Photocyclization asymmetric

Scheme 2 Asymmetric induction in the disrotatory photocyclization of tropolone ethers...

Scheme 8 Asymmetric induction in the Yang photocyclization reaction.

It is important to mention again at this point that a general feature of the solid-state ionic chiral auxiliary approach to asymmetric synthesis is that not all chiral auxiliaries lead to high enantiomeric excesses. A case in point is found in the work of Natarajan et al. on the a-oxoamide-containing salts 43 (Scheme 10) [29]. Like the nonionic a-oxoamides discussed previously (Sect. 2.2), these compounds undergo photocyclization to p-lactam derivatives, and while the prolinamide salt behaves perfectly, leading to p-lactam 44 in 99% ee at 99% conversion, the corresponding 1-phenylethylamine salt affords nearly racemic photoproduct (3% ee at 99% conversion). The reason for this difference is... 

Leibovitch M, Olovsson G, Scheffer JR, Trotter J (1998) An investigation of the Yang photocyclization reaction in the solid state asymmetric induction studies and crystal strac-ture-reactivity relationship. J Am Chem Soc 120 12755-12769... 

Several studies have been undertaken to get optically active helicenes by means of an asymmetric photocyclization. Four types of asymmetric photosynthesis can be distinguished for helicenes ... 

The first possibility is ruled out by the observation that asymmetric photodestruction by irradiation with CPL is much slower than asymmetric photocyclization. Moreover, it results in an optical activity of opposite sign in comparison with the optical activity observed in asymmetric photocyclization of the corresponding 1,2-diarylethylene. Finally, the optical yield and the wavelength of the CPL required for different precursors of [8]-helicene are different and the latter does not show any connection with the CD-Spectrum of [8]-helicene 6S). 

The chemically induced asymmetric photocyclization of the l-(—)-menthyl ester shows a striking temperature dependence 76), a completely reversed ratio is obtained at low temperature. When the chiral group is placed at C(ll) of the benzo[c]phen-anthryl group (77) the effect is of the same order (about 5 % diastereomeric excess) as when the same group is placed at the paraposition of the phenyl group, or when... 

In the case of the formation of the [3-lactam, it was shown that the ability of a compound to crystallize in a chiral space group was increased by using compounds with meta-substituted aryl groups such as 50a-d instead of their ortho- and para-substituted analogues the absolute asymmetric photocyclization of the pyridones resulted in the formation of 51a-d with good ees.[31]... 

Irie et al. reported on the absolute reversible photocyclization reactions of achiral diarylethene 52 in chiral crystal leading to 53 in up to 99% ee. The asymmetric photochromic material has potential application in nonlinear optics, for example, in switchable second harmonic generation devices. [32]... 

Yamamoto, S., Matsuda, K., and Irie, M. (2003) Absolute asymmetric photocyclization of a photochromic diarylethene derivative in single crystals, Angew. Chem. Int. Ed., 42, 1636-1639. 

An asymmetric photocyclization of a-oxoamide 440 in solid state afforded azetidine-2-one 441 (Equation 176) <1996J(P2)61>. The substrate with a chloro group on either ortho- or para-position of the phenyl ring, however, afforded a racemate. Photocyclization of the phenylglyoxamides 442 of enantiomerically pure a-amino acid methyl esters produced azetidin-2-ones 443 and 444 in moderate to high diastereoselectivity, with the m-isomer as the major component (Equation 177) <2002SL131>. 

CONTENTS Preface, Mark Lautens. Photocyclization and Photocycloaddition Reactions of 4- and 2-Pyrones, Frederick G. West. Intramolecular [4+3] Cycloaddition Reactions, Michael Harmata. Lewis Acid Catalyzed [2+2] Cycloaddition Reactions of Vinyl Sulfides and Their Analogues Catalytic Asymmetric [2+2] Cycloaddition Reactions, Koichi Narasaka and Yujiro Hayashi. Vinylboranes as Diels-Alder Dienophiles, Daniel A. Singleton. Preparation and Exo-Selective [4+2] Cycloaddition Reactions of Cobaloxime-Substituted 1,3-Dienes, P... 

Wessig, P., Wettstein, P., Giese, B., Neuburger, M., and Zehnder, M. (1994) Asymmetric synthesis of 3-hydroxy-prolines by photocyclization of N-(2-benzoylethyl)glycinamides. 

Steiner, A., Wessig, P., and Polborn, K. (1996) Asymmetric synthesis of 3-hydroxyprolines by photocyclization of... 

Most of the known asymmetric synthesis reactions equilibrating from pro-chiral racemic mixtures are photocyclizations where ring systems that include one or two asymmetric C atoms are created as a first or the final product. 

In a more recent study, the enamide photocyclization with very similar photosubstrates was examined in the presence of chiral amino alcohols and chiral amines as asymmetric inductors [47]. The achieved enantioselectivities are in the same range as the ones reported by Ninomiya and Naito, but in this approach the asymmetric induction was more effective for the cis products. In cyclopentane at — 40°C, 0.1 equivalents of the most effective inductor, (— )-ephedrine (entity, gave the cis cyclization products with up to 37% ee and the trans products with only 2% ee. The role of the chiral inductor as a Br0nsted acid was supported by flash photolysis experiments. The presence of the chiral amino alcohol led to an increase in the rate of disappearance of a transient that was assigned to the primary cyclization intermediate of type 29, i.e., the chiral inductor accelerates the protonation/deprotonation sequence that reestablishes the aromatic ring. 

Irradiation of tropolone alkyl ether 22 (Scheme 14) led to a 4ir-disrotatory ring closure to yield bicyclo[3.2.0]heptadienone 23 with two chiral centers, while prolonged irradiations led to the formation of a secondary product 24 [76-78]. As the same photocyclization was performed in chirally modified zeolites, it is interesting to compare the asymmetric photochemical behavior of 22 in the distinctly different chiral confined media of zeolites and cyclodextrins. Even in the... 

The host-guest and ionic chiral auxiliary approaches have been most intensively applied for solid-state asymmetric induction. A number of achiral organic compounds could be converted into chiral compounds in high enantioselectivities. However, all the photoreactions in themselves are well-known intramolecular photoreactions photocyclization, [2 + 2] photocyclization, Norrish type II photo-cyclization, di-ir-methane photorearrangement and photoisomerization. New types of asymmetric photoreactions have never been reported. 

Tropolone ethers have served as useful probes of asymmetric induction using several different supramolecular approaches [49]. Solid-state photolysis of 1 1 host-guest complexes of tropolone methyl ether 25 with (R,R)-( — )-4 caused disrotatory [2 + 2] photocyclization to 26 followed by transformation to afford... 

Scheme 24 Absolute asymmetric photocyclization of a succinimide derivative involving a 1,5-hydrogen shift.

Scheme 25 Absolute asymmetric photocyclization involving phenyl migration and radical pair recombination.

Scheme 27 Absolute asymmetric Norrish/Yang photocyclization of adamantane derivative 114.

Scheme 29 Absolute asymmetric photocyclization in a salt crystal formed from two achiral components.

Asymmetric photocyclization to a chiral lactam can be achieved by the use of a chiral metal hydride complex under reductive conditions, showing the enantio-differentiated nature of this type of cyclization. 

Asymmetric photocyclization starting from a chiral enamide also proceeds smoothly under nonoxidative conditions to give a diastereo-differentiated product. 

Furthermore, the chiral metal hydride complex, which is readily prepared from a metal hydride and chiral amine or amino alcohol, has been successfully applied to reductive photocyclization, thus allowing visualization of asymmetric photocyclization (16). 

---