybenzaldehyde with HO-protected 6-chlorohexan-1-ol and lithium afforded after acidic treatment 1,2-dimethoxy-3-(7-hydroxy-heptyl)benzene. Demethylation and bromide formation with boron tribromide gave 1,2-dihydroxy-3-(7-bromohepty0benzene which interacted with lithium-1-octyne in tetrahydrofuran containing hexamethylphosphoric triamide to give after selective catalytic hydrogenation, (15 1)-urushiol (ref. 138). In a different approach this has been synthesised in moderate yield from 2,3-dimethoxyphenyllithium and 1,7-dibromheptane as shown in the following scheme (ref. 139) 

CH St in aromatic hydrocarbons and three-mem-bered rings fall into the same range 

C=C St 1690-1635 Subranges Of variable intensity, weak for highly symmetric compounds, strong for N-C=C and o-c=c 

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The 1,5-dimethylcycloocta-1,5-diene can be partiaUy hydrogenated and the monoene then pyroly2ed to 2,6-dimethylocta-1,7-diene, which can be converted subsequentiy to citroneUol in several steps (138). 

Hydrogenation of monoenes, 123 Hydrogenation of ring A aromatic steroids, 138... 

Enolate trapping, 97 Epoxidation of steroidal dienes, 7 Epoxidation of steroidal monoenes, 2 4, 5/3-Epoxycholestan-3 -ol, 27 5a,6a-Epoxy-5a-cholestan-3 -ol, 82 5a,6a-Epoxy- 17-cy anoandr ost-16-en-3 jS-ol acetate, 20... 

In reductions of dienes to monoenes, it is important to monitor the hydrogen consumed and to stop the reduction at the stoichiometric point few reductions will stop spontaneously. 

Tricarbonylchromium complexes are useful for 1,4-addition of hydrogen to 1,3-dienes to afford monoenes selectively (40,42,43,44). With 1,4-dienes, isomerization into conjugation precedes hydrogenation. Isolated double... 

Dienes can sometimes be completely saturated under conditions where a monoene is not reduced, as illustrated by the data of Reuvers and deCroot (97). In fact, the monoene was still inert at 1200 psig, whereas the diene could be reduced at 28 psig. 

Grevilla pyramidalis A cardol monoene 5-tridecylresorcinol N.W. Australia... 

The oligomerization of cardanol with boron trifluoride etharate as the initiator was studied in detail by Antony et al. [171]. The reaction conditions were optimized by using gel permeation chromatography as 140°C with an initiator concentration of 1%. GPC data indicate conversion of all monoene, diene, and triene components into polymer except the saturated component, indicating participation of all the unsaturated components in polymerization. It is possible that the initiation of po-... 

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

Cyclic dienes and polyenes, monocyclic as well as bicyclic, can be metathesized in the same way as cyclic monoenes. As expected, cyclobutene 27), 1,5-cyclooctadiene, and 1,5,9-cyclododecatriene 28) yield the same polyalkenamer, in this case polybutenamer (1,4-polybutadiene), since these reactants consist of the same base units, i.e.—(CH2)2CH==CH— ... 

There is greater scope for structural variation in the diene based polymers than for the monoene polymers already discussed. The polymers contain units from overall 1,2- and cis- and 1,4-addition. Two mechanisms for overall 1,2-addition may be proposed. These are illustrated in Scheme 4.9 and Scheme 4.10 ... 

Propagation in cyclopolymerization may be substantially faster than for analogous monoene monomers.15" The various theories put forward to account for this observation are summarized in Butler s review.98 A recent theoretical study by Ttiziin et al.u3 looks at the effects of substituents on the rate of the cyclization step. 

The arm-first synthesis of star microgels by initiating polymerization or copolymerization of a divinyl monomer such as diviny lbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers.692 693 The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with NMP,692 6 ATRP69 700 and RAFT.449 701 702 The product contains dormant functionality in the core. This can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP ATRP704). 

A. Andreetta, F. Conti, and G. F. Ferrari Selective homogeneous hydrogenation of dienes and polyenes to monoenes, pp. 204-295. 

Using similar methodology, macrocycle 126 was prepared, as well as the unusual monoene 127 [76]. Considerable debate in the literature over the last thirty years has focused on whether dehydrobenzoannulenes are able to sustain induced ring currents [5al. Although fusion of arenes to the annulenic core provides rigidity and stability, this also weakens the diatropicity/paratropicity of the macrocycle significantly. Until quite recently, the number of planar systems available for study was limited however, with the the addition of 123 and 126, the series of alkyne-linked, tribenzo-fused dehydroannulenes is complete from... 

In cycloadditions two components are commonly involved, and the feasibility of a concerted process will be determined by whether overlap can take place between the HOMO of one component and the LUMO of the other. Thus for a diene plus a monoene,... 

Figure 10.5 SEM images of self-assembled gel from cardanol glycolipids. (a) GLY-2 in cyclohexane (b) EtOH-HjO gel of the monoene derivative I (c) EtOH-H20 gel of GLY-2. Reprinted with permission from [71] (2006) American Chemical Society.

Figurel0.6 Binary self-assembly of cardanyl glycosidesformingvaried nanofibers of varied helicity upon proper mixing of monoene (GLY-1 I) with GLY-2. Reprinted with permission from [72] (2002) Wiley-VCH Verlag GmbH.

Biosynthesis of Unsaturated Fatty Acids. In the mammalian tissues, the forma-tion of monoene fatty acids is only possible. Oleic acid is derived from stearic acid, and palmitooleic acid, from palmitic acid. This synthesis is carried out in the endoplasmic reticulum of the liver cells via the monooxigenase oxidation chain. Any other unsaturated fatty acids are not produced in the human organism and must be supplied in vegetable food (plants are capable of generating polyene fatty acids). Polyene fatty acids are essential food factors for mammals. 

The reason for the different behavior of dienes like 41 and monoenes 37 or 42 is not yet established. It is hard to believe that simple steric factors should make up for the different orientation of the olefin that approaches a metal carbene intermediate. More likely is stereochemical control by an ylide-type interaction between the halogen atom of the (sterically more flexible) monoenes 37 or 42 and the electrophilic metal carbene. 

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