Oxyfunctionalization of Alkenes by Dye-Sensitized Intrazeolite Photooxygenation

Stratakis, Manolis

doi:10.2174/1570179053545387

Cited by 8 publications

(2 citation statements)

References 66 publications

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“…Recent publications report however on the detection of singlet oxygen under these conditions . Similar processes are also observed when the reactions are performed inside nanocavities of zeolites …”

Section: Photo-oxygenationmentioning

confidence: 56%

Photochemical Reactions as Key Steps in Organic Synthesis

2008

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Section: Photo-oxygenationmentioning

confidence: 56%

Photochemical Reactions as Key Steps in Organic Synthesis

2008

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“…The often fascinating regiochemistry and diastereoselection in the ene reaction of singlet oxygen ( 1 O 2 ) with olefins 1 provides an attractive methodology for their selective oxyfunctionalization. 2 Recently, the photooxygenation of alkenes confined in organized media, such as zeolite Y, 3 has gained significant attention due to the remarkable changes in product selectivity compared to the analogous reaction in solution. It has been recognized that for the 1 O 2 ene reactions, steric and electronic interactions contribute significantly to the stability of the product forming transition states.…”

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confidence: 99%

Highly Regioselective and Diastereoselective Photooxygenation of α-Cyclogeranyl Derivatives

Tsangarakis¹,

Zaravinos²,

Stratakis³

2005

Synlett

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Methylene blue-sensitized photooxygenation of several a-cyclogeranyl derivatives affords the exocyclic allylic hydroperoxides in 94-99% regioselectivity, while the diastereoselectivity varies from 50-97% de in favor of the cis isomer, depending on the substituents. By contrast, the ene reaction with N-phenyltriazolinedione affords significantly different regioselectivity and diastereoselectivity.The often fascinating regiochemistry and diastereoselection in the ene reaction of singlet oxygen ( 1 O 2 ) with olefins 1 provides an attractive methodology for their selective oxyfunctionalization. 2 Recently, the photooxygenation of alkenes confined in organized media, such as zeolite Y, 3 has gained significant attention due to the remarkable changes in product selectivity compared to the analogous reaction in solution. It has been recognized that for the 1 O 2 ene reactions, steric and electronic interactions contribute significantly to the stability of the product forming transition states. For example, 4 a 1 O 2 -hydroxy steering effect in combination with 1,3-allylic strain (steric effects) lead to remarkable threo diastereoselectivity in the photooxygenation of chiral allylic alcohols, while by contrast, for the analogous chiral allylic chlorides, repulsive 1 O 2 -halide atom interactions and 1,3-allylic strain as well lead to a highly erythro diastereoselective ene reaction. In addition, a highly diastereoselective and regioselective ene reaction of 1 O 2 with chiral cyclohexadienes has been reported, 5 where steric repulsions and electronic factors control substantially the product diastereoselectivity.In this communication we report a highly regioselective and diastereoselective ene reaction of 1 O 2 with a-cyclogeraniol (1) and its derivatives. Methylene blue-sensitized photooxygenation of 1 in CH 2 Cl 2 affords essentially one out of the three possible regioisomers with >97% cis-diastereoselectivity (1a), as proved by NOE experiments (Scheme 1). Irradiation of the allylic hydrogen atom next to the -OOH functionality in 1a resulted in signal enhancement of the second tertiary allylic hydrogen atom, indicative of a cis stereochemical arrangement. (1) To explore further this fascinating selectivity, we prepared the a-cyclogeranyl derivatives 2-7 6 and performed their photooxygenation reaction. The results are presented in Table 1. 7 The allylic hydroperoxides were purified and characterized after flash column chromatography of the crude reaction mixture. Alternatively, PPh 3 was added to the crude reaction mixture immediately after the photooxygenation and the corresponding allylic alcohols were subsequently purified. Scheme 1 Photooxygenation of a-cyclogeraniolThe reaction was >97% regioselective with exclusive formation of the ene adduct resulting from hydrogen atom abstraction from the allylic methyl group. The only exception was benzyl ether 5 who gave 5% of the tetrasubstituted allylic hydroperoxide formed by abstraction of the tertiary allylic hydrogen atom. The diastereoselectivity varied from 50-97% de,...

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Zeolite NaY‐Promoted Monocyclization of Epoxy Polyene Terpenes: A Unique Route for the Direct Synthesis of Incompletely Cyclized Naturally Occurring Terpenols

et al. 2008

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A variety of epoxy polyene terpenes cyclize readily by confinement within zeolite NaY to form primarily products of monocyclization. The monocyclization pathway is highly predominant, irrespectively of the side chain of the epoxy terpene, while the monocyclic products possess regioselectively an exomethylenic double bond. The selective monocyclization in the case of epoxyfarnesyl acetate, epoxyfarnesylacetone or 2,3-epoxysqualene, provides a direct route to the synthesis of a variety of natural products, such as elengasidiol, farnesiferols B-D, achilleol A, camelliol C and to four farnesylacetonederived metabolites isolated from the brown algae Cystophora monoliformis. The optical rotation of achilleol A derived from the cyclization of (S)-2,3-epoxysqualene matches with that of the natural product, thus the absolute configuration of achilleol A was established as 1S,3R. From the mechanistic point of view, the NaY-promoted cyclization of 9,10-epoxygeranylacetone, selectively deuterium labelled at the C-10 methyl group, is > 97% stereoselective with respect to the topicity of the gem-dimethyl group. This result is in agreement with a concerted mechanism. Finally, we have proved through labelling experiments, for the first time, that the biomimetic transformation of epoxy polyene terpenes to 2,3,4-trimethylcyclohexanones upon acid catalysis is a highly stereoselective process. Thus, the less hindered gem-methyl group on the epoxide functionality becomes a-to the carbonyl in the final isomerized product.

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Oxyfunctionalization of Alkenes by Dye-Sensitized Intrazeolite Photooxygenation

Cited by 8 publications

References 66 publications

Photochemical Reactions as Key Steps in Organic Synthesis

Photochemical Reactions as Key Steps in Organic Synthesis

Highly Regioselective and Diastereoselective Photooxygenation of α-Cyclogeranyl Derivatives

Zeolite NaY‐Promoted Monocyclization of Epoxy Polyene Terpenes: A Unique Route for the Direct Synthesis of Incompletely Cyclized Naturally Occurring Terpenols

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