Rearrangement 1.2- acyl shift

In radical reactions not involving bromine or chlorine on the substrate, rearrangements are much rarer One example is the fluorination of di-tert butyl ketone which produces perfluormated / rt-buty isobutyl ketone [J5] Although isolated yields are poor only the rearranged ketone could be isolated This is perhaps only the second example of a 1,2-acyl shift Low fluorine substrate ratios show that this rearrangement occurs after monofluorination... 

As mentioned earlier (cf. chapter 2.1.1), p,y-enones undergo an 1,3 acyl shift from the Si-state. In sensitized reactions they rearrange to cyclopropyl ketones in the so termed oxadi-pi-methane rearrangement, as illustrated for... 

Photoisomerization of the unsaturated lactone (200) to the 1,5-cyclo-lactone (201) provides the first example of a Type A rearrangement in an j8-unsaturated lactone. An X-ray structure analysis has confirmed the photoisomerization of the /3-y-unsaturated ketone (202) to the cyclopentanone (203) by a 1,3-acyl shift. A similar rearrangement of the 3-oxo-A "° " -compound (204) to the cyclo-butanone (205) is in contrast with earlier reports of the oxo-di- r-methane rearrangement of 3-oxo-A " Lcompounds on direct irradiation. Benzophenone-... 

Photochemical rearrangement by direct irradiation of /fy-unsaturated cyclohexenones resulted in a singlet 1,3-acyl shift. In this manner, irradiation of 2,2-dimethylbicyclo[4.4.0]dec-l (6)-cn-3-one in benzene gave 5-isopropylidenespiro[5.3]decan-l-one (2).57... 

Fries-type rearrangements (Sch. 15) (36) also occur photochemically this is generally a homolytic process, in contrast to the ionic mechanism of the thermal process. Azapropazone undergoes photoinduced 1,3-acyl shift in the solid state and fragmentation in solution (Sch. 16) (37). 

Construction of quaternary stereocenters by enantiocontrolled oxygen to carbon acyl shift is not limited to the azlactone structure. Using the pentaphenylated planar chiral DMAP derivative 79c (Scheme 13.42) Fu and Hills achieved rearrangement of O-acylated oxindoles 84 (Scheme 13.45) and benzofuranones 85 (Scheme 13.46) with very good yields and enantiomeric excesses up to 99% [88]. 

Selective sensitization gave ODPM rearrangement products, whereas direct photolysis resulted in efficient E Z isomerization and inefficient 1,3-acyl shift processes. The results indicated that y,5-double bonds or benzoyl groups present in these systems did not act as intramolecular triplet quencher. The new argument placed by the authors was the charge transfer interaction within the molecule enhances the internal conversion process there by a reduction in the ODPM rearrangement reactivity. 

If rapid bond formation at the stage of the Norrish I fragments competes with decarbonylation, 1,2-acyl shift products are formed as the major components from direct photolysis. In case of the cyclobuteno-annulated bicyclo[2.2.2]octenone 21, 1,2-acyl shift results in the cyclo-butanone 22, whereas ODPM rearrangement (photolysis in acetone) results in the cyclobuteno-diquinane 23 in 44% yield (Sch. 21) [49]. 

The tricyclic compound shown in the following scheme underwent a 1,2-acyl shift (oxa-di-n-methane rearrangement) to form the tetracyclic compound upon irradiation in a Pyrex immersion well in degassed dry acetone as both a solvent as well as a sensitizer. On the... 

Among the examples of [l,3]-sigmatropic rearrangements is the photoinduced [1,33-acyl shift of A-acylenamines, which has been investigated extensively and reviewed in 197868 xhus, the irradiation of /V-acyl enamines, known as enamides, provides a facile synthesis of enaminones. 

Oxa-di-TT-methane rearrangement A photochemical reaction of a P,y-unsaturated ketone to form a saturated a-cyclopropyl ketone. The rearrangement formally amounts to a 1,2-acyl shift and bond formation between the former a and y carbon atoms. 

In addition to the commonly observed photoprocesses of 8-cleavage, decarbonylation, and ring expansion, rearrangements, in particular [1,3]-acyl shifts, and... 

Most p.-y-unsaturated ketones rearrange upon triplet sensitization to cyclopropyl ketones via the so-called oxa-di-TT-methane (ODPM) rearrangement, the result of which is an overall 1,2-acyl shift as schematically shown for (1) -+ (2). p,y-Enone arrangements in which the alkene moiety is further conjugated to either phenyl or C—O sites also undergo this transformation upon direct excitation, i.e. via the singlet excited state. 

In agreement with the findings concerning the reactivity of (38) and (41) (Scheme 8), the 0=C—C— C==C—C==0 chromophore in (8) also undergoes the 1,2-acyl shift on direct excitation. This rearrangement has been shown to occur from the lowest triplet state ( i , ) in the energy range typical of a,P-enones. ... 

In the monocyclic series, the 2,4-cyclohexadienones rearrange to photoproducts of potential synthetic value. However, for efficient 1,2-acyl migration, only a relatively small number of substrates are suitable. These must be highly substituted, for example (48). Whereas on direct excitation in methanol, cleavage to the isomeric ketenes (49) (4> 2 0.42) predominates, the remarkably stereoselective 1,2-acyl shift to the bicyclohexenone (50) is found either in trifluoroethanol or when the dienone is adsorbed on silica gel. The conversion to (50) is followed by a reversible phototransformation to the cross-conjugated dienone (51) and accompanied by aromatization to (52) to a minor extent. Such reactivity has also been verified for tetra- and penta-methylated 2,4-cyclohexadienones. The only photoreaction of the hexamethylated homolog, on the other hand, is ketene formation. ° ... 

Figure 4 Mechanism of trans-protein splicing, (a) Initial association of the intein halves to form a functional intein. (b) Activation of the N-terminal splice-junction via an N-S acyl shift, (c) Formation of a branched intermediate upon transthioesterification. (d) Branch resolution and intein release by succinimide formation. Spontaneous S-N acyl rearrangement yields the processed product with a native peptide backbone.

The conversion of a bicyclo[2.2.1]octenone derivative to the corresponding bicyclo[3.3.0]octenone, a common intermediate in the total synthesis of several iridoid monoterpenes, was achieved by N.C. Chang et al. The target was obtained by sequential application of the Corey-Chaykovsky epoxidation, Demjanov rearrangement and a photochemical [1,3]-acyl shift. 

Unsaturated ketones undergo a formal 1,2-acyl shift from the lowest excited triplet state to afford cyclopropyl ketones (for some early examples cf. Houben-Weyl, Vol. 4/5b, p 794), a reaction which has been termed oxa-di-Jt-methane rearrangement.Acyclic ketones normally do not undergo this reaction, however, an exception is in systems where the C-C double bond is further conjugated, e.g. to a phenyl group. 

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