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Photochemical isomerisation

The add-catalyzed cyclodehydration of (Z)- and ( )-6-hydroxy-a,p-unsaturared ketones offers a mild synthesis of substituted furans. In the case of ( )-olefins, photochemical isomerisation was found to accelerate the reaction <96TL6065>. Reaction of alkynyl(phenyl)iodonium tetrafluoroborates with tropolone in the presence of a base yields 2-substituted furotropones (Scheme 16, <96TL5539>). [Pg.129]

Photochemistry is used commercially in the synthesis of Vitamin A and Vitamin D3, Figure 3.14. Vitamin A is formed via a photochemical isomerisation process and the synthesis of Vitamin D3 involves a photochemical electrocyclic ring-opening step. The industrial relevance of this process is due to the lack of a viable thermal alternative and therefore, if unusual and useful chemistry is only possible via photochemistry, an industrial process may result, due to the high intrinsic value of the chemical product. [Pg.65]

Acyclic azo compounds are obtained by usual routes only as frans-isomers. Photochemical isomerisation provides an easy and effective way to the cis-isomers. The cfs-azo compounds are however thermally quite labile and decompose with elimination of nitrogen 41>, some of them even at —120 °C 5>. Activation energies for the thermal decomposition of cfs-azo-alkanes are of the order of 20... [Pg.67]

Some simple stilbenes isolated from the roots of Polygonum cuspidatum (Polygonaceae) showed moderate inhibitory activity of the bovine thymus P56 " tyrosine kinase activity, when angiotensin 1 was used as a substrate. The most potent were resveratrol (26) and its cis form obtained by photochemical isomerisation. Both compounds showed similar potency against rat brain PKC, whereas resveratrol monoglucosides were less active [81]. [Pg.854]

The photochemical isomerisation of 5-membered aromatic heterocycles is associated with either of two mechanisms depending on the structure of the heterocycle. One of these (scheme 1) involves... [Pg.192]

Photochemical isomerisation to the Z-isomer A synthesis ofmethoxatin Radical isomerisation to the E-isomer Stereospecific Interconversion of E and Z-isomers... [Pg.223]

The acid components required for the synthesis of the ester alkaloids, (+)-trachelanthic and viridifloric acids, were synthesised by stereospecific routes, since the relative stereochemistry of the asymmetric centres in these acids was still uncertain. Eventually they were obtained by appropriate stereospecific icis or trans) hydroxylation of trans-2-isopropylcrotonic acid (34), and its cis-isomer obtained by photochemical isomerisation. After resolution, (+)-trachelanthic acid was identified as the t/irco-2-isopropyl-2,3-dihydroxybutyric acid (35) the corresponding erythro-isomet was identified as viridifloric acid. [Pg.66]

The isomerisation of ( —)-lupinine (5) to (+)-epilupinine (6) has long been known to be possible by chemical methods, e.g. by refluxing natural (-)-lupinine in benzene with metallic sodium, but the yields are low and not consistently reproducible. A preferred method, which consistently gives yields of 20— 25 %, involves the photochemical isomerisation of lupinine in the presence of acetophenone as sensitiser. ... [Pg.88]

Photochemical isomerisation of [6](l,4)naphthalenophane (33) and [6](l,4)an-thracenophane (34) produces the corresponding Dewar valence isomers (35) and (36) respectively, and the efficiency of excited product formation in the adiabatic photocycloreversion of the bridged biplanemer (37) has been determined by the size of its side-chain substituents which influence the interchromo-phore distance in photoproduct (38). ... [Pg.154]

Different cyclophanes exhibit special behaviour in Z,E thermal as well as photochemical isomerisations, because of steric or torsion effects in comparison to azobenzene. Thus the S-C cyclophane is thought to be useful as an optical switch. For this reason its photokinetics have been examined in detail [54,172]. The structure of this compound is given by... [Pg.413]

A DFT-based computational study has established the thermal and photochemical isomerisation mechanisms from the terminal P-bound phosphinidene oxide complex [Ru(tpy)(bpy)(POPh)] to the corresponding O-bound, [Ru(tpy)(bpy)(OPPh)] . Thermal isomerisation was found to be both kinetically and thermodynamically unfavourable, while photoisomerisation can readily take place by either adiabatic or non-adiabatic pathways. The different absorption spectra of the two isomers and the bi-stability of the system make this complex a good candidate for photochromism. ... [Pg.118]

The stability of the 1,3,2,4-dithiadiazolyl radical is extremely sensitive to substituent and is susceptible to both thermal and photochemical isomerisation to the thermodynamically more stable 1,2,3,5-dithiadiazolyl radical (Section 12.1.1.1) via a bimolecular rearrangement." In a number of other instances the radicals appear indefinitely stable in dilute solution, but decomposition appears to occur at high concentrations. As a consequence few 1,3,2,4-dithiadiazolyl radicals have been isolated in the solid state. Of these the phenyl-ene-bridged fe/5(l,3,2,4-dithiadiazolyl) adopts a n -n dimeric structure in the solid state" while the mixed l,2,3,5-/l,3,2,4-fcw(dithiadiazolyl) has recently been reported (Figure 8)." This too adopts a n n bonding mode," although the dimers exhibit an unexpected behaviour with two sets of n -n interactions between 2 and 3 rather than the symmetric 2/2 and 3/3 dimer. [Pg.744]

Fluorescence spectroscopy-based biophysical studies of the interactions of 9, 43 and 44 with phospholipid vesicles (PLVs) demonstrated their propensity to bind to membranes, supporting the theory that these compounds are located in the cell membrane of the producing mollusc. The photochemical isomerisation and electrocyclisation reactions of the precursors 43 and 44 to form tridachiahydropyrone (9) were conducted in the PLVs. The racemic nature of the product isolated from these experiments indicates that if the cell membrane is the biological site of this transformation, then any enantioselectivity conferred in the natural system must be due to the presence of other bilayer components, such as integral membrane proteins, not present in the membrane models here used. This raises a limitation of liposomes as models of biological membranes—they are very simplistic versions of a far more complex system (Fig. 5.1). [Pg.88]

Stilbenes. Have the potential for photochemical isomerisation across the double bond, a reaction which has been widely studied. Addition of apprc riate groups inhibits isoinerisalion and some substituted stilbenes have very high fluorescence yields. Stilbenes are commonly used as optical brighteners and laser dyes, and also find use as phc hois and scintillators. [Pg.187]

Several new syntheses of vinylsilanes have been described. Tris(trimethyl-silyl)aluminium undergoes 5yn-addition to alkynes alternatively the same -isomers can be obtained by photochemical isomerisation of Z-1-alkenyl-silanes. Other methods described involve treatment of the lithium salts of hydrazones with trimethylsilyl chloride, Wurtz-type coupling with vinyl bromides, and reaction of acetylenes with a silyl-copper reagent followed by an electrophile. Using the hydrazone method, a route has been devised for 1,2-carbonyl transposition within ketones (Scheme 17). ... [Pg.242]

As a team, consider the following historic preparation of a tris(l,l-dimethylethyl) derivative of Dewar benzene, B, by the photochemical isomerisation of l,2,4-tris(l,l-dimethylethyl) benzene by van Tamelen and Pappas (1962). B does not revert to A via either a thermal or a photochemical electrocyclic mechanism. Formulate a mechanism for the conversion of A to B and explain the kinetic robustness of B with respect to the regeneration of A. [Pg.633]

Monocyclic aromatic compounds undergo remarkable photochemical rearrangements on irradiation, and provides photochemical isomerisation. For example an aromatic compound o-xylene an irradiation gives a mixture of o-, m-and p-xylenes. [Pg.243]

Mechanisms of the following photochemical isomerisations can be described as conversion of o-xylene into m-xylene and m-xylene into p-xylene, is accompained by 1, 2-alkyl shift. Similarly conversion of o-xylene into p-xylene and vice-versa is accompained by the 1, 3-alkyl group shift. [Pg.243]

There are many examples of photochemical isomerisation of p,7-enones which proceed with [1, 3] acyl shift. Photolysis of 2, 2-dimethylcyclohept -3-enone leads to a [1, 3] sigmatropic shift to a five membered ring ketone. [Pg.267]


See other pages where Photochemical isomerisation is mentioned: [Pg.744]    [Pg.25]    [Pg.474]    [Pg.141]    [Pg.68]    [Pg.287]    [Pg.236]    [Pg.251]    [Pg.250]    [Pg.262]    [Pg.224]    [Pg.42]    [Pg.153]    [Pg.153]    [Pg.210]    [Pg.243]    [Pg.282]    [Pg.48]    [Pg.170]    [Pg.394]    [Pg.537]    [Pg.96]    [Pg.230]    [Pg.242]   
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See also in sourсe #XX -- [ Pg.14 , Pg.239 , Pg.374 ]

See also in sourсe #XX -- [ Pg.242 ]




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Isomerisations

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