Efficient synthesis of cis- and trans-3,4-dihydroxy-3,4-dihydromollugin

An efficient synthesis of naturally occurring compounds isolated from Pentas longiflora, cis-3,4-dihydroxy-3,4-dihydromollugin 2, and trans-3,4-dihydroxy-3,4-dihydromollugin 3 is described. The O-protected mollugins were dihydroxylated using OsO4 to achieve the corresponding cis-dihydroxy derivatives in excellent yield. The synthesis of trans-3,4-dihydroxy-3,4-dihydromollugin was achieved using Oxone in good yield. A mechanism for the formation of cis-3,4-dihydroxymollugin acetonide from the reaction of mollugin with Oxone is proposed.     

Addition of the phenoxathiin cation radical to alkenes and nonconjugated dienes. Formation of (E)- and (Z)-(10-phenoxathiiniumyl)alkenes and (E)- and (Z)-(10-phenoxathiiniumyl)dienes on basic alumina

Addition of phenoxathiin cation radical (PO*+) to acyclic alkenes in acetonitrile (MeCN) solution occurred stereospecifically to form bis(10-phenoxathiiniumyl)alkane adducts. Stereospecific trans addition is ascribed to the intermediacy of an episulfonium cation radical. The alkenes used were cis- and trans-2-butene, cis- and trans-2-pentene, cis- and trans-4-methyl-2-pentene, cis- and trans-4-octene, trans-3-hexene, trans-3-octene, trans-5-decene, cis-2-hexene, and cis-2-heptene. The erythro bisadducts (compounds 6) were obtained with trans-alkenes, while threo bisadducts (compounds 7) were obtained with cis-alkenes. The assigned structures of 6 and 7 were consistent with their NMR spectra and, in one case, 6c (the adduct of trans-4-methyl-2-pentene) was confirmed with X-ray crystallography. Additions of PO*+ to 1,4-hexa-, 1,5-hexa-, 1,6-hepta-, and 1,7-octadiene gave bis(10-phenoxathiiniumyl)alkenes (compounds 8), the assigned structures of which were consistent with their NMR spectra. Each of these adducts lost a proton and phenoxathiin (PO) when treated with basic alumina in MeCN solution. Compounds 6 (from trans-alkenes) gave mixtures of (Z)- (9) and (E)-(10-phenoxathiiniumyl)alkenes (10) in which the (Z)-isomers (9) were dominant. On the other hand, compounds 7 (from cis-alkenes) gave mixtures of 9 and 10 in which, with one exception (the adduct 7c of cis-4-methyl-2-pentene), compounds 10 were dominant. The path to elimination is discussed. The alkenes 9 and 10 were characterized with NMR spectroscopy and, in one case (9a), with X-ray crystallography. Reactions of 8b-d with basic alumina gave mixtures of (E)- (13) and (Z)-(10-phenoxathiiniumyl)dienes (14), in which compounds 13 were dominant. The configuration of the product from 8a (the adduct of 1,4-hexadiene) could not be settled. Noteworthy features in the coupling patterns and chemical shifts in the NMR spectra of some of the adducts and their products are discussed and related to adduct conformations.     

Short, highly efficient syntheses of protected 3-azido- and 4-azidoproline and their precursors

[structure: see text] An improved synthesis of protected cis- and trans-3-azido-L-proline and cis- and trans-4-azido-L- and -D-proline is reported. These compounds have been synthesized from the corresponding hydroxyproline precursors using diphenylphosphoryl azide under Mitsunobu conditions. Short, highly efficient syntheses of these precursors are described, based on a new lactone-opening reaction and p-nitrobenzoate hydrolysis under very mild conditions.     

Diversity-oriented synthesis of enantiomerically pure steroidal tetracycles employing Stille/Diels-Alder reaction sequences

Various steroid analogues were synthesized by Stille coupling of bicyclo[4.3.0]nonenylstannanes cis-/trans-8 and 14 with cyclohexenol triflates 17 and 18 and subsequent Diels-Alder reactions of the resulting dienes. The enantiomerically pure bicyclo[4.3.0]nonenylstannanes cis- and trans-8 were prepared in good yields via the enol triflates cis- and trans-7, obtained from the bicyclo[4.3.0]non-2-en-3-one 5. The alkenylstannane 14 was obtained from the [2+2] cycloadduct 10 a produced from addition of dichloroketene to the enantiomerically pure and protected bishydroxycyclohexadiene 9 a (65 %). Treatment of 10 a with diazomethane, reduction of the dichloromethylene group, and trapping with tributyltin chloride after lithium-for-bromine exchange, yielded the bicyclo[4.3.0]nonenylstannane 14 (23 % over four steps). Stille couplings provided the tricyclic dienes cis-/trans-19 in good yields (73-77 %), whereas the tricyclic diene 20 was obtained in only 34 % yield at best. Diels-Alder reactions of trans-19 with various reactive dienophiles yielded the novel steroidal compounds trans-21 to trans-26 with complete diastereoselectivity. Heating the dienes cis-19 or 20 with maleic acid derivatives provided the corresponding tetracycles cis-23alpha,beta and 27alpha,beta with a cis-C,D ring junction, each as mixtures of two diastereomers. Less reactive dienophiles required higher temperatures to promote the relevant cycloaddition with trans-19 to furnish several stereoisomeric forms of trans-28 and trans-29 in significantly lower yields (31-45 %). The selected steroid analogues trans-22 and trans-23 were deprotected in two steps by using acid catalysis to provide trans-31 and trans-33 (91 and 80 % over two steps). Cyclopropanation of trans-30 yielded the cyclopropasteroid analogue 34 (74 %), treatment of which with trifluoroacetic acid furnished the cyclopropasteroid 35 and the 2-methyl-substituted steroid analogue 36 in 40 and 12 % yield, respectively. Aromatic B-ring steroids 38 (69 %) and 39 (5 %) were accessed by dehydrogenation of trans-24 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.     

Bis(acetylacetonato)ruthenium(II) complexes containing alkynyldiphenylphosphines. Formation and redox behaviour of [Ru(acac)2(Ph2PC triple bond CR)2] (R=H, Me, Ph) complexes and the binuclear complex cis-[{Ru(acac)2}2(micro-Ph2PC triple bond CPPh2)}2

Two equivalents of Ph(2)PC triple bond CR (R=H, Me, Ph) react with thf solutions of cis-[Ru(acac)(2)(eta(2)-alkene)(2)] (acac=acetylacetonato; alkene=C(2)H(4), 1; C(8)H(14), 2) at room temperature to yield the orange, air-stable compounds trans-[Ru(acac)(2)(Ph(2)PC triple bond CR)(2)] (R=H, trans-3; Me=trans-4; Ph, trans-5) in isolated yields of 60-98%. In refluxing chlorobenzene, trans-4 and trans-5 are converted into the yellow, air-stable compounds cis-[Ru(acac)(2)(Ph(2)PC triple bond CR)(2)] (R=Me, cis-4; Ph, cis-5), isolated in yields of ca. 65%. From the reaction of two equivalents of Ph(2)PC triple bond CPPh(2) with a thf solution of 2 an almost insoluble orange solid is formed, which is believed to be trans-[Ru(acac)(2)(micro-Ph(2)PC triple bond CPPh(2))](n) (trans-6). In refluxing chlorobenzene, the latter forms the air-stable, yellow, binuclear compound cis-[{Ru(acac)(2)(micro-Ph(2)PC triple bond CPPh(2))}(2)] (cis-6). Electrochemical studies indicate that cis-4 and cis-5 are harder to oxidise by ca. 300 mV than the corresponding trans-isomers and harder to oxidise by 80-120 mV than cis-[Ru(acac)(2)L(2)] (L=PPh(3), PPh(2)Me). Electrochemical studies of cis-6 show two reversible Ru(II/III) oxidation processes separated by 300 mV, the estimated comproportionation constant (K(c)) for the equilibrium cis-6(2+) + cis6 <=> 2(cis-6(+)) being ca. 10(5). However, UV-Vis spectra of cis-6(+) and cis-6(2+), generated electrochemically at -50 degrees C, indicate that cis-6(+) is a Robin-Day Class II mixed-valence system. Addition of one equivalent of AgPF(6) to trans-3 and trans-4 forms the green air-stable complexes trans-3 x PF(6) and trans-4 x PF(6), respectively, almost quantitatively. The structures of trans-4, cis-4, trans-4 x PF(6) and cis-6 have been confirmed by X-ray crystallography.     

Chemical behavior of the biradicaloid (HO...ONO) singlet states of peroxynitrous acid. The oxidation of hydrocarbons, sulfides, and selenides

Various high levels of theory have been applied to the characterization of two higher lying biradicaloid metastable singlet states of peroxynitrous acid. A singlet minimum (cis-2) was located that had an elongated O-O distance (2.17 A) and was only 12.2 kcal/mol [UB3LYP/6-311+G(3df,2p)+ZPVE] higher in energy than its ground-state precursor. A trans-metastable singlet (trans-2) was 10.9 kcal/mol higher in energy than ground-state HO-ONO. CASSCF(12,10)/6-311+G(d,p) calculations predict the optimized geometries of these cis- and trans-metastable singlets to be close to those obtained with DFT. Optimization of cis- and trans-2 within the COSMO solvent model suggests that both exist as energy minima in polar media. Both cis- and trans-2 exist as hydrogen bonded complexes with several water molecules. These collective data suggest that solvated forms of cis-2.3H(2)O and trans-2.3H(2)O represent the elusive higher lying biradicaloid minima that were recently (J. Am. Chem. Soc. 2003, 125, 16204) advocated as the metastable forms of peroxynitrous acid (HOONO). The involvement of metastable trans-2 in the gas phase oxidation of methane and isobutane is firmly established to take place on the unrestricted [UB3LYP/6-311+G(d,p)] potential energy surface (PES) with classical activations barriers for the hydrogen abstraction step that are 15.7 and 5.9 kcal/mol lower than the corresponding activation energies for producing products methanol and tert-butyl alcohol formed on the restricted PES. The oxidation of dimethyl sulfide and dimethyl selenide, two-electron oxidations, proceeds by an S(N)2-like attack of the heteroatom lone pair on the O-O bond of ground-state peroxynitrous acid. No involvement of metastable forms of HO-ONO was discernible.     

Stereoselective synthesis of functionalized trans-2,5-disubstituted tetrahydrofurans

[reaction: see text] The addition of the titanium enolates of N-acetyl, N-propionyl, and N-bromoacetyl (R)-oxazolidin-2-ones to gamma-lactol 2, derived from (S)-glutamic acid, afforded trans- and cis-2,5-disubstituted tetrahydrofurans (trans/cis ratio: R = H, 2:1; R = Me, 8:1; R = Br, 10:1) after desilylation with aqueous HF/CH3CN. Chromatographic separation and LiBH4 reduction allowed the efficient preparation of the corresponding trans-2,5-disubstituted tetrahydrofuran diols and the recovery of the chiral auxiliary.     

Synthesis,stereochemistry, and isomeric transformations of cis- and trans-1,2-dimethyl-4-aryl-5-aroyl-2-imidazolines

The corresponding trans- and cis-1,2-dimethyl-4-aryl-5-aroyl-2-imidazolines were obtained from complexes of cis- and trans-1-methyl-2-aryl-3-aroylaziridines with BF₃ by heating with acetonitrile. The reaction proceeds with inversion of the configuration of the starting 3-aroylaziridines. In the presence of bases the complexes of cis-1,2-dimethyl-4-aryl-5-aroyl-2-imidazolines readily undergo isomerization to the corresponding trans analogs. The structures of the products were established on the basis of the IR, PMR, and mass spectra and the results of elementary analysis. The configurations of the compounds were determined by means of the Overhauser nuclear effect.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 952–957, July, 1981.     

Reversible photoswitchable wettability in noncovalently assembled multilayered films

Reversible and irreversible photoinduced changes in surface wettability were observed in noncovalently assembled multilayered films. The multilayered films studied were fabricated from a self-assembled monolayer (SAM) consisting of 4-(10-mercaptodecyloxy)pyridine-2,6-dicarboxylic acid on gold, Cu(II) ions complexed to the pyridine head group of the SAM, and either cis- (film 1) or trans- (film 2) stilbene-4,4'-dicarboxylic acid complexed to the Cu(II) ions. Irradiation of film 1 at wavelengths corresponding to the absorption band of the cis-stilbene isomer resulted in an irreversible chemical change and an irreversible increase in wettability, as indicated by surface contact angle and grazing incidence IR measurements. However, no evidence for cis-/trans-photoisomerization was observed. Films 3 and 4, similar to films 1 and 2 in that they consist of an underlying SAM, an intermediate layer consisting of Cu(II) ions, and either cis- or trans-stilbene-4,4'-dicarboxylic acid as the capping ligand, were fabricated with a mixed SAM that contained both 4-(10-mercaptodecyloxy)pyridine-2,6-dicarboxylic acid and 4-tert-butylbenzenethiol. Irradiation of these films at wavelengths corresponding to stilbene isomer absorption bands resulted in reversible cis- to trans- (film 3) and trans- to cis- (film 4) photoisomerization and reversible switching of the surface wettability between a low wetting state (cis-stilbene) and a high wetting state (trans-stilbene). The difference in observed behavior between films 1 and 2 and films 3 and 4 is attributed to the greater surface spacing afforded by the mixed monolayer, which allows greater conformational flexibility and lowers the steric barriers to isomerization.     

Sensitive and accurate quantitation of monoepoxy fatty acids in thermoxidized oils by gas-liquid chromatography

A sensitive and accurate methodology for quantitation of monoepoxy fatty acid methyl esters (FAME) by gas-liquid chromatography is proposed. Analytical problems of interfering compounds, ie, methyl monoester of azelaic acid and methyl docosanoate, were solved by a second methylation step with diazomethane and by elimination of nonpolar FAME by adsorption chromatography, respectively. Six monoepoxy FAME were identified and quantitated in olive and sunflower oils heated at 180 degrees C for 15 h: trans-9,10- and cis-9,10-epoxystearate coming from oleate and trans-12,13-, trans-9,10-, cis-12,13- and cis-9,10-epoxyoleate coming from linoleate. Results demonstrated total recovery of monoepoxy compounds after nonpolar FAME elimination with the additional advantage of sample concentration, which allowed quantitation of monoepoxy FAME in the initial oils. Also, repeatability was excellent as relative standard deviations ranged from 2.2 to 5.1% for on-column injection and from 0.1 to 2.0% for automatic split injection.     

Facile cyanamide-ammonia coupling mediated by cis- and trans-[PtIIL2] centers and giving metal-bound guanidines

Diffusion of ammonia into CH(2)Cl(2) solutions of the dialkylcyanamide complexes cis- or trans-[PtCl(2)(RCN)(2)] (R = NMe(2), NEt(2), NC(5)H(10)) at 20-25 degrees C leads to metal-mediated cyanamide-ammonia coupling to furnish, depending on reaction time, one or another type of novel bisguanidine compound, i.e. the molecular cis- or trans-[PtCl(2){NH=C(NH(2))R}(2)] (cis- and trans-) and the cationic cis- or trans-[Pt(NH(3))(2){NH=C(NH(2))R}(2)](Cl)(2) (cis- and trans-) complexes. Compounds cis- or trans- were converted to cis- or trans-, accordingly, upon prolonged treatment with NH(3) in CH(2)Cl(2). The ammination of the relevant nitrile complexes cis- or trans-[PtCl(2)(RCN)(2)] (R = Et, CH(2)Ph, Ph) in CH(2)Cl(2) solutions affords only the cationic compounds cis- or trans-[Pt(NH(3))(2){NH=C(NH(2))R}(2)](Cl)(2) (cis- and trans-). The formulation of was supported by satisfactory C, H and N elemental analyses, agreeable ESI(+)-MS (or FAB(+)-MS), IR, (1)H and (13)C NMR spectroscopies. The structures of trans-, trans-, cis-, trans-, cis-, and cis- were determined by single-crystal X-ray diffraction disclosing structural features and showing that the ammination gives ligated guanidines and amidines in the E- and Z-forms, respectively, where both correspond to the trans-addition of NH(3) to the nitrile species.     

The remarkable reactivity of high oxidation state ruthenium and osmium polypyridyl complexes

There is a remarkable redox chemistry of higher oxidation state M(IV)-M(VI) polypyridyl complexes of Ru and Os. They are accessible by proton loss and formation of oxo or nitrido ligands, examples being cis-[RuIV(bpy)2(py)(O)]2+ (RuIV=O2+, bpy=2,2'-bipyridine, and py=pyridine) and trans-[OsVI(tpy)(Cl)2(N)]+ (tpy=2,2':6',2' '-terpyridine). Metal-oxo or metal-nitrido multiple bonding stabilizes the higher oxidation states and greatly influences reactivity. O-atom transfer, hydride transfer, epoxidation, C-H insertion, and proton-coupled electron-transfer mechanisms have been identified in the oxidation of organics by RuIV=O2+. The Ru-O multiple bond inhibits electron transfer and promotes complex mechanisms. Both O atoms can be used for O-atom transfer by trans-[RuVI(tpy)(O)2(S)]2+ (S=CH3CN or H2O). Four-electron, four-proton oxidation of cis,cis-[(bpy)2(H2O)RuIII-O-RuIII(H2O)(bpy)2]4+ occurs to give cis,cis-[(bpy)2(O)RuV-O-RuV(O)(bpy)2]4+ which rapidly evolves O2. Oxidation of NH3 in trans-[OsII(tpy)(Cl)2(NH3)] gives trans-[OsVI(tpy)(Cl)2(N)]+ through a series of one-electron intermediates. It and related nitrido complexes undergo formal N- transfer analogous to O-atom transfer by RuIV=O2+. With secondary amines, the products are the hydrazido complexes, cis- and trans-[OsV(L3)(Cl)2(NNR2)]+ (L3=tpy or tpm and NR2-=morpholide, piperidide, or diethylamide). Reactions with aryl thiols and secondary phosphines give the analogous adducts cis- and trans-[OsIV(tpy)(Cl)2(NS(H)(C6H4Me))]+ and fac-[OsIV(Tp)(Cl)2(NP(H)(Et2))]. In dry CH3CN, all have an extensive multiple oxidation state chemistry based on couples from Os(VI/V) to Os(III/II). In acidic solution, the OsIV adducts are protonated, e.g., trans-[OsIV(tpy)(Cl)2(N(H)N(CH2)4O)]+, and undergo proton-coupled electron transfer to quinone to give OsV, e.g., trans-[OsV(tpy)(Cl)2(NN(CH2)4O)]+ and hydroquinone. These reactions occur with giant H/D kinetic isotope effects of up to 421 based on O-H, N-H, S-H, or P-H bonds. Reaction with azide ion has provided the first example of the terminal N4(2-) ligand in mer-[OsIV(bpy)(Cl)3(NalphaNbetaNgammaNdelta)]-. With CN-, the adduct mer-[OsIV(bpy)(Cl)3(NCN)]- has an extensive, reversible redox chemistry and undergoes NCN(2-) transfer to PPh3 and olefins. Coordination to Os also promotes ligand-based reactivity. The sulfoximido complex trans-[OsIV(tpy)(Cl)2(NS(O)-p-C6H4Me)] undergoes loss of O2 with added acid and O-atom transfer to trans-stilbene and PPh3. There is a reversible two-electron/two-proton, ligand-based acetonitrilo/imino couple in cis-[OsIV(tpy)(NCCH3)(Cl)(p-NSC6H4Me)]+. It undergoes reversible reactions with aldehydes and ketones to give the corresponding alcohols.     

Crystal and molecular structures of diastereomeric 2-phosphoryl-, 2-thiophosphoryl-, and 2-selenophosphoryl-substituted 1,3-dithianes

The crystal and molecular structures of five pairs of diastereomeric cis- and trans-2-phosphoryl-, 2-thiophosphoryl-, and 2-selenophosphoryl-5-t-butyl-1,3-dithianes have been determined. For all the examined compounds, all of the basic geometrical parameters, such as bond lengths, bond and torsion angles, and the deformation of a chair conformation of the six-membered heterocyclic rings, have been established. The differences in corresponding bond lengths and valence angles in diastereomeric cis- and trans-2-P-substituted 1,3-dithianes are discussed.     

Si-H and Si-Si activation at Pt: synthesis and reactivity of neutral and cationic silyl complexes

Reactions of [Pt(PEt(3))(3)] (1) with the silanes HSiPh(3), HSiPh(2)Me and HSi(OEt)(3) led to the products of oxidative addition, cis-[Pt(H)(SiPh(3))(PEt(3))(2)] (2), cis-[Pt(H)(SiPh(2)Me)(PEt(3))(2)] (3), cis-[Pt(H){Si(OEt)(3)}(PEt(3))(2)] (cis-4) and trans-[Pt(H){Si(OEt)(3)}(PEt(3))(2)] (trans-4). The complexes cis-4 and trans-4 can also be generated by hydrogenolysis of (EtO)(3)SiSi(OEt)(3) in the presence of 1. Furthermore, the silyl compounds cis-4 and trans-4 react with B(C(6)F(5))(3) and CH(3)CN by hydride abstraction to give the cationic silyl complex trans-[Pt{Si(OEt)(3)}(NCCH(3))(PEt(3))(2)][HB(C(6)F(5))(3)] (8). In addition, the reactivity of the complexes cis-4, trans-4 and 8 towards alkenes and CO was studied using NMR experiments.     

Enzymatic desaturation of fatty acids: delta11 desaturase activity on cyclopropane acid probes

The formation of methylenecyclopropanes by enzymatic desaturation of 11-cyclopropylundecanoic acid (1) and its disubstituted derivatives cis- and trans-3-5 has been investigated using the Delta(11) desaturase of Spodoptera littoralis as model enzyme. Gas chromatography coupled to mass spectrometry analyses of methanolyzed lipidic extracts from tissues incubated with each probe revealed that all the cyclopropyl fatty acids were transformed into the corresponding 11-cyclopropylidene acids, except for compound trans-5 (5b), which was not desaturated at C11. The formation of methylenecyclopropane 9 as the only reaction product from 1 indicates that a potential radical intermediate is too short-lived to allow rearrangement reactions. Information on the Delta(11) desaturase substrate binding domain is provided considering the cyclopropyl probes 3-5 as conformationally restricted analogues of the straight-chain substrates.     

Ligand discrimination in the reaction of nitrones with [PtCl2(PhCN)2]. Selective formation of mono-oxadiazoline and mixed bis-oxadiazoline complexes under thermal and microwave conditions

[2+3] Cycloaddition of nitrones to the nitrile ligands in the complexes cis- or trans-[PtCl2(PhCN)2] occurs under ligand differentiation and allows for selective synthesis of complexes of the type cis- or trans-[PtCl2(oxadiazoline)(PhCN)]. Microwave irradiation enhances the reaction rates of the cycloaddition considerably and further favours the selectivity towards the mono-cycloadduct with respect to thermal conditions, because the first cycloaddition is accelerated to a higher extent than the second one. Reaction of the trans-substituted mono-oxadiazoline complexes with a nitrone different from the one used for the first cycloaddition step gives access to mixed bis-oxadiazoline compounds of the composition trans-[PtCl2(oxadiazoline-a)(oxadiazoline-b)]. The corresponding cis-configured complexes, however, do not undergo further cycloaddition. All reactions described occur without isomerisation of the stereochemistry around the platinum center, independently of whether thermal or microwave heating is applied.     

Stereospecific synthesis of cis- and trans-4-amino-3-hydroxythiophanes

The reduction of 4-benzoylamino- and 4-carbethoxyamino-3-ketothiophanes proceeds stereospecifically to form only trans-4-benzoylamino- and trans-4-carbethoxyamino-3-hydroxythiophanes, respectively, from which trans-4-amino-3-hydroxythiophane is obtained by alkaline hydrolysis. It was established that acid hydrolysis of trans-4-carbethoxyamino-3-hydroxythiophane leads only to trans-4-amino-3-hydroxythiophane, while acid hydrolysis of trans-4-benzoylamino-3-hydroxythiophane is accompanied by inversion to form cis-4-amino-3-hydroxythiophane. Derivatives of the cis- and trans-isomeric pairs of 4-amino-3-hydroxythiophanes were synthesized.Translated from Khimiya Geterotsilicheskikh Soedinenii, No. 12, pp. 1609–1613, December, 1970.     

Gas-phase ozonolysis of alkenes: formation of OH from anti carbonyl oxides

Gas-phase ozone-alkene reactions are known to produce the hydroxyl radical (OH) in high yields. Most mechanistic studies to date have focused on the role of syn carbonyl oxides; however, OH production from ethene ozonolysis indicates a second, poorly understood OH-forming channel, which may contribute to OH production in the ozonolysis of substituted alkenes as well. Using laser-induced fluorescence, we have measured OH and OD yields from the ozonolysis of two partially deuterated alkenes, cis- and trans-3-hexene-3,4-d2. OD is formed from both alkenes, indicating a pathway of hydroxyl-radical formation involving vinylic hydrogens, accounting for one-third of total OH formation from cis-3-hexene. The lack of a significant kinetic isotope effect suggests this pathway is the "hot acid" channel, arising from rearrangement of anti carbonyl oxides. Measured yields also allow for the estimation of syn:anti carbonyl oxide ratios, approximately 50:50 for trans-3-hexene and approximately 20:80 for cis-3-hexene, qualitatively consistent with our understanding of ozonide decomposition pathways.     

Hetero-Diels-Alder reactions of alpha-aryl-beta-monohalo-alpha-nitrosoethylenes: diastereoselective synthesis of 6-substituted 3-aryl-4-halo-5,6-dihydro-4H-1,2-oxazines.

Treatment of alpha-aryl-beta-bromo(or chloro)-alpha-nitrosoethylene, prepared in situ from alpha-monobromo(or chloro)ketoximes and sodium carbonate in ether at rt, with allytrimethylsilane afforded exclusively trans-(4S,6S)- and trans-(4R,6R)-3-aryl-4-halo-6-[(trimethylsilyl)methyl]-5,6-dihydro-4H-1,2-oxazines 10 albeit in low yields. Similar treatment of beta-halo-alpha-nitrosoethylenes with ethyl vinyl ether, however, gave single stereoisomers, i.e., cis-(4S,6S)- and cis-(4R,6R)-6-ethoxy-4-halo-5,6-dihydro-4H-1,2-oxazines 11, in moderate to good yields. The result is in contrast to the reported predominant formation of trans-11a by a radical reaction. On the other hand, similar reactions with tert-butyl vinyl ether at 30 degrees C gave diastereomeric mixtures of cis-(4S,6S)-, cis-(4R,6R)-, trans-(4S,6R)-, and trans-(4R,6S)-6-(tert-butoxy)-4-halo-5,6-dihydro-4H-1,2-oxazines 12. In contrast to compounds 11, the major isomers have (4S,6R) and (4R,6S) configurations. The tendency of a [4 + 2] cycloaddition reaction is consistent with that observed in the Diels-Alder reaction with inverse-electron demand. The stereochemistries of compounds 10-12 were assigned on the basis of the (1)H NMR coupling constants, which were unambiguously determined by the decoupling experiments. All reactions leading to compounds 10-12 proceed with very high regioselectivity. Diastereoselectivity and high regioselectivity are understood in terms of the frontier orbital method. It has been found that cis-12g is isomerized to a mixture of stereoisomers in favor of the trans-isomer in the presence of HClO(4) (72%) in CHCl(3) at rt.     

Synthesis of cis- and trans-3-aminocyclohexanols by reduction of β-enaminoketones

We describe a protocol developed for the preparation of β-enaminoketones derived from 1,3-cyclohexanediones, and their subsequent reduction by sodium in THF-isopropyl alcohol to afford cis- and trans-3-aminocyclohexanols.