Z-Isomerisations

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

The isomers exhibit different absorption spectra The E-form displays an intense 7i7i band in the near UV range and a broad, symmetry-forbidden n7i band in the visible region. On isomerisation to the Z-form, the nn band is shifted to shorter wavelengths, with a concomitant increase of the nn band. The E->Z isomerisation engenders a pronounced structural change of the molecule, which is manifested in an increase of dipole moment (see also Section 8.8.1). 

Starting from l,3,5-tris(bromomefhyl)benzene as core unit, Vogtle et al. constructed the first dendrimer with intramolecularly accumulated azobenzene entities in the periphery by convergent synthesis (Fig. 8.25) it expectedly exhibits complex photoswitchable properties owing to the possibility of intramolecular E/Z isomerisation [79, 80]. 

A facile synthesis of two naturally occurring 3-deoxyanthocyanidins from 2,4,6-triacetoxybenzaldehyde has been described <03S1878>. A one-step route to pyranoanthocyanins, molecules that have been found in red wine <03TL4887>, involves initial reaction under aqueous conditions of cinnamic acids at the electrophilic 4-position of anthocyanins followed by an intramolecular cyclisation and decarboxylation <03TL7583>. A tri-( )-caffeoyl anthocyanin present in the blue petals of morning glory, Ipomoea tricolor, is resistant to ,Z-isomerisation by UV-B irradiation, a property that may be important in plant survival <03TL7875>. 

Luan, F., Mosandl, A., Munch, A. and Wiirst, M. (2005) Metabolism of geraniol in grape berry mesocarp of Vitis vinifera L. Cv. Scheurebe demonstration of stereoselective reduction, E/Z-isomerisation, oxidation and glycosilation, Phytochemistry, 66(3), 295-303. 

Vinyl anions derived from vinylsilanes have continued to develop as useful synthons. The lithio derivative (201), prepared by lithium-bromine exchange shows an excellent level of stereointegrity if kept below 0°C and reacts with a wide range of electrophiles. The organomagnesium derivative (202) was also examined but this anion shows a higher propensity towards E/Z isomerisation.176 Configuration instability is a drawback to the use of anions such as (201) and... 

Thermal electrocyclic ring opening of 2-oxopyran-5-carbaldehyde followed by E/Z isomerisation and electrocyclisation is proposed to account for their conversion to 3- and 5-acetylpyran-2-ones <97CC1293>. 

Joachim Buddrus from the Technical University of Berlin developed a variant of the Wittig reaction, by using an epoxide as HBr scavenger this method produces carotenoids ofhigh purity. [71] Thermal (Z/ )-isomerisation and crystallisation follow simultaneously under solvent-exchange (chloroform/ethanol). The yield of 93 % is the best that has been achieved in Wittig reactions of this type. 

HSQC measurements carried out by Nidiry " provided conclusive evidence for E- Z isomerisation of E- and Z-octadecyl p-coumarates and allowed the author to assign H and NMR signals to both isomers and to determine vicinal H-H couplings across a double bond and within an aromatic ring. 

The 6k conrotatory mode of electrocyclisation is promoted imder photoehemi-cal conditions. Furthermore, a photochemical [4 + 2] cycloaddition of 9 with singlet oxygen ( 02) would furnish tridachiahydropyrone B (12), with a further E/Z isomerisation leading to tridachiahydropyrone C (13). These observations, coupled with the fact that the molluscs reside in areas exposed to direct sunhght, underpin the biosynthetic hypothesis proposed by Moses et al. [10]. 

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