Photochemically Induced Cycloadditions

Stereoselective [2+2] photocycloadditions of alkenes are a powerful tool for the synthesis of cyclobutanes.However, in diastereoselective reactions cyclic enones have been intensively examined with moderate success. By double stereodifferentiation an increase in selectivity was achieved. 

In [2+2] photocycloadditions of a,(3-unsaturated ketals derived from C2-symmetric 1,2-diols tartaric acid derivatives (58) proved to be efficient to yield the cycloadducts in up 

In photocycloadditions of 1,2-dioxine-4-ones A derived from menthone with unsymmetrically substituted alkenes moderate regioselectivity was observed. However, the cycloaddition reaction with cyclopentene proceeded highly stereoselective. Chiral bicyclic lactams diastereoselectively react with ethylene furnishing optically active cyclobutanes.  

Reactions of a,P-unsaturated ester derivatives with alkenes have been investigated leading to cyclobutanones with moderate stereoselectivity 

The photochemically induced [2+2] cycloaddition (Patemo-Btlchi reaction) of carbonyl compounds with alkenes gives oxetanes.  

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Thermally and photochemically induced cycloadditions of 2,3-butanedione with 1,1-diethoxylene show different regiochemistry [194], This is expected on the basis of the above explanation. Perhaps electron transfer occurs between reactants before the bond formation starts. 

As shown below, the enantiomeric excess achieved in the 6n-cyc-lisation reaction of amide 22 to the tetrahydrophenathridinone 23 remained below 60% ee. While this is still an impressive value for a photochemical reaction in solution, the observed enantiomeric excess is clearly inferior to the values achieved in the different photochemically induced cycloaddition reactions presented in Sect. 2.2 and Sect. 2.3. As pointed out previously, complete chirality transfer from the template to the substrate is only possible if a complete complexation of the sub-... 

Thermally and photochemically induced cycloadditions of 2,3-butanedione with... 

The photochemically induced [2 + 2] cycloaddition of selenophene with dimethylmaleic anhydride gives a 1 1 adduct (140), but attempts to form an oxetane by photoreaction with benzophenone failed (80JHC1151). 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

A number of photochemically induced 4 + 2 cycloadditions have been observed/131-150 This reaction can be either a concerted ( 4, + 20) or a ( 40 4- 2S) addition. The stereochemical consequences of the allowed reactions are as follows ... 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

In a similar approach, Garner et al. (78) made use of silicon-based tethers between ylide and dipolarophile during their program of research into the application of azomethine ylides in the total asymmetric synthesis of complex natural products. In order to form advanced synthetic intermediates of type 248 during the asymmetric synthesis of bioxalomycins (249), an intramolecular azomethine ylide reaction from aziridine ylide precursors was deemed the best strategy (Scheme 3.84). Under photochemically induced ylide formation and subsequent cycloaddition, the desired endo-re products 250 were formed exclusively. However, due to unacceptably low synthetic yields, this approach was abandoned in favor of a longer tether (Scheme 3.85). 

Trimethyloxazole 257 undergoes photochemically induced [2 + 2] cycloaddition with aromatic and aliphatic aldehydes to provide bicyclic oxazolines 258 with excellent regiochemical and stereochemical control. Diastereoselec-tivities from 75-99% can be achieved, which is the first reported example of a Paterno-Biichi reaction involving an oxazole. The oxetane cycloadducts can be hydrolyzed to a-amino-(3-hydroxy ketones. Other oxazoles have not been evaluated to determine if they undergo the photochemical cycloaddition (Scheme 8.71). 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

Intramolecular cycloaddition, particularly in y,unsaturated ketones,348 has also been reported. Recent examples include the photochemically induced conversion of the cyclohexanone (397) to the two possible adducts (398 and 399)349 ancj the formation of the oxetans (400) from 5-acylnorbomenes (401 ).350 The introduction of an ether oxygen into the unsaturated ketone... 

The first and the most simple is the observation that the common thermally induced reactions have transition structures involving a total of (4n+2) electrons. We saw in the last chapter that [4+2], [8+2], and [6+4] thermal cycloadditions are common, and that [2+2], [4+4], and [6+6] cycloadditions are almost only found in photochemically induced reactions. The total number of electrons in the former group are (4n+2) numbers, analogous to the number of electrons in aromatic rings. 

The sensitized photochemically-induced [2 + 2] cycloadditions of silyl enol ethers with acrylonitrile or methyl acrylate have been found, because of the regiospecificity of the... 

Cyclic enones with ring sizes of six-to-eight carbons can be photochemically induced to undergo [4+2] cycloadditions via isomerization to a strained trans isomer (Schs. 22-24). Irradiation of 2-cycloheptenone 99 leads to [2+2] dimerization of an intermediate r -2-cycloheptenone 100, but if this irradiation is conducted with an excess of cyclopentadiene 32 at —50 °C, a single [4+2] adduct 101 is isolated in very high yield [65,66]. The somewhat less strained nms-2-cyclo-octenone can be generated and trapped by subsequent addition of a cyclopentadiene [67,68]. Extension of this reaction to intramolecular examples has recently been reported [69]. 

Cycloaddition of diazoalkanes to 6-6 bonds was among the first reactions tested on the carbon cages41 and affords isolable fullerene-fused pyrazolines as primary adducts. Photochemically induced extrusion of N2 from the latter yields 6-6-closed methanofullerenes having the functionalized bond embedded in a cyclopropane substructure, whereas the thermal process affords 6-5 open homofullerenes in which a methylene group is bridging the open junction between a six- and a five-membered ring.15-17... 

Appreciable solvent effects are also obtained in photochemically induced [2 -I- 2]-cycloadditions and cycloreversions [390-392], Examples are given in Eqs. (5-143) [390] and (5-144) [680],... 

The orbitals overlap suprafacially on both ir-systems (Fig. 8.35) thus, the [4s4-2s]-cycloaddition reaction is thermally allowed (see the rules summarized in Table 8.1). However, the rules are reversed when the reaction is photochemically induced. 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

Thiocarbonyl compounds participate in a wide variety of reactions leading to cyclic compounds. In the majority of cases such cycloaddition reactions are thermally or photochemically induced and occur with a full or high degree of stereoselectivity. 

Photochemically induced [2 + 2] cycloaddition is of extraordinary importance in organic synthesis,as this is a method ideally suited for the preparation of sterically congested compounds. The reaction may occur by a concerted mechanism allowed by rules of orbital symmetry, or, more often, via a biradical pathway. For preparative purposes, the most widely exploited is the enone-alkene photochemical [2 + 2] cycloaddition. This reaction proceeds with high regioselectivity, although its stereoselectivity might be low. The first example of the utilization of this reaction for the synthesis of a natural compound, a-cariophyllene 385, was described by Corey (Scheme 2.129). Adduct 386, formed as a mixture of stereoisomers in high yield from simple precursors, was further transformed via the tricyclic intermediate 387 into the... 

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