Thermal isomerisation

The reaction of 2-bromo-5-nitrothiazole with weakly basic secondary aliphatic amines gave the expected 2-amino products. The isomeric 5-bromo-2-nitrothiazole with such amines gave mixtures of the expected 5-amino products along with 2-aminated 5-nitrothiazole rearrangement products. A mechanism was proposed which involves the slow thermal isomerisation of the 5-bromo-2-nitrothiazole to the much more reactive 2-bromo isomer which competes, in the case of relatively weak amine nucleophiles, with direct but slow displacement of the 5-bromo group to form the normal displacement product <96JHC1191>. 

Nguyen, M., Erancis, D., and Schwartz, S., Thermal isomerisation susceptibility of carotenoids in different tomato varieties, J. Sci. Food Agric., 81, 910, 2001. 

Enthalpies of gas-phase thermal isomerisations of 1-methyl- to 2-methyltetrazoles have been reported to be small (2.1-3.9 kJ mol ) (90ZPH656). 

Cernia et al.553) examined the effect of exposure on m-trans-isomerisation, using a high intensity mercury arc lamp with no filtering specified. Under these aggressive conditions irradiation at minus 196 °C produced a rapid increase in free-spin concentration and irradiation at temperatures up to 120 °C increased the rate of isomerisation above this temperature the thermal isomerisation is the dominant reaction. 

Chan FIWS, Levett G, Matthew JA (1978) Thermal isomerisation of methyl linoleate hydroperoxides. Evidence of molecular oxygen as a leaving group in a radical rearrangement. J Chem Soc Chem Commun 756-757... 

This photochemical/thermal isomerisation of the ruthenium-containing ring is accompanied by a dramatic geometrical change of the compound. Molecular modelling studies [92] suggest that the distance between the two... 

Fig. 16 Thermal isomerisation of the photoisomer 33pJoto and the photochemical back reaction. The pyridine ligand moves from an external position to an intra-ring location while the - (CH2)i8 - fragment undergoes a folding/unfolding process...

Various substituted naphtho[2,l-f>]thiopyran-l -ylidene-9H-thioxanthene derivatives 23 have been synthesised using diazo-thioketone coupling as the key step. The compounds function as light-driven molecular motors and studies of the photochemical and thermal isomerisation processes indicate a four-step unidirectional 360° rotation about the double bond of the upper half of the molecule with respect to the lower half <02JA5037>. 

A similar sigmatropic rearrangement occurs in the thermal isomerisation of the C-seco-triene (22), derived by photolysis of methyl dehydroursolate acetate 21), to give the triene (23) [15]. This hydrogen transfer is completely stereospecific in... 

The anti-rachitic factors resulting from the ultraviolet irradiation of cholesterol (40) and ergosterol (41) namely vitamin Dj (42) cholecalciferol and Dj, ergocalciferol (43) respectively are important dietary materials the chemistry of which was only elucidated by the investigations of many chemists (ref.41). Vitamin Dj is most easily derived by semi-synthesis from cholesterol through formation firstly of 7-dehydrocholesterol by reaction with N-bromosuccinimide followed by dehydrobromination with collidine. Ultraviolet light irradiation affords previtamin Dj which is thermally isomerised to the endo compound shown and thence to the exo... 

Gajewski, J.J. Hydrocarbon Thermal Isomerisations, Academic New York, 1981. 

Epimandol has been detected in the neutral extracts of Larix europaea and in the bark of the Sitka spruce. Cupressus resins have been examined by g.l.c. and imbricatolic acid (5) identified in all but Cupressus sempervirens. This acid co-occurs with the cis- and trans-communic acids and with sandaracopimaric acid. A -Communic acid has been isolated as its methyl ester from Callitris columellaris. During this work the thermal isomerisation of the conjugated diene of communic acid was noted. The structures and stereochemistry of some... 

Otfier Addition Reactions.—Hydroboronation of cholest-5-ene, with thermal isomerisation of the borane derivative prior to oxidation, afforded a mixture of cholestanols, reflecting the varying steric hindrance to substitution at positions in rings a and b (no products corresponding to migration of boron past C-8 were found). As expected, the equatorial 3 - and 2a-positions are the most favoured,... 

The isolation of the biologically active metabolite of vitamin D3 in a pure state, and the determination of its structure as 25-hydroxycholecalciferol (401) has been rapidly followed by its synthesis via irradiation of 25-hydroxycholesta-5,7-dien-3j -ol by three independent groups. The first synthesis of pre-calciferola (405) has been achieved by reaction of the lithium derivative of the chloro-ketone (402) with the en-yne (403) to give (404) which was treated with bis(ethylenediamine)chromium(ii) and then partially reduced catalytically. The thermal isomerisation of (405) (as 3,5-dinitrobenzoate) to vitamin D3 represents the first non-photochemical synthesis. 

NGUYEN M, FRANCIS D and SCHWARTZ s (2001), Thermal isomerisation snsceptibihty of carotenoids in different tomato varieties , J Sci Food Agric, 81, 910-917. 

With tert-buiy acetylene, the f/ -acyl Tp W fra/ - -OCCH = CH Bu (CO)2 forms as the major product, along with small amounts of the f/ -vinyl and alkylidyne complexes, Tp W( / -C Bu = CH2)(CO)2 and Tp W( = CCH2 Bu) (CO)2, respeetively. With Me3SiC = CCH3, the allyl complex Tp" W(ri -syn-CH2CHCHSiMe3)(CO)2 is produced in addition to a significant amount of the alkylidyne complex Tp W( = CEt)(CO)2, which is perhaps produced by hydrolysis of an f/ -vinyl intermediate. Thermal isomerisation of the -allyl complex to the... 

The cyclopropyl-hydrido complex 415 exhibits appreciable thermal instability, such that it rearranges quantitatively in ambient temperature benzene solutions to the rhodacyclobutane complex Tp Rh(cr,cr -CH2CH2CH2)(CNCH2 Bu) (154, Scheme 34, Section II-C.l). Thermal isomerisation of 154 (65 °C, 2.5 h) affords a 7i-propene complex (153) that is able to activate benzene. Alternatively, in the presence of excess CNCH2 Bu, 154 thermally inserts two equivalents of the isonitrile to ultimately afford the rhodacyclohexane complex 427 (Scheme 34). 

A similar synthesis of tropolone started from cyclopentadiene and tetrafluoroethylene the bicyclic adduct was thermally isomerised into a tetrafluorocycloheptadiene which was in turn converted into tropolone, the overall yield being 20% [254]. 

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