Sigmatropic rearrangements irradiation

The thiol 48 (with R R, R = H) undergoes intramolecular cychsation in THF, in the presence of AIBN under UV irradiation, to give a 2-phosphonothiolane [36], a phosphorus and sulfur analogue of proline. More recently, an asymmetric version of the sequence, [2,3]-sigmatropic rearrangement and radical cychsation, has been carried out (see Sect. 5.1.1.) [41]. 

The conjugated diene dienoestrol (65) was irradiated at 254 nm in 90% aqueous methanol. Rotation and cis-trans photoisomerization gave (66) which underwent a photochemical [1, 5]sigmatropic rearrangement to give (67). Photocyclization followed by enol-keto tautomerism then gave the isolated dihydrophenanthrene dione (68) [56]. 

Moreover, by irradiation of 2-acetyI-2,5,6-trimethyl-3,4-aihydro-2//-pyran in pentane, a 92% conversion was obtained yielding a cis/trans mixture of 1,2-diacetyl-1,2-dimethylcyclobutane (3, 25%) and 3-methylbut-3-en-2-one (32%) as major products by 1,3-sigmatropic rearrangement or via diradical intermediates.44... 

Because they involve two electron pairs, [ 1,3] sigmatropic rearrangements are photochemically allowed and thermally forbidden. An example that occurs upon irradiation is shown in the following equation ... 

The 1,3-dipolar intermeditate V/149 is then transformed to the ten-mem-bered dichlorolactone V/151 by a [3.3] sigmatropic rearrangement. The side product, V/152, is obtained by a [2+2] cycloaddition [72] [74]. Under standard conditions, the dichlorolactone, V/151, was converted to phoracantholide I (V/130). Dechlorination of V/151 gave V/153 which was converted to phoracantholide J, V/154, by irradiation. Both phoracantholides are occurring naturally, see Chapter VII.2. The same technique has been applied to the synthesis of other products [73] [74],... 

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. 

For the parent (20 l,3,S-hexatriene (1), the main pathways competing with the desired electrocycliza-tion to (2) are the isomerization of the central double bond to give the ( )-triene (50) as well as die formation of other cyclized products (51) and (52) and the vinylallene (53). The sensitized irradiation of isomeric trienes results primarily in rapid (Z)-(E) isomerization about the central double bond and dimerization. By contrast to singlet state photochemistry, no electrocyclic or sigmatropic rearrangements occur under sensitized conditions. ... 

Reactions of Oxazoles. 2-Amino-4-methyloxazole (359) forms the rearranged hydro-peroxide (360) by the action of hydrogen peroxide. Thermolysis of 2- (3,3-di-methylallyloxy)-4,5-diphenyloxazole yields the oxazolin-2-one (361) with inversion of the allyl group by [ , ] sigmatropic shift irradiation, on the other... 

Ibotenic acid undergoes an interesting photoreaction comparable to a Lossen rearrangement of hydroxamic acid (Scheme 12). When irradiated in an aqueous solution with a low-pressure mercury lamp, it is transformed to muscazone with a yield of up to 40%. This occurs most probably via an unstable keto-lactam which then undergoes a [1,3] sigmatropic rearrangement 46). 

Many charge-accelerated reactions have been observed in [3,3] sigmatropic rearrangements [10], In the Claisen-Johnson rearrangement, a combination of micro-wave irradiation and acidic catalysis (KSF day) has been used successfully by Jones with several cyclic allylic alcohols [1 Ij. Rate acceleration (9 min versus 12.5 h for conventional heating) and better yields were observed (Scheme 6.3). 

Oxabicyclo[3.2.0]heptanes are also produced in the CuOTf-catalyzed photocycloadditions of allyl 2,4-hexadienyl ethers (eq 24). The CuOTf-catalyzed photocycloadditions of bis-2,4-hexadienyl ethers are more complex. Thus UV irradiation of 5,5 -oxybis[( )-l,3-pentadiene] in THF for 120 h produces vinylcy-clohexene and tricyclo[3.3.0.0 ]octane derivatives (eq 25). However, shorter irradiations reveal that these products arise by secondary CuOTf-catalyzed rearrangements of 6,7-divinyl-3-oxabicyclo[3.2.0]heptanes that are the primary photoproducts (eq 26). UV irradiation of the divinylcyclobutane intermediates in the presence of CuOTf promotes formal [1,3]- and [3,3]-sigmatropic rearrangements to produce a vinylcyclohexene and a 1,5-cyclooctadiene that is the immediate precursor of the tricyclo[3.3.0.0 ]octane. 

The first step is an electrocyclic ring-opening reaction, and the second step is a [1,7] sigmatropic rearrangement. This sequence of steps not only occurs in our bodies but is also utilized commercially to enrich milk with vitamin D3. All milk sold in the United States is irradiated with UV light, which converts the 7-dehydrocholesterol present in the milk into vitamin D3. 

Thus a solution of tridachiahydropyrone (9) was dissolved in CDCI3, degassed by purging with argon, and then irradiated. The photochemical sigmatropic rearrangement to form phototridachiahydropyrone (93) was monitored over time by analysis of the NMR spectrum of the reaction mixture. The relative intensities of the oleflnic proton signals of the two compounds were used in order to determine the ratio of 9 93 at various intervals. The results are depicted in Fig. 2.1. 

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