Cycloaddition photochemical induction

Application of the same procedures to other ring closures shows that 4 + 4 and 2 + 6 ring closures and openings require photochemical induction while the 4 + 6 and 2 + 8 reactions can take place only thermally (see 15-52). In general, cycloaddition reactions allowed thermally are those with 4n + 2 electrons, while those allowed photochemically have 4n electrons. 

It should be noted, however, that the 1,3-dipolar cycloaddition chemistry of diazo compounds has been used much less frequently for the synthesis of natural products than that of other 1,3-dipoles. On the other hand, several recent syntheses of complex molecules using diazo substrates have utilized asymmetric induction in the cycloaddition step coupled with some known diazo transformation, such as the photochemical ring contraction of A -pyrazolines into cyclopropanes. This latter process often occurs with high retention of stereochemistry. Another useful transformation involves the conversion of A -pyrazolines into 1,3-diamines by reductive ring-opening. These and other results show that the 1,3-dipolar cycloaddition chemistry of diazo compounds can be extremely useful for stereoselective target-oriented syntheses and presumably we will see more applications of this type in the near future. 

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). 

Herzog, H., Koch, H., Scharf, H.-D., and Runsik, J. (1986) Chiral induction in photochemical reactions V. Regio- and diastereoselectivity in the photochemical [2 + 2] cycloaddition of chiral cyclenone-3-carboxylates with l,l -diethoxyethene. Tetrahedron, 42, 3547-3558. 

Among these reactions, the photochemical cycloadditions of C=C bom which can create up to four asymmetric carbons during the photochemical sti are particularly interesting, and numerous synthetic applications of this react have been reported. Advances in the understanding of the origin of asymmefa induction, during addition of alkenes with carbonyl derivatives, cyclic enom and aromatic compounds, will be discussed in detail. 

Another stereochemical aspect of these Diels-Alder reactions which has been studied by the Vedejs group is the facial selectivity in cycloadditions of chiral thioaldehydes. For instance, thioaldehyde (184), generated by the photochemical method, added to cyclopentadiene to give exo adducts (185) and (186) Jong with endo isomers (187) and (188) (Scheme 24). As was the case for achiral thioaldehydes, the endo adducts predominated (-9 1). The facial selectivity obtained can be rationalized via a Felkin-Anh or Comforth model for asymmetric induction. 

A patent has been lodged dealing with the photochemical synthesis of the thiolactones (82) from the thioacids (83). The cycloadduct (84) is the sole product from the irradiation of 1,1-diethoxyethene with biacetyl in non-polar solvents. The authors suggest that an electron transfer process is operative and that the cycloaddition therefore proceeds via the zwitterion (85). A study of chiral induction in the photochemical synthesis of oxetanes using the chiral... 

A menthol-based route to grandisol is described by H.-D. Scharf using the [2+2] cycloaddition of ethylene with an enantiomerically pure menthyl-substituted furanone as the photochemical key step. The asymmetric induction of this reaction is relatively poor, the diastereomeric photoproducts, however, can be easily separated. Both enantiomers of grandisol can be synthesized using this methodology. 

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. 

A comparison between single and double induction in the photochemical [2 + 2] cycloadditions of 3-oxo-l-cyclohexen-l-carboxylates with the ketene acetals was investigated89 90. Single induction using (-)-menthol as chiral auxiliary [R1 or R2 = (-)-menthyl] gave low selectivities only. A combination of these single inductions [R1 = R2 = (-)-menthyl] leads to the intermediacy of a matched reaction pair which gave an increased diastereoselectivity (d.r. 27.5 72.5) whereas a mismatched" reaction pair [R1 = ( + )-menthyl, R2 = (-)-menthyl] resulted in a decrease (d.r. 41.5 58.5). A more pronounced contrast in double induction results when the two chiral auxiliaries arc not equivalent [R1 = (—)-8-phenylmenthyl, R2 = (—)-menthyl]. 

Ring and open-chain derivatives of L-erythritol 1,4-dicinnamate have been irradiated yielding products of photochemical asymmetric synthesis, in which intramolecular 2-1-2 cycloaddition occurs to give L-erythritol esters of cyclobutane-1,2-dicarboxylic acid the 2,3-di-O-methyl derivative of l-erythritol gave a high degree of asymmetric induction, whereas the 2,3-0-isopropylidene derivative was much less stereospecific, and gave the opposite cyclobutane enantiomer. ... 

Hoffmann, N., Buschmann, H., Raabe, G., and Scharf, H. D., Chiral induction in photochemical reactions. 15. Detection of stereoelectronic effects by temperature dependent measurements of the diastereoselectivity in the photosensitized [2-1-2]-cycloaddition. Tetrahedron, 50,11167-11186,1994. 

---