Efficient chirality transfer in the SmI2-mediated cyclization of aldehydo beta-alkoxyvinyl sulfoxides: asymmetric synthesis of 3-hydroxyoxanes

Stereoselective syntheses of 3-hydroxyoxanes were achieved via efficient chirality transfer in the SmI2-mediated cyclization reactions of aldehydo beta-alkoxyvinyl sulfoxides.     

Formation of organomagnesium compounds via EtMgBr-mediated radical cyclization of allyl beta-iodoacetals

[reaction: see text] Treatment of allyl beta-iodoacetals with ethylmagnesium bromide in THF provided tetrahydrofuran derivatives in good yields. On the other hand, the reaction in DME provided tetrahydrofuranylmethylmagnesium compounds in good yields.     

Vinyl radical cyclization in the synthesis of natural products: seychellene

The use of vinyl radical cyclization in the simplification of synthesis design is probed with the sesquiterpene seychellene as a target. The construction is particularly effective because of very high stereoselectivity in the alkylation of a 2,2,2-bi- cyclo-5-octene-2-one system and because of the control offered by the specific location of the double bond in the product of the vinyl radical cyclization.     

Cyclopentane synthesis and annulation: Intramolecular radical cyclization of acetals

A general procedure for cyclopentane synthesis based on free-radical cyclization of thioacetals is described. This permits the rapid assembly, by intramolecular annulation, of various ring systems bearing useful functionality for use in total synthesis.     

Convenient synthesis of 2-benzazepines via radical cyclization

Synthesis of 2-benzazepines was readily achieved via 7-endo radical cyclization of N-o-bromobenzylitaconamides or N-o-bromobenzylmethacrylamides which were prepared in two steps from commercially available benzaldehydes, amines, and alpha,beta-unsaturated acids.     

Stereoselection in the Prins-pinacol synthesis of acyltetrahydrofurans

[reaction: see text]. Depending upon the nature of the alkene and allylic substituents, acid-promoted rearrangements of acetals derived from anti allylic diols give 12 or stereoisomeric acyltetrahydrofurans 13. Stereoelectronic effects of the allylic substituents and the extent of bonding in the Prins cyclization transition state are central features of a proposed new model for predicting stereoselection in the Prins-pinacol synthesis of acyltetrahydrofurans.     

Stereoselective synthesis of 2,4,5-trisubstituted piperidines via radical cyclization

A novel approach to 2,4,5-trisubstituted piperidines is reported, involving the 6-exo cyclization of stabilized radicals onto α,β-unsaturated esters. Only two of the four possible diastereoisomers are observed, with diastereomeric ratios ranging from 3:2 to 40:1 when the radical stabilizing group is vinyl or phenyl. Cyclization of a (triethylsilyl)vinyl-stabilized radical gives the corresponding piperidine radical as a single diastereoisomer that may either be trapped by tributyltin hydride to afford the 2,4,5-trisubstituted piperidine or undergo a second 5-endo cyclization onto the (triethylsilyl)vinyl substituent to produce the 3,5,7-trisubstituted octahydro[2]pyrindene as a single diastereoisomer.     

Regioselective synthesis of heterocyclic scaffold by aryl radical cyclization

Regioselective synthesis of a number of coumarin‐annulated pentacyclic heterocycles have been achieved by tri‐n‐butyltin hydride‐mediated aryl radical cyclization. The products are formed as a mixture of cis‐ and trans‐ forms which were successfully separated by careful silica gel flash chromatography.     

An unusual stereochemical outcome of radical cyclization: synthesis of (+)-biotin

An enantioselective synthesis of (+)-biotin 1 starting from naturally available cysteine is described. The key steps are the unusual stereochemical outcome of radical cyclization of compound 10 to prepare 5,5-fused system 11, and the introduction of C4-sidechain at C₆ in 13 via a Grignard reaction.     

Diastereoselective synthesis of cyclopentapyridazinones via radical cyclization: synthetic studies toward halichlorine

The pyridazinone ring system serves as an excellent scaffold for the diastereoselective preparation of novel cis-fused cyclopentapyridazinones utilizing the directed 5-exo radical cyclization approach. This overall approach was successfully employed in the preparation of a functionalized aza-spirocycle.     

Conversions of allyl aryl sulfoxides under pummerer rearrangement conditions

In the presence of acetic acid, allyl phenyl sulfoxide forms -acetoxyallyl phenyl sulfide and 2-acetoxymethyl-2,3-dihydrobenzothiophene; under the same conditions allyl 1-naphthyl sulfoxide gives 2-methyl-2,3-dihydronaphtho-[1,2-b] thiophenoxide.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 747–749, June, 1985.     

Regioselective synthesis of quinolone-annulated sulfur heterocycles by aryl radical cyclization

The tin hydride-mediated cyclizations of a number of sulfides and sulfones under mild, neutral conditions, have been investigated accompanied by some amount of β-scission product for sulfides. The sulfides were derived from 4-mercaptoquinolone and 2-bromobenzyl bromides by phase transfer catalyzed reaction and the corresponding sulfones were prepared by treatment of the sulfides with m-CPBA at room temperature. The sulfides and the corresponding sulfones were then reacted with ⁿBu₃SnH-AIBN to give regioselective quinolone-annulated sulfur heterocycles.     

5-Exo-dig radical cyclization of enediynes: the first synthesis of tin-substituted benzofulvenes

[reaction: see text] Bu(3)Sn-mediated 5-exo-dig radical cyclization of diaryl enediynes provides a mild and efficient approach to tin-substituted fulvenes. Further synthetic opportunities opened by this process and general factors responsible for the observed regio- and stereoselectivity are outlined.     

Free radical intramolecular cyclization in the synthesis of heterocycles (review)

The possibilities of homolytic cyclization for the synthesis of heterocyclic compounds are examined. Factors affecting the yield and structure of the resulting products are discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1011–1034, August, 1993.     

Total synthesis of (+)-seco-C-oleanane via stepwise controlled radical cascade cyclization

An asymmetric concise total synthesis of the (+)-seco-C-oleanane 1 was accomplished. The successful route to this natural product involves as the key step a stepwise regio- and stereocontrolled catalytic radical polyene cascade cyclization from preoleanatetraene oxide (16), a process mediated by Cp(2)TiCl. The use of this single-electron-transfer complex permits mild cyclization conditions without using unnecessary prefunctionalizations and stops the process at the bicyclic level. Theoretical data revealed high activation energy for the third ring closure, which would account for the control of the cyclization. This process also led to natural (-)-achilleol B, camelliol A, and (+)-seco-β-amyrin as minor compounds.     

Thiophenol-mediated intramolecular radical cyclization: an efficient method for the synthesis of benzoxocine derivatives

A new, efficient, high yielding method for the synthesis of benzoxocine derivatives has been developed via a thiophenol-mediated intramolecular 8-endo radical cyclization. This method allowed the synthesis of the backbone of several sesquiterpenes.     

Highly stereoselective prins cyclization of silylmethyl-substituted cyclopropyl carbinols to 2,4,6-trisubstituted tetrahydropyrans

The homoallyl cation formed from a cyclopropyl carbinol that was vicinally substituted by a silylmethyl function underwent smooth Prins cyclization with aldehydes and ketones to form 2,4,6-trisubstituted tetrahydropyrans with very high stereoselectivity.     

A new synthesis of allyl sulfoxides via nucleophilic addition of sulfinyl carbanions to group 6 Fischer carbene complexes

[reaction: see text] A novel synthesis of allyl sulfoxides has been developed. Primary alpha-lithiosulfinyl carbanions react with group 6 Fischer carbene complexes to give allyl sulfoxides as products. The Fischer carbene complex experiments involve a 1,2-addition of two molecules of sulfinyl carbanion to give an intermediate that, after a beta-elimination, furnishes the mentioned product.