Stereoisomerization photochemical

Photochemical equilibration of the 3 2 stereoisomeric mixture of N-acyl vinylogous ureas in DMF by irradiation at 300 nm in a Ray-onet reactor equipped with a stannous chloride filter solution at 37 °C for 4 h leads to a 6 1 mixture of trans-Aa b and c .v-Aa b paly-toxins. The total synthesis of palytoxin (1) is now complete. 

The process by which a stereochemically inactive center is converted to a specific stereoisomeric form. In most cases, the reacting center is prochiral. Such processes can occur with reactions involving an optically active reagent, solvent, or catalyst (eg., an enzyme). The reaction produced by such a process is referred to as an enantioselective reaction. In principle, use of circularly polarized light in photochemical reactions of achiral reactants might also exhibit asymmetric induction. However, reported enantioselectivities in these cases have been very small. 

Although hole-catalyzed (cycloaddilions involving radical cation intermediates) and PET (photochemical electron transfer) mixed [2 -I- 2] cycloadditions have been reported from electron-rich alkenes, the only report of a cyclodimerization is that of (E)-4-(prop-l-enyl)anisole which gives stereoisomeric mixtures of the head-to-head dimers 1 and 2.12... 

C. Photochemical Stereoisomerizations of Aryl, Vinyl and Acyl Cyclopropanes... 

Like proton transfer, photoisomerization is a fundamentally important photochemical process. The two most important forms of photoisomerization are valence isomerization and stereoisomerization. The latter is probably the most common photoinduced isomerization in supramolecular chemistry. It may occur in systems in which the photoactive component has unsaturated bonds which can be excited, and this effect may be exploited for optical switching applications. A number of interfacial supramolecular complexes capable of undergoing cis-trans photoisomerization have been studied from this perspective - some examples are outlined in Chapter 5. 

Photochemical cyclocondensation of l,3-diacetylimidazolin-2-one with 3,4-dihydro-2-methoxy-2//-pyran in acetone solution gives a mixture of four stereoisomeric [2 + 2] cycloaddition products... 

Diphenylazirine (372) undergoes a photochemically induced addition of methyl-dithiobenzoate in benzene, via the nitrile ylide (373), affording a mixture of the two stereoisomeric Az-thiazolines (375 Scheme 230) (72TL4087). 

Aromatic C-H bonds are not broken in radical halogenation, because they are a little stronger than aliphatic C-H bonds When benzene reacts photochemically with chlorine, a radical addition process takes place, and the mixture of stereoisomeric hexachloro-cydohexanes (5.78) includes one isomer which has powerful insecticidal properties but which, unlike some chlorinated insecticides, is readily biodegradable. 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

N-phenylhydroxylamine, and a 1,5-transfer of a methoxycarbonyl group appears to be responsible for the conversion of the 3(2H)-indolone (27) into the isomer (28), a process which is probably initiated by photochemically induced E-stereoisomerization. 

Photochemical ring opening of linearly conjugated cyclohexadienones affords dienylketenes (145), which react in one of the following ways recycli-zation to the original or to a stereoisomeric cyclohexadienone, formation of bicyclo[3.l.0]hexenones (146), or addition of a protic nucleophile to yield substituted hexadienecarboxylic acids (147) (Quinkert et al., 1979). 

The reactive intermediate o-quinodimethane (84) has been generated in room-temperature solutions from l,2-bis[(phenylseleno)methyl]benzene (85) by sequential, two-colour laser irradiation. A third laser was then used to induce photorearrangement of (84) to benzocyclobutene. Owing to the high thermal reactivity of (84), room-temperature photoreactions of (84) have not previously been observed. Diphenyl diselenide (Ph2Se2) has been shown to be an effective catalyst - an alternative to iodine - for the photochemical stereoisomerization of carotenoids. Unlike iodine, Ph2Se2 tolerates the presence of Hunig s base in the isomerization of acid-sensitive carotenoids. In... 

Intramolecular hydrogen abstraction by a photochemically generated alkoxyl radical has been employed in remote functionalisation of non-activated carbon atoms,and the stereoisomeric a-iodo epoxides (128) are formed by irradiation of 5-hydroxy-5/3-cholest-3-ene (129) in benzene containing mercury(II) oxide and iodine. 

The studies reported last year on the stereoisomeric control of the photo-induced Diels Alder reaction of maleic anhydride with homo-chiral anthracene derivatives such as (46) have been extended using 320-400 nm radiation, and this gives the head-to-tail anthracene dimer as well as the previously reported adduct (47) with excellent diastereoselectivity. The thermal and photochemical retroaddition process has been examined and the results suggest that this facile process may promote the anthracene as a new chiral auxiliary. 

Ultraviolet irradiation of pyridines can prodnce highly strained species that can lead to isomerised pyridines or can be trapped. The three picolines and the three cyano-substituted pyridines constitute photochemical triads irradiation of any isomer, in the vaponr phase at 254 nm, results in the formation of all three isomers. From pyridines and from 2-pyridones 2-azabicyclo[2.2.0]-hexadienes and -hexenones can be obtained in the case of pyridines these are nsnally nnstable and revert thermally to the aromatic heterocycle. Pyridone-derived bicycles are relatively stable, 4-alkoxy- and -acyloxy-pyridones are converted in particnlarly good yields. Irradiation of iV-methyl-2-pyridone in aqueous solution prodnces a mixture of regio- and stereoisomeric 4n pins 4n photo-dimers. ... 

In the presence of Fe2(CO)g and CH3—CHBr—CO—CHBr—CH3, tellurophene undergoes photochemical additions leading to 2-substituted derivatives (4), and a mixture of the stereoisomeric 2,3-dihydrobicyclic compounds (5) <85PS(25)245>. 

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