Intramolecular reactions of hydroperoxides and oxetanes: stereoselective synthesis of 1,2-dioxolanes and 1,2-dioxanes

[reactions: see text] The 5-exo openings of oxetanes by hydroperoxides proceed rapidly and stereospecifically to furnish 1,2-dioxolanes. The corresponding 6-exo cyclizations are slower and proceed with moderate stereoselectivity. In the case of hydroperoxy acetals, 5-exo nucleophilic transfer of alkoxide competes effectively with 6-exo attack by the hydroperoxide.     

Radical-induced formation of some siloles and diazasiloles

The Bu(3)Sn radical-induced reaction of o-iodobenzylvinylsilanes and o-iodoallylsilanes leads to cyclic products in yields of 40-60%. These regioselective cyclizations occur with high preference for a 5-exo and a 7-endo mode with a 6-exo mode being absent. An example for ring closure via a 7-exo mode is described.     

A study of aryl radical cyclization in enaminone esters

Aryl radical cyclization in N-phenyl, N-benzyl, and N-phenethyl enaminone esters 1a-f was studied. N-Benzyl and N-phenethyl enaminones afforded 5-exo and 6-exo cyclization products, respectively, but radical cyclization did not occur in N-phenyl enaminones. The rate constants for the 5-exo and 6-exo cyclization processes in secondary enaminones were estimated as being on the order of 10(7) s(-1) at 353 K; since DNMR experiments showed the rate constant for rotation around the enaminone C3-N bond to be on the order of 10(4) s(-1) at this temperature, the initial enaminone configuration is maintained throughout the cyclization process. PM3 calculations suggested that the nonoccurrence of endo and 4-exo cyclizations is due to the corresponding transition structures involving significant distortion of the conjugated enaminone system.     

Preference of beta-lactam formation in Cu(I)-catalyzed intramolecular coupling of amides with vinyl bromides

A general and highly efficient synthesis of 4-alkylidene-2-azetidinones was achieved by the Cu(I)-catalyzed intramolecular C-N coupling of amides with vinyl bromides. This 4-exo ring closure was found to be fundamentally preferred over other modes (5-exo, 6-exo, and 6-endo) of cyclization under copper catalysis. Tandem C-N bond formation was then successfully developed to allow the convenient generation of medium-sized lactams.     

Stereoselective cascade reactions that incorporate a 7-exo acyl radical cyclization

[reaction: see text] Radical cascades that feature a 7-exo acyl radical cyclization followed by a 6-exo or 5-exo alkyl radical cyclization proceed with very good yields and diastereoselectivities. Two stereocenters are created by the reaction, and a single isomeric product was obtained from each of the five substrates examined. The relative configurations of the products are consistent with cyclizations occurring via chairlike or pseudochairlike transition states.     

Thermodynamic and strain effects in the competition between 5-exo-dig and 6-endo-dig cyclizations of vinyl and aryl radicals

Electronic and structural factors controlling the competition between 5-exo-dig and 6-endo-dig cyclizations of sp2-radicals were analyzed using a combination of available experimental data and computation. Although the stereoelectronically favored 5-exo pathways usually has the lower activation energy, formation of a new aromatic ring not only makes the 6-endo process favorable thermodynamically in conjugated systems but also lowers its activation barrier to the extent where the 5-exo/6-endo selectivity is controlled by subtle factors such as the different sensitivity of the two pathways to strain effects in polycyclic systems. In particular, the stronger sensitivity of the 5-exo pathway to strain leads to a crossover in selectivity. The 6-endo cyclization is kinetically favored in smaller (and strained) cycles, whereas the 5-exo cyclization has lower barriers in the larger rings.     

Alkoxy radical cyclizations onto silyl enol ethers relative to alkene cyclization, hydrogen atom transfer, and fragmentation reactions

This study examines the chemoselectivity of alkoxy radical cyclizations onto silyl enol ethers compared to competing cyclizations, 1,5-hydrogen atom transfers (1,5-HATs), and β-fragmentations. Cyclization onto silyl enol ethers in a 5-exo mode is greatly preferred over cyclization onto a terminal alkene. The selectivity decreases when any alkyl substitution is present on the competing alkene radical acceptor. Alkoxy radical 5-exo cyclizations displayed excellent chemoselectivity over competing β-fragmentations. Alkoxy radical 5-exo cyclizations onto silyl enol ether also outcompeted 1,5-HATs, even for activated benzylic hydrogen atoms. In tetrahydropyran synthesis, where 1,5-HAT has plagued alkoxy radical cyclization methodologies, 6-exo cyclizations were the dominant mode of reactivity. β-Fragmentation still remains a challenge for tetrahydropyran synthesis when an aryl group is present in the β position.     

Iodocyclization reactions for the desymmetrization of cyclohexa-1,4-dienes

Fifteen examples are presented showing that various modes of cyclization (5-endo, 5-exo, 6-endo, 6-exo, and 7-endo) can be used for the desymmetrization of cyclohexa-1,4-dienes. All take place with complete diastereocontrol and good yield.     

An unequivocal1H NMR structural assignment of TOX8 and TOX9, the two most abundant toxaphene congeners in marine mammals

The structure of the two most abundant toxaphene congeners has unequivocally been established by 500 MHz¹H NMR spectroscopy as 2-endo,3-exo,5-endo, 6-exo,8,8,9,10,10-nonachlorobornane (TOX9) and as 2-endo,3-exo,5-endo,6-exo,8,8,10,10-octachlorobornane (TOX8). Semiempirical calculations (AM1 and PM3-MNDO) were carried out for both structures. The distance information found by nuclear Overhauser enhancement (NOE) for the protons is in agreement with the energy minimized AM1 and PM3-MNDO structures. For these definitively established NMR data for TOX8 and TOX9, together with literature data for other toxaphene isolates, a set of rules has been derived for¹H chemical shifts in polychlorinated bornane structures. A set of rules is also proposed for assigning systematical nomenclature to NMR-derived polychloro bornane structures.     

An unequivocal1H NMR structural assignment of TOX8 and TOX9, the two most abundant toxaphene congeners in marine mammals

The structure of the two most abundant toxaphene congeners has unequivocally been established by 500 MHz¹H NMR spectroscopy as 2-endo,3-exo,5-endo, 6-exo,8,8,9,10,10-nonachlorobornane (TOX9) and as 2-endo,3-exo,5-endo,6-exo,8,8,10,10-octachlorobornane (TOX8). Semiempirical calculations (AM1 and PM3-MNDO) were carried out for both structures. The distance information found by nuclear Overhauser enhancement (NOE) for the protons is in agreement with the energy minimized AM1 and PM3-MNDO structures. For these definitively established NMR data for TOX8 and TOX9, together with literature data for other toxaphene isolates, a set of rules has been derived for¹H chemical shifts in polychlorinated bornane structures. A set of rules is also proposed for assigning systematical nomenclature to NMR-derived polychloro bornane structures.     

Metal-catalyzed regioselective oxy-functionalization of internal alkynes: an entry into ketones, acetals, and spiroketals

[reaction: see text] Platinum(II) and an unusual cationic gold(I) complex were identified as mild catalysts for the room temperature cycloisomerization or tandem hydroalkoxylation/acetal formation of unactivated internal alkynols. Under the appropriate conditions, 5-endo, 5-exo, 6-endo, and 6-exo cycloisomerization modes are all available.     

Cyclizations of N-stannylaminyl radicals onto nitriles

[reaction: see text] Stannylaminyl radicals derived from radical reactions of Bu(3)SnH with azidoalkylmalononitriles exhibit highly efficient 5- and 6-exo cyclization onto either nitrile group to give aminoiminyl radicals that in turn are reduced to amidines or undergo successive 5-exo cyclization onto an internal alkene.     

Regiochemistry of copper(I)-mediated cyclization reactions of halo-dienamides

Reaction of 2-substituted dienamides with catalytic amounts of copper halide/tripyridylamine (TPA) furnishes either 5-exo or 6-endo products with the outcome dependent upon the radical initiating unit. Reaction of 3-substituted dienamides produces beta-lactams via a 4-exo cyclization with termination of the reaction occurring via either halogen atom transfer, trapping with oxygen, elimination, or radical-radical coupling depending upon the diene.     

Stereoselective cyclization reactions of IBX-generated alkoxyamidyl radicals

In this paper, a method for the generation of alkoxyamidyl radicals is presented. These N-centered radicals can efficiently be formed starting from the corresponding acylated alkoxyamines using IBX as an oxidant. Stereoselective 5-exo and 6-exo reactions with these N-heteroatom-centered radicals leading to isoxazolidines and [1,2]oxazinanes are discussed. The N-O bond in the heterocycles can readily be cleaved with SmI(2) to provide N-acylated 1,3-amino alcohols.     

Synthesis of 2-substituted (+/-)-(2r,3r,5r)-tetrahydrofuran-3,5-dicarboxylic acid derivatives

An efficient synthesis of 2-substituted (+/-)-(2R,3R,5R)-tetrahydrofuran-3,5-dicarboxylic acid derivatives has been developed. Starting from 5-norborne-2-ol, the key intermediate (+/-)-methyl 5,6-exo,exo-(isopropylidenedioxy)-2-oxabicyclo[2.2.1]heptane-3-exo-carboxylate (15) was synthesized in an efficient six-step sequence. The key transformation is the base-catalyzed methanolysis-rearrangement of (+/-)-6,7-exo,exo-(isopropylidenedioxy)-4-exo-iodo-2-oxabicyclo[3.2.1]octan-3-one (14). Further manipulation of the 3-substituent of (+/-)-methyl 5,6-exo,exo-(isopropylidenedioxy)-2-oxabicyclo[2.2.1]heptane-3-exo-carboxylate (15) followed by deprotection of the diol moiety and ring opening catalyzed by RuCl(3)/NaIO(4) gave the title compounds in good yield.     

Pushing radical cyclization from regioselective to regiospecific: cyclization of amidyl radicals controlled by vinylic halogen substitution

Efficient and regiospecific 6-exo, 7-endo, 7-exo, and even 8-endo amidyl radical cyclizations can be accomplished by the direct reactions of unsaturated N-H amides if they bear a vinylic halogen (Cl, Br, I) substituent. This remarkable halogen-substitution effect could be rationalized in terms of lone pair-lone pair electron repulsion between the N radical and the vinylic halogen atom, in addition to the well-known steric and radical-stabilizing effect. [reaction: see text]     

Generation of alpha-phosphonovinyl radicals and development of a new route to highly functionalized vinylphosphonates and vinylphosphonate-incorporated carbocyclic or heterocyclic compounds via a radical trapping sequence

The first direct generation of synthetically useful alpha-phosphonovinyl radicals was achieved by treatment of alpha-phosphonovinyl halides with a tributyltin radical. The alpha-phosphonovinyl radicals 2a-d were trapped with electron-rich olefins and an electron-deficient olefin to produce alpha-functionalized vinylphosphonates 3a-f in 16-55% yields. The alpha-phosphonovinyl radicals 7e-g containing the YCH2CH=CH2 (Y = O, CH2, S) substituent at the beta-position afforded mixtures of 5-exo and 6-endo cyclization products, 5e-g and 6e-g, in good yields. The 5-exo/6-endo product ratios increase in the following order of the beta-substituent: OCH2CH=CH2 > CH2CH2CH=CH2 > SCH2CH=CH2. The effects of the beta-substituents upon the cyclization reaction were discussed. Radical cyclization of alpha-phosphonovinyl radicals bearing functional groups such as geranyloxy, geranylthio, and (2-cyclohexen-1-yl)thio groups at the beta-position afforded 5-exo, 5-exo and 6-endo, and cis-fused-5,6-ring cyclization products incorporating an alpha,beta-unsaturated phosphonate unit within the ring, respectively, in good yields. The alpha-phosphonovinyl radical 20 underwent tandem radical cyclization-radical cyclization to produce a mixture of two isomeric bicyclo[4.3.0]nonenes including a vinylphosphonate moiety in high yield.     

Kinetic and theoretical study of 4-exo ring closures of carbamoyl radicals onto C=C and C=N bonds

Carbamoyl radicals were generated from oxime oxalate amides, and the kinetics of their 4-exo cyclizations onto C=C and C=NO bonds, leading to beta-lactam-containing species, were studied by EPR spectroscopy. DFT computations with model carbamoyl radicals predicted 4-exo ring closures onto C=NO bonds to be facile, especially when tert-butyl substituents were present. The reverse ring-opening reactions were predicted to have much higher activation energies. Experimental evidence also favored slow reverse ring opening.     

Indium-mediated atom-transfer and reductive radical cyclizations of iodoalkynes: synthesis and biological evaluation of HIV-protease inhibitors

Novel indium-mediated radical cyclization reactions of aliphatic iodoalkynes have been studied. Treatment of iodoalkynes with a catalytic amount of In (0.1 equiv) and I(2) (0.05 equiv) promotes atom-transfer 5-exo cyclization to give five-membered alkenyl iodides. In contrast, reaction with In (2 equiv) and I(2) (1 equiv) yields reductive 5-exo cyclization products via the same 5-exo cyclization. Both processes are most likely initiated by low-valent indium species. To demonstrate versatility of these reactions, optically active HIV protease inhibitors were synthesized by this reductive cyclization method. Among them, several products, which contain a hydroxyethylamine dipeptide isostere as a transition state-mimicking substructure, proved to possess potent activity (IC(50) = 5-39 nM) against a wide spectrum of HIV strains, including multidrug-resistant variants.     

8-endo versus 7-exo cyclization of alpha-carbamoyl radicals. A combination of experimental and theoretical studies

Atom transfer radical cyclization reactions of N-(4-pentenyl)iodoacetamides were investigated. The reactions were efficiently promoted by BF3.OEt2. For N-alkenyl-substituted iodoamides, excellent regioselectivity in favor of 8-endo cyclization was observed, while both 7-exo and 8-endo cyclization products were formed with the 8-endo cyclization preferred in the cases of N-(2-allylphenyl)-substituted iodoamides. Density functional theory calculations at the B3LYP/6-31G level revealed that both the s-trans and the s-cis conformational transition structures were feasible for the 8-endo cyclization of N-alkenyl-substituted alpha-carbamoyl radicals while 7-exo transition structures were much less stable. For the cyclization of N-(2-allylphenyl)-substituted alpha-carbamoyl radicals, the transition structures for 8-endo and 7-exo cyclizations were of comparable energy. These results were in excellent agreement with the experimental observations.