Photocycloadditions asymmetric induction

Scheme 20. Intramolecular [2 + 2] photocycloaddition Asymmetric induction when the cyclenone is linked to the alkenyl chain at an asymmetric carbon.

Ethyl 4-[2-(4-pyridyl)ethenyl]cinnamate (5 OEt) crystals (j8-type packing) gives an optically active dimer through a topochemical [2-1-2] photocycloaddition (enantiomeric effect >90%). The asymmetric induction is ex-... 

Scheme 4 Asymmetric induction in an intramolecular [2+2] photocycloaddition reaction using optically pure coordinatoclathrate host 9b...

Upon irradiation with ultraviolet light Fischer chromium-carbene complexes 16 react as if they were ketenes, although evidence for the generation of free ketenes has not been observed (see Section 1.4.2.). Photocycloadditions of these complexes onto chiral enamines show significant asymmetric induction.22... 

One of the first examples of high asymmetric induction in the intramolecular [2 + 2] photocycloaddition in which the chiral center located at the side chain found in Winkler and coworkers113 approach to the synthesis of (—)-histrionicotoxin alkaloid 240. Irradiation of 237 is the key step in the synthetic strategy. The isomer 238 was formed in... 

Recently, Carreira and coworkers126 obtained very high asymmetric induction (83-100%) in intramolecular [2 + 2] photocycloadditions of 1,3-disubstituted allenes with enones 289 and 290 (Scheme 63). 

Photochemical (4 + 2) cycloaddition of anthracene and dimethyl fumarate or maleate have been reported by Kaupp [301], The reaction requires a high concentration of dimethyl fumarate or maleate owing to the short lifetime of the excited singlet state of anthracene and its facile dimerization. However, irradiation of a benzene solution of (9-anthryl)methyl methyl fumarate or maleate resulted in the intramolecular (4 + 2) photocycloaddition efficiently [302], Asymmetric induction in this intramolecular photocycloaddition of 291b-c was also investigated (Scheme 83). 

In solution, the great conformational mobility of molecules does not favor tight transition states. For a good asymmetric induction, there is a need to decrease this mobility and to introduce the chiral information as close as possible from the prochiral centers. An efficient way to introduce the chiral environment in asymmetric [2 4- 2]-photocycloadditions is to connect chiral auxiliaries to the reactants. When removable chiral auxiliaries are attached to the starting molecules before the photochemical step, photocycloadditions proceed diastereoselectively. After removal of the chiral auxiliary, the overall process can be considered as a byroad for an enantioselective reaction. 

Eycken and coworkers [121] employed ji-CDx as a chiral template for asymmetric induction in the intramolecular meta-photocycloaddition [126,127] of 4-phenoxy-1-butenes 48 and 50 in the solid state. (Scheme 19.)... 

Amido-substituted methylenecyclopropanes can be used in [2 + 2] photocycloadditions with chromium-alkoxycarbene complexes. (5)-3-(2-Methylenecyclopropyl)-4-phenyloxazolidin-2-one (14) was prepared from pentacarbonyl[A-phenylglycinyl(cyclopropyl)carbene]chro-mium(O) by ring closure with sodium hydride and diphenyl carbonate. Photolysis of a variety of chromium-alkoxycarbene complexes 15 in the presence of two equivalents of optically active enecarbamate under carbon monoxide produced optically active cyclobutanones. The cis- and trani-products, 16 and 17, were formed with a high degree of asymmetric induction at the position a to the oxygen. ... 

Another illustration of the role of the conformational mobility on the level of the observed asymmetric induction is shown by the intramolecular photocycloaddition of cinnamyl derivatives of vinyl glycine 73 into a mixture of 74 and 75 (Scheme 17). In the sensitized reaction and for steric reasons, the attack of -77,-77 excited styrene on the vinyl oxazolidinone 76 gives mainly 77 [55]. 

An interesting use of removable chiral auxiliaries in photocycloaddition reactions concerns imminium salts. With cyclic enones, the observed asymmetric induction does not result from an approach of the double bonds in parallel planes because of the triplet nature of the reactive excited state. In contrast, the corresponding imminium salts react through their singlet excited state, and an approach of the reactants in parallel planes is now required during the cycloaddition process. For chiral imminium salts 130 derived from a cyclohexenone and a pyrrolidine having a C2 axis of symmetry, the intramolecular [2 -I- 2] photocycloaddition process occurs with a de up to 82%. As expected, the stereochemis-... 

The degree of asymmetric induction in the photocycloaddition reaction can be quite high with substrates containing a stereogenic center. Winkler, Scott and Williard have reported that irradiation of 1-tryptophan-derived vinylogous amide 107 led to the isolation of ketoimine 108 in 91% yield as a single diastereomer . Closure to the tetracyclic portion of the aspidosperma ring system 109 was achieved in two steps by formation of the silyl enol ether with LDA and r-butyl dimethylsilyl triflate followed by treatment with tetrabutylammonium fluoride (TBFA). Conversion of 109 to 110 with >97% optical purity was then achieved. As 110 is an intermediate in Biichi s synthesis of vindorosine, the sequence outlined in Scheme 26 represents a formal total synthesis of vindorosine. 

Many other methods of asymmetric induction in photochemistry, such as template-induced enantioselective photoreactions,591 asymmetric photochemistry in zeolites,592 or simple photoreactions of a chiral starting material, parallel their ground-state chemistry counterparts. Diastereoselective photocycloaddition in the Patemo Biichi reaction of chiral reactants (Case Study 6.17) may serve as an additional example. 

Very few asymmetric [4+2] photocycloadditions have been described in the literature [11]. However, reported examples are usually found to proceed with good levels of selectivity. The intramolecular [4+2] photocycloaddition of the diaryl-ethene derivative (114) was found to proceed smoothly with good diastereoselec-tivity (de = 86.6% in toluene at -40 C) (Scheme 27) [155]. When the dielectric constant was increased slightly, the de was found to increase this was also the case as the temperature was lowered. The mechanism of the asymmetric induction is described in detail. 

A preliminary examination of asymmetric induction in photochemical [2 + 2] cycloaddition reaction of an enone to an alkene employed a chiral auxiliary attached to the alkene component74. The photocycloaddition of 2-cyclopentenone to the optically active ketene acetal 1 led both to oxetanes and cyclobutanes in the ratio 6.5 3.5 with a total yield of 60%. The m-[2C + 2C] addition of the chiral alkene I to 2-cyclopentenone was completely regioselective and gave four diastereomeric head-to-tail cycloadducts 2-4 in the ratio 6 29 33 32. 

An excellent agreement between the d.r. (7/8) and the enantiomeric excess of the lactone 9 is shown for all auxiliaries. 8-Phenylmenthyl is the most effective chiral group in achieving asymmetric induction. It is assumed that the enoate 6a adopts the conformation shown in 10. The phenyl ring sterically blocks the C(a)-S7-face and consequently the photocycloaddition is directed predominantly to the C( )-Re-face to give the adduct 7a which on reduction led to lactone (+)-9. 

---