Photochemical syntheses stereoselective synthesis

Scheme 70 Stereoselective synthesis of (3-lactams by photochemical rearrangement...

A stereoselective synthesis of compounds possessing an oxepane ring fused to a diquinane or a bicyclo[4.2.0]octane moieties using photochemical reactions of a common precursor 115 (Scheme 51) is developed the latter compound is related to sterpurane, another class of bioactive sesquiterpenes <2003TL475>. 

Carless, H.A.J., Swan, D.I.,and Haywood, D.J. (1993) Stereoselective synthesis of tetrahydrofuran-3-ols by photochemical 8-hydrogen abstraction of (3-allyloxy-carbonyl compounds. Tetrahedron, 49, 1665-1674. 

Until recently organic photochemistry has only partially focused on stereoselective synthesis, one of the major challenges and research areas in modern organic synthesis. This situation has dramatically changed in the last decade and highly chemo-, regio-, diastereo- as well as enantioselective reactions have been developed. Chemists all over the world became aware of the fascinating synthetic opportunities of electronically excited molecules and definitely this will lead to a new period of prosperity. Photochemical reactions can be performed at low temperatures, in the solid or liquid state or under gas-phase conditions, with spin-selective direct excitation or sensitization, and even multi-photon processes start to enter the synthetic scenery. 

Photochemical reaction of bifunctional ketoenamines in CC14 with maleic and fumaric acids and esters can be used for stereoselective synthesis of cis and trans alkyl 7-azabicyclo[2.2.1]heptanecarboxylates in good yield330 (equation 248). 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

Interest in the synthesis of desether muscarine derivatives (6) has developed owing to their activity and specificity, which parallel those of the muscarines. In order to overcome deficiencies in overall yield of a previous photochemical synthesis, a new stereoselective route has been devised (Scheme 1). The readily available cyclo-pentene amide (3) was epoxidized to give (4), which upon treatment with lithium dimethylcuprate provided the amido-alcohol (5). Metal hydride reduction followed... 

Rac/meso-isomers of bridged bis(indenyl) zirconium dichlorides can interconvert photochemically.796 This interconversion has been utilized in the stereoselective synthesis of ansa-zirconoccnc binaphtholate stereoisomers.797 Specifically, the rac-meso-mixtures 1034 induced by irradiation in toluene react with 1 equiv. of the dilithium salt of racemic binaphthol to give the racemic binaphtholate complex 1035 (Scheme 253). Analogous reactions with 1 equiv. of the Z (+) enantiomer of dilithium binaphtholate afford the enantiomerically pure binaphtholate complex. The structure of the racemic binaphtholate complex, Me2Si(2-Me-4-But-C5H2)2Zr(binaphtholate), has been crystallographically determined. 

A stereoselective synthesis of aminoalkyl-substituted P-lactams (41) has been developed, the key step of which is [2+2] cycloaddition of imines to ketenes generated photochemically from diazoketones (42), which are derived from protected a-amino acids. A number of steroidal diazoketones related to progesterone have been synthesized as potential photoaffinity labelling reagents for the mineralocorticoid receptor. ... 

Preparation of Phosphines by Addition of P-H to Unsaturated Compounds. -This route has not received much attention over the past year. A stereoselective synthesis of tris(Z-styryl)phosphine is offered by the addition of phosphine to phenylacetylene in a superbasic system (HMPA-H20-K0H)." In a similar vein, the reaction of phosphine with styrene and a-methylstyrene in a superbasic medium (DMSO-KOH) provides a route to the primary phosphines, (2-phenylethyl)phosphine and (2-methyl-2-phenylethyl)phosphine, respectively. 7 Transition metal phosphine complexes have been shown to catalyse the a-hydroxylation, P-cyanoethylation, and P-alkoxycarbonylethylation of phosphine. 71 Addition of primary phosphines to acrylic esters has been used for the synthesis of the phosphines (80).7 A similar addition of diphenylphosphine to acrylic esters and amides has given a series of hydrophilic phosphines (81). 72 The bis(phosphorinanyl)ethane (82) is formed in the photochemical addition of l,2-bis(phosphino)ethane to 1,4-pentadiene. ... 

As base. For vicarious substitution of m-dinitrobenzene with carbon nucleophiles under photochemical conditions, TBAF serves as the base. The nitroaldol condensation with a-amino aldehydes constitutes the key step of a stereoselective synthesis of 1,3-diamino-2-alkanols. ... 

Synthesis of those insect aggregation pheromones having bridged structures may have important economic consequences. Multistriatin (120) is an aggregation pheromone of a European elm bark beetle. A stereoselective synthesis "" has been reported from tartaric acid. A delightfully simple synthesis " of frontalin (121), a pheromone of the pine beetle, is by photolysis of heptane-2,6-dione in methanol-titanium chloride. We also note further photochemical studies... 

The unsatisfactory results generally obtained in the stereoselective synthesis described in Section 3.2.1 and the limits of photochemical routes underline once more the need of developing nonphotochemical enantioselective procedures. Even if asymmetric synthesis is a key step in many synthetic procedures of organic chemistry, and has nowadays reached a high level of sophistication, only sporadic cases have been reported in the field of thiahelicenes. This might also be due to the complex synthetic methods known for the preparation of thiahelicenes. Hence, efficient enantioselective synthesis of heterohelicenes stiU represents a challenging goal. 

J. Damen and D.G. Neckers, Stereoselective synthesis via a photochemical template effect,... 

One aspect of the current work on chemical control of insects centers on the use of sex pheromones as lures. Recently, chemists at the U.S. Department of Agriculture s Boll Weevil Research Laboratory isolated and synthesized components of the sex attractant of the male boll weevil, a pest responsible for a prodigious loss of cotton crop in the United States. A stereoselective synthesis of the active pheromone was later carried out at the Zoecon Corporation, and is outlined below. A key feature of this synthesis is the photochemical cycloaddition of ethylene to 3-methylcyclohex-2-enone, followed by oxidative disconnection of the resulting c/s-fused bicycio[4.2.0] octane (A) thereby generating the cis disposed side-chains. 

Nishimura, J., Takeuchi, M., Takahashi, H., and Sato, M., Intramolecular [2 -I- 2]-photocycloaddition. 9. Stereoselective synthesis of naphthalenophanes and effects of the tether length and substituents on the photochemical reaction courses of vinylnaphthalenes. Tetrahedron Lett., 31, 2911, 1990. 

Wessig, R, Stereoselective synthesis of novel chimerical amino acids via a photochemical key step, Synlett, 1465, 1999. 

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

Enantiomerically enriched formyl irimclhyleiiemethane -irontricarbonyl complexes add to allylzinc bromides with high stereoselectivity. This was used, after photochemical deprotection, in an enantioselcctive synthesis of the insect pheromone (-)-ipsdienol4s. 

Recent research deals with stereoselective 1,3-dipolar cycloadditions of nitrones for the syntheses of alkaloids and aza heterocycles asymmetric synthesis of biologically active compounds such as glycosidase inhibitors, sugar mimetics, /3-lactams, and amino acids synthesis of peptido-mimetics and peptides chemistry of spirocyclopropane heterocycles synthesis of organic materials for molecular recognition and photochemical applications. 

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