Photocycloaddition asymmetric

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-... 

Zamojski and coworkers have explored the use of the furan-carbonyl photocycloaddition in asymmetric synthesis, with somewhat limited success88. Irradiation of chiral glyoxylate derivative 196 [R = (R)(—)-menthyl and (R)(—)-8-phenylmenthyl] afforded... 

Asymmetric [2 + 2] photocycloadditions. Acid-catalyzed condensation of t-butyl acetoacetate with (— )-menthone (1) results in two dioxacyclohexenones, 2 and 3, which are easily separable. Photocycloaddition of ( — )-2 with methylcyclo-butene gives mainly the adduct (4) which has the cis, anti, cis-configuration at the... 

In addition to bimolecular [2-1-2] photocycloaddition reactions similar to the one discussed above [11], absolute asymmetric syntheses employing spontaneously grown chiral crystals have been studied for a number of unimolecular photorearrangement reactions as well. The first report of such an investigation dealt with the photoisomerization of dibenzobarrelene diester 6 to the corresponding dibenzosemibullvalene compound 7 [12], an example of the well-known di-7i-methane (Zimmerman) photorearrangement. Achiral diester 6 crystallizes spontaneously from cyclohexane in the now familiar chiral space... 

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

In cycloadditions of enones to alkenes novel strategies have been adopted for ring expansion of the cycloadducts, either by the choice of appropriate alkenes, e.g. 2-(trimethylsiloxy)buta-1,3-diene,70 vmv-2-trimethylsiloxybuten-2-oales71 or 3,3-dimethylcyclopropene,72 or by using 3-oxo-l-cyeloalkene-l-carboxylates as enones.73 Asymmetric [2 + 2] photocycloaddition of cyclopent-2-enone to a (+ )-dihydrofuran acetonide constitutes the cornerstone of the synthetic strategy in the first total synthesis of the novel antitumor metabolite ( )-echinosporin.74 The cycloaddition product 25 from treatment of 2-(2-carbomethoxyethyl)-2-cyclopentenone (24) with ethene has been used as a precursor for the preparation of tricyclo[4.2.0.01,4]octane.75... 

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... 

Recently, the same group104 has reported the first total synthesis of the novel antitumor metabolite (—)-echinosporin 224. The synthesis is based on an asymmetric [2 + 2] photocycloaddition, which constitutes the cornerstone of the synthetic strategy (Scheme 48). 

High stereoselectivity was found in the first example of intramolecular [2 + 2] photocycloaddition discovered by Ciamician and Silber111 in the irradiation of camphor 233 to carvon camphor 234. Generally, if the tethered alkenyl side chain is connected to the enone via an asymmetric center, the configuration of this center plays an important role on the diastereofacial selectivity. For example, high stereoselectivity was found in the irradiation of the diketone 235 or its corresponding enol acetates112, when substituents on the alkenyl side chain affect the selectivity of the enolization of 235 but not the diastereofacial selectivity (Scheme 50). 

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). 

It will be interesting to follow the developments on the highly efficient and more stereoselective photocycloaddition and photoaddition to aromatic rings from the viewpoints for the synthesis of more complex compounds, including natural products. In addition, the chiral induction in the excited states should be more attractive projects in the near future. Although some excellent reviews about the asymmetric photochemical reactions have been reported in recent years [490-492], the highly enantioselective or diastereoselective photocycloaddition and photoaddition have been reported in only limited cases. 

Kohmoto et al. reported an example of absolute asymmetric synthesis involving the intramolecular [4 + 4] photocycloaddition of 9-anthryl-/V-(naphthylcarbonyl)-... 

The dimerization of thioformylketene was investigated by B3LYP and G3MP2B3 methods. The 4 + 4-pathway has the lowest energy barrier and calculations suggest that the reaction is pseudopericyclic.231 The stereospecific intramolecular 4 + 4-cycloaddition reaction between cyclohexadiene iron tricarbonyl complex and appended dienes (198) generates cyclooctadiene tricyclic adducts (199) (Scheme 56).232 The first example of an asymmetric intermolecular 4 + 4-photocycloaddition reaction in solution between 9-cyanoanthracene and chiral 2-methoxy-l-naphthamides has been reported. The frozen chirality is effectively transferred to the optically active product.233... 

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... 

Smith s synthesis of (—)-echinosporin stands out for its good stereo-and regiocontrol at every stage. Its large number of highly chemoselective reactions, and its use of an asymmetric [2 + 2]-photocycloaddition reaction for constructing the m-fused 6,5-bicyclic core are also commendable. The latter reaction provides a beautiful illustration of how properly harnessed photochemistry can be decisively deployed for the stereocontrolled synthesis of complex natural products. 

A very useful extension of the de Mayo reaction has been recently introduced by Blechert et al. (Scheme 6.26) [78]. The retro-aldol fragmentation was combined with an intramolecular enantioselective allylation (asymmetric ring-expanding allylation) catalyzed by a chiral Pd complex. Bicycloheptane 68, for example, was accessible by intermolecular [2 + 2]-photocycloaddition of cyclopentenone 67 with allene. Further transformation in the presence of Pd2(dba)3 (dba = dibenzylideneacetone) and the chiral oxazoline ligand 69 (tBu-phox) resulted in the enantioselective formation of cycloheptadione 70. 

Faure, S., Piva-Le-Blanc, S., Bertrand, C., Pete, J.-P., Faure, R., and Piva, O. (2002) Asymmetric intramolecular [2 + 2] photocycloadditions a- and (3-Hydroxy acids as chiral tether groups. Journal of Organic Chemistry, 67, 1061-1070. 

Jarosz, S. and Zamojski, A. (1982) Asymmetric photocycloaddition between furan and chiral alkyl glyoxylates. Tetrahedron, 38, 1447-1451. 

The last two chapters by Hoffmann and Ramamurthy deal with a collection of photochemical reactions with arenes, the ortho-, meta- and para photocycloadditions and with a conceptually exciting concept in organic photochemistry, the use of contrained media. Retigeranic acid (16, by formal asymmetric synthesis) was synthesized via a fabulous reaction sequence involving an intramolecular meta photocycloaddition as key step [16]. 

The first report concerning an asymmetric Paterno-Buchi reaction with a chiral carbonyl component was reported in 1979 by Gotthardt and Lenz [128]. The photocycloaddition of the enantiomerically pure menthyl ester of phenylglyoxylic acid 133 with 2,3-dimethyl-2-butene gave the oxetane 134 with a diastereomeric excess of only 37% (Sch. 45). 

Until now, only few attempts were made to control the absolute configuration of the chiral polycyclic products. When salicylic esters of type 77 (Sch. 14) possessing an asymmetric alcohol moiety as chiral auxiliary instead of the n-butyl group are irradiated, the corresponding products can be isolated with 17% ee [50]. Better results can be obtained with the additionally acylated substrates 79a-c (Sch. 15). Compounds 80a-c were isolated with diastereomeric excesses up to 90% [29,56]. These compounds were formed via [2+2] photocycloaddition yielding the intermediates X. A thermal rearrangement leads to 80a-c. Compounds 81a-c are produced in the same time via a further photochemical rearrangement but due to thermal reversibility of the last step, 80a-d could be isolated with yields up to 90% when the reaction was completed. 

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