Stereoselective cyclization

Morpholinone 177, derived from allyl glycine, cyclized stereoselectively under UV irradiation (A > 342 nm) in the presence of 9,10-anthracene dicarbonitrile (ADC) and biphenyl (PB) to give compound 176 as a single diastereo-isomer <2001JOC6896> (Scheme 26). 

Pandey and co-workers developed two photosystems useful for initiating one-electron reductive chemistry and applied them to activate a, 3-unsaturated ketones. The resulting carbon-centered radicals cyclize stereoselectively with proximate olefins. Their concept involved a secondary and dark electron transfer from... 

Sulfonyl-substituted homoallylic alcohols tmdergo 5-endo-trig cyclization reactiorrs on treatment with base, with cyclization stereoselectively depending on double bond geometry, to give substituted tetrahydrofurans <99T13471>. 

Intramolecular conjugate addition of carbamates to a,fi-unsaturated esters.3 In the presence of potassium f-butoxide or NaH, allylic carbamate esters 1 cyclize stereoselectively to traws-oxazolidones (2). 

Key-step in the mechanistic scenario is a primary electron transfer process involving a sacrificial electron donor as exemplary shown for the triphenylphosphine case in Sch. 28. The 9,10-dicyanoanthracene radical anion (DCA -) thus generated undergoes a secondary thermal electron transfer to the unsaturated ketone. The resulting carbon-centered radical or radical anionic intermediate, subsequently cyclizes stereoselectively with a proximate olefin. The observed 1,2- 77 -stereochemistry of the C-C bond formation step contrasts with the commonly observed -stereoselectivity of 5-hexenyl radical cyclizations. As sacrificial electron donors, the... 

The tricarbonylchromium complexes of (o-chlorophenyl) but-3-en-l-yl ethers undergo carbonylative cyclization stereoselectively." Homologation of benzylic alcohols to form arylacetic acids is achieved by Pd(0)-catalyzed carbonylation using HI and CO. ... 

The lack of correlation between cyclization stereoselectivity and the steric energy of the products (calculated by the MMX force field)... 

The cis-sclcctivc introduction of a cyclopentyl ring via a radical cyclization is the key step in the synthesis of ( )-silphinene. an angular fused triquinane48. Conversion of readily available 3,5.5-trimethylcyclohexenone to the radical precursor 3, followed by Barton deoxygenation16 generates the radical, which cyclizes stereoselectively to the tricyclopentanoid skeleton 4. 

Electron transfer to 2-(3-butenyl)cyclopentanone using zinc-chlorotrimethylsilane generates a radical that cyclizes stereoselectively in a 5-exo mode to cfv-annulated cyclopentanols31. Similar to the cyclizations of the 2-(3-butenyl)cyclopentyl radical, a strong preference for the 1,5-ris-substituted diastereomer is observed. 

Principles of 4-Penten-l-oxyI Radical Cyclizations -Stereoselectivity, Regioselectivity, and Theoretical Considerations... 

The 1,2-5y -configured radical 45 does not cyclize stereoselectively. Both dia-stereomeric bromomethyl-substituted tetrahydrofurans 3,5-/ra 5-44 and 3,5-cis- 44 are obtained in equal amounts from the photoreaction of pyridinethione 43 and BrCCh (Scheme 11). This observation is explained by the transition state model for intermediate 45, where the methyl and the benzoyloxy substituent should both compete for / . eMconformationally flexible intermediate 45. 

A reaction of the dinitrile (83) with hydrogen chloride in ether yielded the enaminonitrile (84). From the unsaturated (85), a dark blue immonium salt (86) was obtained <80JCR(S)233>. A new metabolite from Streptomyces olivaceus has been shown to be (25)-l-oxo-2,3-dihydropyrrolizin-3-carboxylic acid (88b) by total synthesis. Compound (87) was cyclized stereoselectively with phosphorus pentoxide in toluene. Partial racemization occurred during the hydrolysis of (88a) <81TL3293>. 

A new method for the conversion of aldopyranosides to 5-thioaldopyranosides is exemplified in Scheme 3 the isomeric acyclic intermediates, e.g. compound 33, could be separated, inverted at C-5 and cyclized stereoselectively, thus offering access to several different products. 5-Thio-D-arabinose (34) has been synthesized from D-arabinose in six standard reaction steps. Synthesis of 5-thio-L-fucopyranose, as the tetraacetate 35, from D-arabinose required chain-extension at the non reducing end which was achieved by diastereoselective reaction of aldehyde 36 with methyl lithium the D-aftro-product 37 was then treated sequentially with tosyl chloride and potassium thioacetate. A number of aryl a-5-thiofucosides, as well as p-nitrophenyl a-l,5-dithiofucoside, were prepared from 1-acetate 35 directly or via a trichloroacetimidate. These compounds were used to demonstrate the importance of sulfur in the ring and oxygen at the... 

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