Photodeconjugation, unsaturated esters

The interesting pyranone (1) has been obtained by photodecarboxylation of (2) using low-temperature matrix isolation techniques (Bleasdale et al.). Bart-nik et al. have described a simple photodecarboxylation route to azabicyc-lobutanes such as (3). Attention is drawn to a new photochemical synthesis of p-lactones (Aoyama et al.) and -lactams (see ref. 50 in Part III, Chapter 2). Skinner and Weedon have reported an interesting method for photodeconjugation of a,p-unsaturated esters e.g., (4) (5). Polar solvents are required,... 

Since the early observations of photodeconjugation of conjugated aliphatic carbonyl and carboxyl derivatives [19], the reaction has been generalized to unsaturated esters [20], lactones [21], acids [22], amides [20n], aldehydes [23], aliphatic enones [24], and medium-ring unsaturated enones [25]. 

Synthetic applications of the diastereoselective photodeconjugation of unsaturated esters 13 and 17 were explored and compared with the corresponding enantioselective approach. Highly enantioselective syntheses... 

The E)- Z) photoisomerization of a, 8-unsaturated esters/ cinnamic esters/ butenoic esters/ anddienoic esters is catalyzed by Bp3-OEt2 ovEthylahiminum Dichloride. The latter two reactions also involve the photodeconjugation of a, 8-unsaturated esters to /3,)/-unsaturated esters. The BFs-MeOH complex is used for the isomerization of 1- and 2-butenes to form equal quantities of cis- and tran5-but-2-enes/ the BF3-OEt2-acetic acid complex is not as effective. 

Piva O, Mortezaei R, Henin F, Muzart J, Pete JP. Highly enantioselective photodeconjugation of a,p-unsaturated esters, origin of the chiral discrimination. J. Am. Chem. Soc. 1990 112 9263-9272. 

Asymmetric Photodeconjugation Principle Enantioselective Photodeconjugation of a,P-Unsaturated Esters and Lactones Diastereoselective Photodeconjugations. .. 70-7... 

Because the synthesis of chiral fluoro compounds is of great interest, due to their biological properties, the asymmetric photodeconjugation of a-fluoro-a,P-unsaturated esters 46 has been explored. Despite the high acidity of the hydrogen on the C2 atom, the diastereoselective isomerization to the P.y-unsat-urated isomers is effective with levels of induction for 47 similar to those for non-fluorinated substrates (Scheme 25). 

Mortezaei, R., Henin, F, Muzart, J., and Pete, J. R, Enantioselective photodeconjugation of a,P-unsaturated esters in the presence of catalytic amounts of a chiral-inducing entity. Tetrahedron Lett., 26, 6079, 1985. 

Bargiggia, F. and Piva, O., Diastereoselective photodeconjugation of chiral a,(i-unsaturated esters. Tetrahedron Asymm., 12,1389, 2001. 

This chapter covers organocatalytic processes where the enantio-determining step involves a proton transfer. Most of the organocatalytic processes outlined herein share a key step in common, i.e. the enantioselective protonation of an enolate or enol intermediate species obtained in situ from various precursors. The main organocatalytic approaches reported in the literature may be classified according to the nature of these precursors (Scheme 3.1). Special emphasis will be giveu to decarboxylation of malonates, addition of protic nucleophiles (NuH) to keteues or to a,P-unsaturated carbonyl compounds. We will also focus on tautomerisation of enols formed in situ via photodeconjugation of a,P-unsaturated esters and on the protonation of silyl enolates. Finally, a last section will be devoted to other miscellaneous substrates. 

The stereochemical outcome of numerous fundamental chemical transformations such as decarboxylation of malonates, tandem hetero-Michael addition and nucleophilic addition to ketenes, photodeconjugation of unsaturated esters, pro-... 

Buffer catalysis has been applied to induce chiral induction by enantio-selective protonation remarkable enantiomeric excess was achieved in the photodeconjugation of a,/3-unsaturated ketones and esters by using chiral catalysts for the ketonization of photoenols in aprotic solvents.29... 

Scheme 4. Unsaturated lactones, esters, enones and -aminoalcohols considered in enantioselective photodeconjugation reactions.

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