The role of the sulfinyl group on the course of the reactions of 3-p-tolylsulfinylfuran-2(5H)-ones with nitrones. synthetic uses of cycloreversion processes

[reaction: see text] The reaction of enantiopure 3-p-tolylsulfinylfuran-2(5H)-ones (2a and 2b) with cyclic (4) and acyclic (6) nitrones afforded furoisoxazolidines (5 and 7) in high yields under mild conditions. The reactivity of the dipolarophile was dramatically enhanced by the sulfinyl group, which modulated the pi-facial selectivity (it was complete for reactions from 2b, yielding only the anti adducts) and was the main controller of the endo/exo selectivity. Cycloreversion processes from the resulting sulfinyl furoisoxazolidines proceeded readily and were to be considered to account for an improvement in the selectivity (facial and endo/exo) and even for an inversion of it when the composition of the reaction mixtures obtained under kinetic and thermodynamic conditions were quite different.     

1,3-dipolar cycloadditions of diazoalkanes to activated sulfoxides: influence of Lewis acids

[reaction: see text] The reactions of diazomethane and diazoethane with (S)-3-p-tolylsulfinylfuran-2(5H)-one (3) and its 4-methyl derivative (4) have been studied. The sulfinyl group was able to completely control the pi-facial selectivity of all these reactions, which decreased when the polarity of the solvent increased and could be inverted in the presence of Lewis acids, Yb(OTf)(3) being the most efficient catalyst. This behavior made possible the stereodivergent synthesis of diastereoisomeric pyrazolines in almost quantitative yields and de's higher than 98%. The endo/exo selectivity was also complete in reactions of 3 with diazoethane, whereas 4 afforded an easily separable 1:1 mixture of diastereoisomers. Steric factors accounts for the endo/exo selectivity, whereas electrostatic interactions must also be considered to explain the facial selectivity.     

Pyrrolo[2,1-b]thiazole derivatives by asymmetric 1,3-dipolar reactions of thiazolium azomethine ylides to activated vinyl sulfoxides

1,3-Dipolar reactions of thiazolium azomethine ylides to enantiopure cyclic and acyclic vinyl sulfoxides provide an efficient access to polyfunctionalized pyrrolo[2,1-b][1,3]thiazoles in a highly regio- and stereoselective manner. Regioselectivity can be inverted by modifying the position of the sulfinyl group at the double bond of the sulfinylfuranones. The sulfoxide is the main controller of the endo selectivity of these processes as well as of the pi-facial selectivity in reactions of (Z)-3-p-tolylsulfinylacrylonitriles. In contrast, the pi-facial selectivity in reactions of 5-alkoxy-3-p-tolylsulfinyl furan-2(5H)-ones is mainly controlled by the configuration at C-5, affording the anti adducts with respect to the alkoxy group as the major or exclusive adducts.     

First asymmetric cycloaddition of carbonyl ylides to vinyl sulfoxides and furan-2(5H)-ones

The Rh2(OAc)4-catalyzed reactions of o-(methoxycarbonyl)-alpha-diazoacetophenone with enantiomerically pure 5-ethoxy-3-p-tolylsulfinylfuran-2(5H)-ones 1a and 1b afford 4,10-epoxybenzo[4,5]cyclohepta[1,2-c]furan-3,9-diones 6a and 6b, in good or moderate yields and in a completely regioselective way. The pi-facial selectivity is complete for 1a, which only yields anti-6a adducts, and very high for 1b. The endo stereoisomers are favored with respect to the exo ones in both reactions. The sulfinyl group significantly increases the reactivity of the dipolarophile as it has been demonstrated by studying the behavior of 5-methoxyfuran-2(5H)-one (3).     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Asymmetric synthesis of pyrrolo[2,1-a]isoquinoline derivatives by 1,3-dipolar cycloadditions of stabilized isoquinolinium N-ylides with sulfinyl dipolarophiles

Enantiomerically pure pyrrolo[2,1-a]isoquinoline derivatives are obtained by 1,3-dipolar reactions of isoquinolinium azomethine ylides with enantiopure 3-p-tolylsulfinylacrylonitriles, tert-butyl (2E)-4,4-diethoxy-2-p-tolylsulfinylbut-2-enoate, and 5-ethoxy-3-p-tolylsulfinylfuran-2(5H)-ones. Reactions evolve through the anti conformation of the ylide with complete regioselectivity. The facial selectivity is completely controlled by the configuration of the sulfinyl sulfur for acyclic dipolarophiles, whereas it is high (dr 83/17 or 89/11) but controlled by the C-5 configuration for sulfinylfuranones. Complete endo selectivity is observed with cyclic dipolarophiles and substituted acrylonitriles, but it is low with butenoate. The sulfinyl group also exerts a positive influence on the dipolarophilic reactivity toward these ylides.     

Intramolecular photoarylation of alkenes by phenyl cations

Acetone-sensitized irradiation of various o-chlorophenyl allyl ethers in polar solvents led to either (dihydro)benzofurans or chromanes. The reaction appeared to involve photoheterolysis of the aryl-Cl bond followed by phenyl cation addition onto the tethered double bond either in 5-exo or 6-endo modes. The adduct cation gave the end products by deprotonation; addition of chloride anion or of the solvent, depending on the structure; and the conditions used. Preference for the 5-exo mode increased in passing from medium polarity (methylene chloride, ethyl acetate) to high polarity solvents (aqueous acetonitrile, methanol, 2,2,2-trifluoroethanol), for which this was often the exclusive path. The same compounds underwent photohomolysis when irradiated in cyclohexane, and radical cyclization was one of the process occurring. Substitution of a methylene group for the ether oxygen atom made 6-endo cyclization by far the main path in a related o-chlorophenylbutene. Again, the selectivity was higher in polar protic solvents. The results are discussed in terms of in cage ion pair versus free phenyl cation reactions.     

Regio- and diastereoselectivity studies on the photocycloaddition of ketene diethyl acetal to chiral 2(5H)-furanones

The photochemical [2+2] cycloaddition of 1,1-diethoxyethylene to chiral 2(5H)-furanones is investigated. The effect of the substituents of the lactone and the polarity of the solvents on the chemical yield, regioselectivity, and facial diastereoselectivity is evaluated. The reactions in ether proceed with excellent regioselectivity and good yields. Hydrolysis of the ketal group of the major cycloadducts afforded enantiopure cyclobutanones fused to γ-lactones.     

Photochemistry of 5-aminoquinoline in protic and aprotic solvents

Photophysical properties of 5-aminoquinoline (5AQ) have been investigated in various non-polar and polar (protic and aprotic) solvents using steady state and time resolved fluorescence. In aprotic solvents, the spectral maxima depend on the polarity. However, in protic solvents both the fluorescence intensity as well decay time show decrease depending on the hydrogen bonding ability of the solvent. The results suggest that photochemistry 5AQ is quite sensitive towards the polarity as well as protic character of the solvent.     

The rearrangement route to 2-azabicyclo[2.1.1]hexanes. Solvent and electrophile control of neighboring group participation

The reactions of N-(alkoxycarbonyl)-2-azabicyclo[2.2.0]hex-5-enes 5 with halonium ion electrophiles were studied in polar and nonpolar aprotic solvents and also in protic media with the aim of controlling nitrogen neighboring group participation. Specifically, for bromonium ions nitrogen participation is facilitated by the polar aprotic solvent nitromethane and by the poorly nucleophilic protic solvent acetic acid. Alkene 5b and bromine/nitromethane afford only the rearranged anti,anti-5,6-dibromo-2-azabicyclo[2.1.1]hexane 6b, and NBS/acetic acid gives an 8:1 mixture favoring rearranged 5-bromo-6-acetate 6f. Conversely, pyridinium bromide perbromide/CH(2)Cl(2) is selective for only unrearranged 5,6-dibromide 7. Iodonium and phenylselenonium ions react with alkenes 5 to give only unrearranged 1,2-addition products 9 and 10, regardless of solvent. Chloronium and fluoronium ions react with alkenes 5 to give 4-aminomethyl-3-hydroxycyclobutene 11, derived by ring cleavage.     

Cerium(IV) ammonium nitrate catalyzed synthesis of α-dehydro-β-amino esters

α-Dehydro-β-amino esters have been synthesized regioselectively from acetates of Baylis-Hillman adducts with amines in the presence of a catalytic amount of ceric ammonium nitrate (CAN) in good yield. The regioselectivity does not differ with respect to the polarity of the solvent.     

Synthesis and dienophilic behavior of (S)-2-cyano-3-(p-tolylsulfinyl)-1,4- benzoquinone

Hydrocyanation of 2-(p-tolylsulfinyl)-1,4-benzoquinone (2) followed by oxidation with PhI(OAc)(2) gives 2-cyano-3-(p-tolylsulfinyl)-1,4-benzoquinone (1). The generation of 1 in the presence of cyclic and acyclic dienes affords the Diels-Alder adducts with a complete chemo- (only reaction with the sulfinyl-substituted double bond takes place), regio- (controlled by the cyano group), and endo selectivity (with respect to the quinone moiety), whereas the pi-facial selectivity is dependent on the structure of the diene.     

Structure and photophysics of 2-(2'-pyridyl)benzindoles: the role of intermolecular hydrogen bonds

The photophysical properties of two isomeric 2-(2'-pyridyl)benzindoles depend on the environment. Strong fluorescence is detected in nonpolar and polar aprotic solvents. In the presence of alcohols, the emission reveals an unusual behavior. Upon titration of n-hexane solutions with ethanol, the fluorescence intensity goes through a minimum and then increases with rising alcohol concentration. Transient absorption and time-resolved emission studies combined with ground- and excited-state geometry optimizations lead to the conclusion that two rotameric forms, syn and anti, coexist in alcohols, whereas in nonpolar and aprotic polar media, only the syn conformation is present. The latter can form cyclic complexes with alcohols, which are rapidly depopulated in the excited state. In the presence of excess alcohol, syn --> anti rotamerization occurs in the ground state, promoted by the cooperative action of nonspecific and specific effects such as solvent polarity increase and the formation of hydrogen bonds to both donor and acceptor sites of the bifunctional compounds.     

Asymmetric synthesis of rubiginones A(2) and C(2) and their 11-methoxy regioisomers

Convergent enantioselective syntheses of angucyclinone-type natural products rubiginones A(2) (2) and C(2) (1) and their 11-methoxy regioisomers 3 a and 3 b have been achieved by using two domino processes from a common enantiomerically pure 1-vinylcyclohexene 4. Key steps in the synthesis of this diene were the stereoselective conjugate addition of AlMe(3) on (SS)-[(p-tolylsulfinyl)methyl]-p-quinol (9) and the elimination of the beta-hydroxy sulfoxide fragment, after oxidation to sulfone, to recover a carbonyl group. The first domino sequence comprised Diels-Alder reaction with a sulfinyl naphthoquinone followed by sulfoxide elimination. An efficient opposite regioselection in the cycloaddition step was achieved in the convergent construction of the tetracyclic skeleton using a sulfoxide at C-2 or C-3 of the dienophiles 5 or 6, derived from 5-methoxy-1,4-naphthoquinone. The second domino process, triggered by oxygen and sunlight, allowed the transformation of the initial tetracyclic adducts into the final products after B ring aromatization, silyl deprotection and C-1 oxidation.     

Influence of the Sulfinyl Group on the Chemoselectivity and pi-Facial Selectivity of Diels-Alder Reactions of (S)-2-(p-Tolylsulfinyl)-1,4-benzoquinone

Diels-Alder reactions of (S)-2-(p-tolylsulfinyl)-1,4-benzoquinone (1a) with cyclic (cyclopentadiene and cyclohexadiene) and acyclic dienes (1-[(trimethylsilyl)oxy]-1,3-butadiene and trans-piperylene) under different thermal and Lewis acid conditions are reported. Chemoselectivity (reactions on C(2)-C(3) versus C(5)-C(6) double bonds) is mainly related to the cyclic (on C(5)-C(6)) or acyclic (on C(2)-C(3)) structure of the diene. The high pi-facial selectivity observed could be controlled by choosing adequate experimental conditions.     

Solvent dependence of singlet oxygen / substrate interactions in ene-reactions, (4+2)- and (2+2)-cycloaddition reactions

Product formation of singlet oxygen reactions with simple olefins occurring as ene-reactions, (4+2)- and (2+2)-cycloaddition reactions is independent on solvent polarity. Thus, 2,3-dimethyl-2-butene (1) and 2-methy]-2-butene (3), 1,3-cyclohexadiene (6), and benzvalene (8) yield allylic hydroperoxides (2) and(4) (54%) + (5) (46%), endoperoxide (7), and dioxetane (9), respectively. The rates of the ene-reactions and (4+2)-cycloaddition reactions are only slightly dependent, those of the (2+2)-cycloaddition reaction, however,are clearly dependent on solvent polarity. “Physical” quenching of singlet oxygen by the olefins is negligible, but substantial by the sensitizer tetraphenylporphin (TPP) in chlorinated solvents.     

Exciplex and radical ion intermediates in the photochemical reaction of 9-cyanophenanthrene with 2,3-dimethyl-2-butene

The photochemical reaction of 9-cyanophenanthrene and 2,3-dimethyl-2-butene, first reported by Mizuno, Pac and Sakurai, has been reinvestigated. The formation of a [2+2]-cycloadduct via a singlet exciplex is the exclusive reaction in the nonpolar solvents benzene and ethyl acetate. Photochemical behavior in polar solvents is far more complicated than previously reported. Mechanisms consistent with the effects of solvent polarity, methanol concentration, methanol deuteration, and light intensity upon product yields are proposed. Formation of a 9-cyanophenthrene anion radical and 2,3-dimethyl-2-butene cation radical is the primary photoinitiated process in polar solvent. The cation radical can undergo deprotonation to yield an allyl radical or nucleophilic attack by methanol to yield a methoxyalkyl radical. Covalent bonding of these radicals and the 9-cyanophenanthrene anion radical gives rise to the acyclic adducts obtained in polar solvents. The anion radical can also be protonated, leading ultimately to the formation of 9,10-dihydro-9-cyanophenanthrene.     

Theoretical study of the reaction mechanism and solvent effect on the regioselectivity of 1,3‐dipolar cycloaddition reaction between azide and acetylene derivatives

The reaction mechanism and solvent‐dependant regioselectivity of 1,3‐dipolar cycloaddition reactions between azide and acetylene derivatives have been studied using computational methods. The two possible reaction transition states were located. Geometry and NBO analysis found that the reactions take place along a synchronous and concerted mechanism for TS1 and an asynchronous and less concerted mechanism for TS2 . SCRF analysis found that TS2 is more sensitive to the polarity of solvent. In less polar solvent such as CCl₄, the difference of activation barriers of the two transition states is small. However, when the reactions were conducted in water, the activation barriers for TS2 increase which leads to the observed regioselectivity. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:203–207, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20236     

Selectivity in the photodimerization of 6-alkylcoumarins

Coumarin and 6-alkylcoumarins (alkyl = C(1) to C(16)) were photodimerized in homogeneous solvents differing in polarity and in aqueous micellar solutions. The four possible photodimers, syn head-to-head (hh), anti head-to-head, syn head-to-tail (ht), and anti head-to-tail, were identified through a combination of X-ray analysis and NMR spectroscopy. In 6-methylcoumarin the concentration-corrected dimerization (quantum) yield increases with decreasing concentration of the educt; anti-hh was formed exclusively in nonpolar solvents and upon triplet sensitization and was the main product under all conditions except for ionic micellar systems, which direct to preferred syn-hh dimerization. Long alkyl substituents, however, lead to anti-hh in polar solvents and in micelles, too. Predominating ht dimer formation was observed for nonsubstituted coumarin in polar solvents only. Thus, syn/anti and hh/ht selectivity can be steered by varying the 6-alkyl substituent. Syn-hh photodimers of 6-methylcoumarin can be photochemically split into the monomers; they partly proved thermally unstable against acids, bases, methanol, and on SiO(2) surfaces.     

Diels-Alder Addition of Dicyclopentadiene with Cyclopentadiene in Polar Solvents

Diels-Alder addition of dicyclopentadiene and cyclopentadiene in polar solvents has been studied to produce tricyclopentadiene（TCPD） that is a potential high-density fuel precursor. GC and MS analysis shows that the adducts contain two isomers, namely exo- and endo-TCPD. Theoretical simulation shows that although the transition state of endo-TCPD has a lower activation energy, exo-TCPD is thermodynamically preferred. Polar solvents can accelerate the reaction rate and improve the exo/endo ratio of TCPD because the transition state of exo-TCPD has a higher polarity than that of endo-TCPD. The solvent effect follows the order of polarity： benzyl methanol〉cyclohexanone〉toluene. The conversion rises when the temperature ranges from 120 to 150 ℃, but the selectivity of TCPD slightly decreases. Increasing the pressure can improve the conversion but the exo/endo ratio of TCPD is unchanged. The apparent kinetics in different solvents was determined via nonlinear regression. The activation energies are 99.47, 101.15, and 107.32 kJ/mol for benzyl methanol, cyclohexanone, and toluene, respectively. The optimal reaction conditions are as follows： benzyl methanol as solvent, temperature 150 ℃, and pressure 900 kPa. After an 11-hour reaction, a conversion of 58.0%, a TCPD selectivity of 95.7%, and an exo/endo ratio of 1/5.3 has been obtained.