Photocyclization, enantioselective

Table 15 Enantioselective photocyclization of anilides and glyoylamide in their 1 1 inclusion complexes with 10b or 10c...

Optically active oxaziridines are useful reagents for the enantioselective oxidation of olefins 37 39). The following three preparative methods to make this reagent available have been reported enantioselective oxidation of an imine by (-)-peroxycam-phoric acid 37,38), photocyclization of a nitrone which has a chiral substituent39), and photocyclization of a nitrone in an optically active solvent 39). However, an... 

On the other hand, stereo- and enantioselective photocyclization reactions of N-benzoylformylpiperidine (74a), N-benzoylformylmorpholine (74b), and N-benzoyl-formylhexamethyleneimine (74c) were achieved by irradiation of the respective inclu-... 

Enantioselective photocyclization of 74 occurred efficiently in the inclusion complex with 2a. In particular, the selectivity is very high in the case of 74a. However, control is inefficient in the 1 2 complex 85 composed of 2a and 74c. The host guest ratio probably depends on the packing of the components in the crystal. The packing is... 

X-Ray crystal structural studies32) (Fig. 13 and Scheme 8 which refers to the crystal structure) showed that one molecule of 93 is held in a fixed conformation determined by two hydrogen bonds and by neighboring host molecules which prevent free rotation about the CO—CO single bond in 93. Free rotation about this bond would enable the production of the two possible enantiomers. The fixed conformation of the guest molecule by the chiral host molecule causes the least molecular motion during the photocyclization reaction and the high enantioselectivity. 

The most exciting enantioselective photochemical conversion of a a-oxoamide to a P-lactam has been found in the case of N,N-diisopropylbenzoylformamide (96) which gives P-lactam 97. In the photocyclization of plain 96 in the solid state, optically active P-lactam 97 of high optical purity was obtained in high chemical yield. Thus no optically active host compound is necessary for the enantioselective reaction 48>. 

Attempts have been made for the enantioselective synthesis of (—)-yohim-bane. Kametani et al. (216) reported the total synthesis of key intermediate (-)-368 starting from L-tryptophan, using enamide photocyclization. However, the optical purity of (—)-17-methoxyhexadehydroyohimbane obtained was only 17% owing to the partical racemization of intermediates throughout the reaction sequence. 

Figure 42 Results on the enantioselective Norrish-Yang photocyclization of trans-A-terf-butyl-1-methy Icy clo hexyl aryl ketone. Note that the same isomer of the chiral inductor favors opposite isomers within NaY and NaX zeolites.

Optically active hosts 12 [10] and 13 [10] are also useful for an enantioselective photocyclization of 9 [11], The mechanism of the enantioselective reaction of 9 in an inclusion complex with 12 has also been studied by x-ray analysis [12],... 

B. Enantioselective Intramolecular Photocyclization Reactions of Achiral Molecules in Inclusion Complex With a Chiral Host Compound... 

Intramolecular [2 + 2] photocyclization reactions of 2- /V-(2-propcnyl)amino]cy-clohex-2-enones (67) are also controlled enantioselectively by carrying out irradiation in inclusion complex with a chiral host compound. When inclusion crystals of 67 with 12 are irradiated in the solid state, optically active 9-azatricyclo[5.2.1.0]-decan-2-ones (68) were obtained in the chemical and optical yields indicated in Table 3 [30],... 

Enantioselective photocyclization of 2-(arylthio)-3-methylcyclohexan-l-ones (75) to dihydrobenzothiophene derivatives 77 was also achieved in inclusion crystals using chiral host molecules. Photoirradiation of the 1 1 inclusion crystals of 75g with (-)-12b as a water suspension gave the corresponding photocyclization product (+ )-m-77g of 82% ee in 83% yield. Similar photoirradiation of the... 

Enantioselective photocyclization of /V-al lyl furan-2-carboxanilidc (78) in its inclusion crystals with the optically active host compound 12b was accomplished successfully. More interestingly, ( )-79 and (+)-79 were obtained selectivity... 

Photocyclization reaction of the V-(aryloylmcthyl)-8-valerolactams (80) occur stereoselectively and enantioselectively in their inclusion crystals with optically... 

The photocyclization reaction of acrylanilide to 3,4-dihydroquinolinone was first reported in 1971, and its application to alkaloid synthesis has long been studied [44], Although stereo- and enantio-control are important in this reaction, no such attempt has been reported, except one enantioselective photocyclization of 1-(methylacryl)-A-methylanilide (85) in benzene-ether containing (i )-bis(/>tolu-oyl)tartaric acid to give ( )-3-methyl-A-methyl-3,4-dihydroquinolinone (87) of 12-16% ee [45],... 

Interestingly, the enantioselective conversion of 88a in the inclusion complex crystal with 12b into (+)-89 proceeds by a partial single-crystal to single crystal photocyclization manner [47],... 

Optically pure P-lactam derivatives can also be prepared by an enantioselective photocyclization reaction of 2-pyridone in its inclusion complex with a chiral host. For example, irradiation of the 1 1 inclusion complex of 1 and 4-methoxy-/V-inethyl-2-pyridone (33) in the solid state gave (—)-34 of 100% ee... 

Figure 1 Enantioselective photocyclization reactions of 31 in a 1 1 inclusion compound with 1.

The next example deals with the enantioselective photocyclization of a-tropolone alkyl ethers [182]. Cyclization occurs through an allowed 4e -disrotation that can result in opposite optical isomers depending on the direction of rotation (Sch. 37). Racemic products are obtained in solution, but irradiation of crystalline inclusion complexes of a-tropolone alkyl... 

Highly enantioselective photocyclization of l-alkyl-2-pyridones, in the solid state, to P-lactams 29 has been achieved in inclusion crystals with optically active host compounds <02OL3255>. 

Surprisingly, the photocyclization is enantioselective with amino acid derivatives 180 of phthalimide. For example, an inversion of the starting stereogenic center on the pyrrolidine ring was observed from the corresponding proline derivatives 182 (Scheme 47) [110]. 

Ninomiya and Naito established an enantioselective variant of their enamide photocyclization based on chiral proton donors. The enamide photocyclization... 

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. 

The efficiency of these chiral host compounds has been shown in highly enantioselective photocyclization and photocycloaddition reactions of prochiral lactams. These substrates, for example 2-quinolone derivatives, are expected to coordinate to lactam 44 with its NH-group as the hydrogen donor and the carbonyl group as the hydrogen acceptor, as depicted in Scheme 15. In this complex, any... 

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. 

Scheme 4 Enantioselective photocyclization via ylide intermediates in host-guest crystals.

Scheme 5 Enantioselective photocyclization of acrylanilides in their host-guest crystals.

Scheme 7 Enantioselective [2 4- 2] photocyclization of tropolones in supramolecular solids.

Furthermore, tropolone methyl ether 25 included in NaY and RbY zeolites that had been modified with optically pure norephedrine smoothly underwent enantioselective [2 + 2] photocyclization to afford the primary photoproduct 26 in around 40% ee (Scheme 7) [53]. The best results obtained are up to 78% ee... 

Scheme 8 Enantioselective [2 + 2] photocyclization of pyddones in their inclusion crystals.

Scheme 11 Enantioselective Norrish/Yang photocyclization by using an ionic chiral auxiliary or a chiraliy modified zeolite.

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