Stereochemical and spectroscopic studies on the reaction of allylstannanes with aldehydes

Development of a Double Allylboration Reagent Targeting 1,5-syn-(E)-Diols: Application to the Synthesis of the C(23)-C(40) Fragment of Tetrafibricin

Interest in the synthesis of the C(23)-C(40) fragment 2 of tetrafibricin prompted us to develop a new method for the synthesis of 1,5-syn-(E)-diols. Toward this end, the kinetically controlled hydroboration of allenes 6, 33, ent-39, 42 and 45 with the Soderquist borane 25R were studied. Tetrabutylammonium allenyltrifluoroborate 45 gave superior results and was utilized in a double allylboration sequence with two different aldehydes to provide the targeted 1,5-syn-(E)-diols in generally high yields (72-98%), and with high enantioselectivity (>95% e.e.), diastereoselectivity (d.r. >20:1), and (E)/(Z) selectivity (>20:1). This new method was applied to the synthesis of the C(23)-C(40) fragment 2 of tetrafibricin.

γ-Silylated α,β-unsaturated amides — Preparation by [1,5]-sigmatropic rearrangement and use as masked dienolate equivalents in carbonyl condensations

The reaction of lithium dienolates derived from N,N-dialkylsenecioamides (1a–1c) with triorganosilyl electrophiles occurs initially at the oxygen atom predominantly, and is followed by an O → C silicon migration to afford the γ-silylated senecioamides (4a–4h). The γ-silylated senecioamide Z-4a undergoes fluoride-ion-mediated condensations with aromatic aldehydes to give kinetic α-(6) and thermodynamic γ-(5) condensation product patterns comparable to lithium dienolates. The TiCl4-mediated reactions with aldehydes gives α-products (6) in a highly syn-selective manner. Possible transition-state models for the syn-selective condensations are discussed and a chair-like transition state featuring bidentate coordination to titanium (11) is proposed.

Temporary silicon connection strategies in intramolecular allylation of aldehydes with allylsilanes

Three gamma-(amino)silyl-substituted allylsilanes 14a-c have been prepared in three steps from the corresponding dialkyldichlorosilane. The aminosilyl group has been used to link this allylsilane nucleophile to a series of beta-hydroxy aldehydes through a silyl ether temporary connection. The size of the alkyl substituents at the silyl ether tether governs the outcome of the reaction on exposure to acid. Thus, treatment of aldehyde (E)-9aa, which contains a dimethylsilyl ether connection between the aldehyde and allylsilane, with a range of Lewis and Brønsted acid activators provides an (E)-diene product. The mechanism of formation of this undesired product is discussed. Systems containing a sterically more bulky diethylsilyl ether connection react differently: thus in the presence of TMSOTf and a Brønsted acid scavenger, intramolecular allylation proceeds smoothly to provide two out of the possible four diastereoisomeric oxasilacycles, 23 (major) and 21 (minor). A diene product again accounts for the remaining mass balance in the reaction. This side product can be completely suppressed by using a sterically even more bulky diisopropylsilyl ether connection in the cyclization precursor, although this is now at the expense of a slight erosion in the 1,3-stereoinduction in the allylation products. The sense of 1,3-stereoinduction observed in these intramolecular allylations has been rationalized by using an electrostatic argument, which can also explain the stereochemical outcome of a number of related reactions. Levels of 1,4-stereoinduction in the intramolecular allylation are more modest but can be significantly improved in some cases by using a tethered (Z)-allylsilane in place of its (E)-stereoisomer. Oxidation of the major diastereoisomeric allylation product 23 under Tamao-Kumada conditions provides an entry into stereodefined 1,2-anti-2,4-syn triols 28.

Stereoselective synthesis of 2,4,5-trisubstituted tetrahydropyrans using an intramolecular allylation strategy

A highly stereoselective route to 2,4,5-trisubstituted tetrahydropyrans is reported. The key step employs an intramolecular allylation of a (Z)-allylsilane onto an aldehyde under Brønsted acid activation. Complete 1,4-stereoinduction accounts for the formation of only two out of the possible four THP products. The level of 1,3-stereoinduction is optimal when the reaction is carried out in an apolar solvent, which is in accord with electrostatics being key to controlling this aspect of the stereoselectivity.

Chiral allylic and allenic metal reagents for organic synthesis

This account traces the evolution of our work on the synthesis of chiral allylic and allenic organometal compounds of tin, silicon, zinc, and indium and their application to natural product synthesis over the past quarter century.

2-Phenylthio-3-bromopropene, A Valuable Synthon, Easily Prepared by a Simple Rearrangement

[reaction: see text] Treatment of allyl phenyl sulfide with bromine, followed by aqueous sodium hydroxide, provides a good yield of 2-phenylthio-3-bromopropene 2 via a mechanism that is elucidated by isolation of the 1,3-dibromo-2-(phenylthio)propene intermediate 7. Three uses of 2 as an annulating agent for cycloheptanone are illustrated, demonstrating that this reagent is a synthetic equivalent of acetonyl halide and an alpha-halo vinyllithium. It is also readily converted to the useful synthons 2,3-bis(phenylthio)propene 16 and 2-phenylthio-3-(phenylsulfonyl)propene 17.

A computational study of the mechanism for the transmetalation of 2-trimethylstannylbuta-1,3-diene with SnCl4

Electronic structure calculations were performed at the B3LYP/6-31G level to identify the stationary structures on the potential energy surfaces for the transmetalation of 2-trimethylstannylbuta-1,3-diene with SnCl(4). The reaction pathways were characterized by locating the transition states on the intrinsic reaction coordinate. The calculations showed that the reaction between the reactant and SnCl(4), which generates 1-trichlorostannylbuta-2,3-diene via transmetalation, has a low energy barrier of 78.1 kJ.mol(-)(1). The following isomerization process is the rate-controlling step. It turned out that the isomerization process from 1-trichlorostannylbuta-2,3-diene to 2-trichloro-stannylbuta-1,3-diene via transmetalation with SnCl(4) is more energetically favorable than other possible isomerization processes.

Stereoselective synthesis of tetrahydropyrans via formal [4 + 2]-cycloaddition: a comparison of allylsilane and crotylsilane

The reaction of a series of beta-(triethylsilyloxy)aldehydes with several allylsilanes and crotyldimethylphenylsilane is described. Aldehydes possessing an alpha-stereocenter afforded tetrahydropyrans as mixtures of two diastereomers with allylsilane, but only a single diastereomer was observed in the case of crotylsilanes. The reaction time for crotylsilanes was longer than that for allylsilanes likely due to the increased steric hindrance. Allylsilanes afforded tetrahydropyrans in 34-67% yields, and crotylsilanes provided products in 0-62% yields.

Mechanistic studies on the B(C(6)F(5))(3) catalyzed allylstannation of aromatic aldehydes with ortho donor substituents

Mechanistic studies on the B(C(6)F(5))(3) catalyzed allylstannation of isomeric substituted benzaldehydes are reported. Confirming a report by Maruoka et al., good (5:1) to excellent (>20:1) selectivities for ortho over para isomers are observed when 1:1 mixtures (X = OMe, Cl, F, OTBS) are allylstannated with C(3)H(5)SnBu(3) in the presence of B(C(6)F(5))(3) (2.5% per CHO). The best selectivities are observed for the anisaldehydes. Multinuclear NMR studies on solutions of B(C(6)F(5))(3) and C(3)H(5)SnBu(3) (1:1 to 1:5) show that the borane abstracts the allyl group from the organotin reagent, forming an adduct (C(6)F(5))(3)B...CH(2)CHCH(2)SnBu(3), 1, or ion pair [(C(6)F(5))(3)BCH(2)CH=CH(2)](-)[Bu(3)SnCH(2)CHCH(2)SnBu(3)](+), 2, depending on the reagent ratio. These compounds are important in the mechanism of Lewis acid catalyzed 1,3-isomerization of substituted allyl stannanes. When allyltin reagent is added to solutions of B(C(6)F(5))(3) and ortho-anisaldehyde (1:5) at -60 degrees C, conversion to the stannylium ion pair [Bu(3)Sn(ortho-anisaldehyde)(2)](+)[o-ArCH(allyl)OB(C(6)F(5))(3)](-), o,o-4, is observed. The structure of this species was confirmed by (1)H, (11)B, (19)F, and (119)Sn NMR spectroscopy and by forming related ion pairs (o-5 and o,o-5) utilizing the [B(C(6)F(5))(4)](-) counteranion via reaction of [Bu(3)Sn](+)[B(C(6)F(5))(4)](-) with aldehyde. The anion in o,o-4 is formed via direct allylation of the ortho-anisaldehyde/B(C(6)F(5))(3) adduct o-3, while the cation arises upon aldehyde ligation of the resulting tributylstannylium ion. The crystal structure of the related derivative ortho-C(6)H(4)(OMe)CHO x SnMe(3)BF(4), 6, showed that the aldehyde binds the tin nucleus only through the carbonyl oxygen. Similar reactions using para-anisaldehyde show that formation of p,p-4 occurs at a much slower rate, again demonstrating the preference for the ortho substituted substrates. For similar experiments using benzophenone, however, formation of the ion pair [Bu(3)Sn(Ph(2)CO)(2)](+)[(C(3)H(5))B(C(6)F(5))(3)](-), 8, was observed, illustrating the differences subtle changes in substrate can bring. Ion pair 8 is formed via the trapping of 1 by the benzophenone substrate. In the presence of excess aldehyde and allyltin reagent, ion pair o,o-4 catalyzes the allylstannation of aldehyde to give the product stannyl ether. Several lines of experimental evidence suggest this is the true catalyst in the system. The chemoselectivity observed thus does not rely on classical chelation control in any way. Rather, we propose that the ortho donor group stabilizes the developing positive charge at the beta carbon of the allyl group and the tin atom during the allylation event. This stabilization renders the ortho substituted substrates kinetically favored toward allylation irrespective of the Lewis acid employed.

Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.

Zirconium tetrachloride-formaldehyde sigma-complexes: A computational and spectroscopic investigation

We have carried out a combined theoretical-experimental study of the structures and energies of ZrCl(4)-aldehyde complexes using (13)C NMR spectroscopy and a DFT (B3LYP) computational approach. The computational investigation has demonstrated the existence of different types of complexes: a 1:1 complex (H(2)CO-ZrCl(4)), various 2:1 complexes ((H(2)CO)(2)-ZrCl(4)), and several dimeric species. The analysis of the energies involved in the formation of the various complexes has indicated that the dimeric species should correspond to the only adduct observed in the (13)C NMR spectra (carbonyl resonance at 226.96 ppm) when a 1:1 ZrCl(4)/aldehyde molar ratio is used, while the 2:1 complex should be responsible for the signal at 224.30 ppm that is recorded when this molar ratio is 1:2.

Isomerization reaction of olefin using RuClH(CO)(PPh(3))(3)

When methyl 5-(tert-butyldiphenylsilyl)oxy-2-pentenoate was refluxed in toluene in the presence of RuClH(CO)(PPh(3))(3) (5 mol %), double-bond migration took place to afford methyl 5-(tert-butyldiphenylsilyl)oxy-4-pentenoate in high yield. This means that the double bond conjugated with the ester moiety migrates to a deconjugated position by a ruthenium catalyst. We planned to prepare an enol ether from alpha,beta-unsaturated compounds having an ether moiety in a tether using ruthenium-catalyzed isomerization of the double bond. As a result, silyl or benzyl enol ether was obtained from the alpha,beta-unsaturated ester having alcohol protected by the silyl or benzyl group in a tether in high yield. In this reaction, double bond migration of alpha,beta-unsaturated ketone and alpha,beta-unsaturated amide took place to produce deconjugated compounds. Moreover, the double bond of alpha, beta-unsaturated ester having a triple or double bond in a molecule migrated to produce conjugated enyne and diene. On the other hand, treatment of a bis-metalated compound having an alpha, beta-unsaturated ester moiety or the double bond in a tether with RuClH(CO)(PPh(3))(3) gave allyl bis-metalated compound in good yield. These compounds are useful units in synthetic organic chemistry.

A new strategy for synthesis of attached rings

The first investigation of the use of pinacol-terminated Prins cyclizations to form attached rings is reported. Treatment of triisopropylsilyl ethers of (Z)- or (E)-[2-(6,6-dimethoxyhexylidene)cyclohexanol with SnCl4 provides bicyclic products having attached rings. The approach is synthetically useful in the Z alkylidenecyclohexane series, proceeding selectively by a pathway involving carbon migration in the pinacol rearrangement step, to provide methoxy epimers of 1-(2-methoxycyclohexyl)cyclopentylcarboxaldehyde. Reaction of the corresponding E stereoisomer is more complex and yields a mixture of products resulting from both hydride and carbon migration in the pinacol rearrangement step.Key words: Prins cyclization, pinacol rearrangement, attached rings.

A three-component coupling strategy for tetrahydrofuran synthesis: application of the diisopropyl tartrate modified (E)-gamma-(dimethylphenylsilyl)allylboronate as an alpha,gamma-allyl dianion equivalent

[reaction: see text] A highly convergent three-component coupling strategy for the stereocontrolled synthesis of 2,3,5-trisubstituted tetrahydrofurans is described. After allylboration of the first aldehyde with 1, the chiral, nonracemic allylsilanes 2 are coupled with a second aldehyde or ketone with Lewis acid catalysis to give tetrahydrofurans 3 or 4 with excellent selectivity. The 2,5-stereochemistry is controlled by operating under nonchelate (e.g., 3) or chelate (e.g., 4) conditions.

New Lewis-Acidic Molybdenum(II) and Tungsten(II) Catalysts for Intramolecular Carbonyl Ene and Prins Reactions. Reversal of the Stereoselectivity of Cyclization of Citronellal

New Mo(II) complexes BnEt(3)N(+)[Mo(CO)(4)ClBr(2)](-) (A) and Mo(CO)(5)(OTf)(2) (B) and their W(II) congeners D and E have been developed as catalysts for the title reactions. Unlike other Lewis acids, the latter catalysts exhibit cis-stereoselectivity in the cyclization of citronellal (1 --> 3 with A and 1 --> 5 with B). Isotopic labeling allowed formulation of the reaction mechanism, according to which these complexes act as bulky Lewis acids, eta(1)-coordinated to the carbonyl oxygen. The stereochemistry appears to be controlled by the protruding ligand L(p), which dictates the boatlike transition state III. The kinetically formed cis-alkenol 3 can be equilibrated by [Mo(CO)(4)Br(2)](2) (C) or ZnCl(2) to its trans-epimer 2 via a retro-ene reaction.