Allylic Silanes Research Articles

AgOAc‐Catalyzed Aldehyde Allylation Using Allyldimethyl(2‐pyridyl)silane.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Inter‐ and Intramolecular Differentiation of Enantiotopic Dioxane Acetals Through Oxazaborolidinone‐Mediated Enantioselective Ring‐Cleavage Reaction: Kinetic Resolution of Racemic 1,3‐Alkanediols and Asymmetric Desymmetrization of meso‐1,3‐Polyols.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reactions of Silylmethyl Allylic Silanes: Synthesis of Tetrahydrofurans

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of functionalized bicyclo[3.2.1]octan-6-ones for diterpenoids: allylsilane directed Pummerer reaction: insertion reactions of diazoketones

The sulfide 5 underwent Pummerer cyclization to give 6 , whereas the allyl silane analog 10 produced the bicyclo[3.2.1]octanones 11 and 11a . Extension of this methodology to 15 resulted in 16 without the necessity for allyl silane activation. The intermediate diazoketone 14 on treatment with BF 3·OEt 2 gave 17 , 18 and 19 , whereas the saturated adduct 22 on treatment with Rh 2(OAc) 4 gave 23 .

Total synthesis of (+)-4,5-deoxyneodolabelline.

The first total synthesis of the marine dolabellane diterpene (+)-4,5-deoxyneodolabelline (1) has been accomplished. The highly efficient approach is characterized by the convergent assembly of dihydropyrans 2ab and cyclopentylsilanes 3ab. Allylic silane 3a was prepared from 2-methyl-2-cyclopentenone via a copper-catalyzed 1,4-addition followed by diastereoselective alkylation of the resulting enolate. A chemical resolution of racemic cyclopentanone 8 was effected by (S)-CBS-catalyzed borohydride reduction. Direct hydroxymethylation of the enantioenriched ketone 8 to form allylic alcohol 14 was achieved by a Stille palladium-catalyzed cross-coupling from the cyclopentenyl triflate 13. Treatment of the corresponding allylic phosphate 15 with trimethylsilylcopper reagent provided for displacement with allylic transposition yielding the exocyclic allylsilane 3a with excellent diastereoselectivity. Dihydropyrans 2a and 2b were prepared from optically pure acyclic acetals via ring-closing metathesis. Coupling of 3a and 2a or 2b via the carbon-Ferrier protocol gave trans-2,6-disubstituted dihydropyrans 30 and 35 with complete stereoselectivity. Vanadium-based pinacol coupling reactions were explored for closure of the medium-sized carbocycle to yield syn-diol 33. X-ray diffraction studies of the monobenzoate 34 have provided unambiguous stereochemical assignments. Substantial ring strain accounted for the lack of alkene products typical of reductive elimination using TiCl(3) and zinc-copper couple (McMurry) conditions leading to 37. Finally, the natural product 1 was obtained via Swern oxidation of the diol 37.

Development of the [3 + 2] Annulations of Cyclohexenylsilanes and Chlorosulfonyl Isocyanate: Application to the Total Synthesis of (.+‐.)‐Peduncularine.

The synthesis of (±)-peduncularine was accomplished using the [3 + 2] annulation of an allylic silane with chlorosulfonyl isocyanate to assemble the bicyclic core of the alkaloid. The stereochemistry of the annulation product was employed to control the installation of the indolylmethyl side chain at C-7 with complete stereoselectivity.

Synthesis of (.+‐.)‐5‐epi‐Citreoviral and (.+‐.)‐Citreoviral and the Kinetic Resolution of an Allylic Silane by a [3 + 2] Annulation.

[reaction: see text] The [3 + 2] annulation reaction of allylsilane 1 with an alpha-keto ester provided the highly substituted tetrahydrofuran 2 as a single diastereomer in high yield. The synthesis of (+/-)-5-epi-citreoviral and (+/-)-citreoviral has been accomplished with this annulation reaction as the key step. Using the pantolactone-derived alpha-keto ester, the allylsilane 1 has been resolved with high enantiomeric purity.

Preparation and reactivity of imino glycals: stereocontrolled, divergent approach to imino sugars.

The synthesis of 3,4,6-tri-O-acetyl imino D-glucal 2 from D-glucal is reported. This imino glycal participates in a variety of Lewis acid mediated carbon-carbon bond forming reactions by allylic displacement of the C-3 acetate group by added nucleophiles. Allyl silanes, trimethylsilyl enol ethers, alkenes and dialkyl zinc reagents serve as suitable reaction partners. In all the cases studied, the beta-anomer is predominant. Using imino glycal 8, epimeric at C-5, it is established that the configuration at C-5 of the piperidine ring plays a major role in controlling the stereochemical outcome. These results are rationalised by invoking the intermediacy of a conjugated N-acyliminium ion. A short stereocontrolled synthesis of (+)-deoxoprosophylline is achieved using this chemistry. Additionally, imino glucal 2 is transformed into bromo piperidine 16, whose X-ray crystal structure is determined. Bromide 16 participates in palladium catalysed Stille and Suzuki cross-couplings allowing access to C-2 substituted imino sugars 17 and 18. In other studies, imino sugar C-glycosides 21 and 22 are made by combining the Lewis acid mediated carbon-carbon bond forming reactions with stereospecific dihydroxylations.

Regio- and stereoselective functionalization of electron-deficient alkenes by organosilicon compounds via photoinduced electron transfer

Regio- and stereoselective photoallylation of electron-deficient alkenes by use of allylic silanes via photoinduced electron transfer has been described. Similar photoinduced functionalization reactions such as arylmethylation, alkylation, and silylation can be achieved by using a variety of organosilicon compounds. These photoreactions proceed via radical cations of organosilicon compounds and radical anions of electron-deficient alkenes as reactive intermediates. The key step of the photoreactions is the attack of carbon radicals, which are generated from the radical cations of organosilicon compounds, on the radical anions of alkenes. The mechanism of the regio- and stereoselective photofunctionalization is discussed.

A Novel Axially Chiral 2,2′‐Bipyridine N,N′‐Dioxide. Its Preparation and Use for Asymmetric Allylation of Aldehydes with Allyl(trichloro)silane as a Highly Efficient Catalyst.

AbstractFor Abstract see ChemInform Abstract in Full Text.

AgOAc-Catalyzed Aldehyde Allylation Using Allyldimethyl(2-pyridyl)silane

Abstract Silver salts having relatively strong Ag–O bond were found to promote the allylation of aldehydes with allyldimethyl(2-pyridyl)silane. Catalytic allylation is also possible when AgOAc was employed. A novel cooperative activation of allyl(2-pyridyl)silane has been implicated from several control experiments.

Preparation of New Ferrocenylmonophosphine Ligands Containing Two Planar Chiral Ferrocenyl Moieties and Their Use for Palladium-Catalyzed Asymmetric Hydrosilylation of 1,3-Dienes

Bis{(Rp)-2-[(1S)-1-methoxyethyl]ferrocenyl}arylphosphines (S,Rp)-9 (aryl=4-MeOC6H4 (9a), Ph (9b), 4-CF3C6H4 (9c), 3,5-(CF3)2C6H3 (9d)), which contain two planar chiral ferrocenyl moieties, were prepared via (Rp)-1-bromo-2-[(1S)-1-methoxyethyl]ferrocene ((S,Rp)-8). Asymmetric hydrosilylation of linear 1,3-dienes such as deca-1,3-diene (10a) with trichlorosilane in the presence of a palladium catalyst coordinated with 9d gave allylic silanes of up to 93% ee.

On the Mechanism of the Silicon‐Tethered Reductive Pauson—Khand Reaction.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic Asymmetric Cross‐Coupling.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Inter- and intramolecular differentiation of enantiotopic dioxane acetals through oxazaborolidinone-mediated enantioselective ring-cleavage reaction: kinetic resolution of racemic 1,3-alkanediols and asymmetric desymmetrization of meso-1,3-polyols.

Acetals derived from racemic 1,3-alkanediols undergo kinetic resolution in chiral oxazaborolidinone-mediated ring-cleavage reaction with nucleophiles such as enol silanes and allylic silanes. Enantioselectivity of the reaction is affected by nucleophiles, the N-sulfonyl groups of oxazaborolidinones, and the substituents attached to the acetal carbon. For disubstituted acetals rac-1 and for trisubstituted acetal rac-2, derived from syn-2,4-dimethyl-1,3-pentanediol, satisfactory enantioselectivity is obtained by using methallylsilane 7b,c as a nucleophile in combination with N-mesyloxazaborolidinone 4a. On the other hand, enantioselective reaction of trisubstituted acetal rac-3b, derived from anti-2,4-dimethyl-1,3-pentanediol, is realized by using silyl ketene acetal 5e in combination with N-tosyloxazaborolidinone 4b. The reaction conditions optimized for the kinetic resolution, or enantiomer differentiating reaction, of the racemic acetals are successfully applied to asymmetric desymmetrization of meso-1,3-polyols through intramolecular differentiation of the enantiotopic acetal moieties of the bis-acetal derivatives. The utility of the ring-cleavage reaction as a method for enantioselective terminal differentiation of prochiral polyols is demonstrated in convergent asymmetric synthesis of pentol derivative 35 corresponding to the C(19)[bond]C(27) ansa-chain of rifamycin S.

Development of the [3 + 2] Annulations of Cyclohexenylsilanes and Chlorosulfonyl Isocyanate: Application to the Total Synthesis of (±)-Peduncularine

The synthesis of (+/-)-peduncularine was accomplished using the [3 + 2] annulation of an allylic silane with chlorosulfonyl isocyanate to assemble the bicyclic core of the alkaloid. The stereochemistry of the annulation product was employed to control the installation of the indolylmethyl side chain at C-7 with complete stereoselectivity.

Synthesis of (+/-)-5-epi-citreoviral and (+/-)-citreoviral and the kinetic resolution of an allylic silane by a [3 + 2] annulation.

[reaction: see text] The [3 + 2] annulation reaction of allylsilane 1 with an alpha-keto ester provided the highly substituted tetrahydrofuran 2 as a single diastereomer in high yield. The synthesis of (+/-)-5-epi-citreoviral and (+/-)-citreoviral has been accomplished with this annulation reaction as the key step. Using the pantolactone-derived alpha-keto ester, the allylsilane 1 has been resolved with high enantiomeric purity.

Intramolecular coupling of allyl carboxylates with allyl stannanes and allyl silanes: a new type of reductive elimination reaction?

The palladium-catalyzed intramolecular coupling of allyl stannanes with allyl carboxylates provides a general synthesis of five- and six-membered-ring carbocycles. The intramolecular coupling leads selectively to trans five-membered carbocycles and cis six-membered carbocycles, regardless of the cis or trans configuration of the allylic functions in the starting material. For example, the stereoselective synthesis of 10-epi-elemol demonstrated the cis configuration of the six-membered carbocycles. The related Oppolzer cyclization leads to lower yields, or fails completely, with substrates substituted at C-3 of the allyl and/or alkene terminus. The palladium-catalyzed intramolecular coupling of allyl silanes with allyl trifluoroacetates allows the synthesis of trans five-membered-ring carbocycles and requires the use of a bicyclic phosphite as the ligand. DFT calculations suggest that the preferred pathway for the intramolecular allyl/allyl coupling is by formation of the Cbond;C bond between the C-3 termini of the allyl ligands of bis(eta(3)-allyl)palladium complexes.

A novel axially chiral 2,2'-bipyridine N,N'-dioxide. Its preparation and use for asymmetric allylation of aldehydes with allyl(trichloro)silane as a highly efficient catalyst.

[reaction: see text] New axially chiral 2,2'-bipyridine N,N'-dioxides were obtained by a new method that does not involve any procedures for the separation of enantiomers. One of the dioxides, (R)-3,3'-bis(hydroxymethyl)-6,6'-diphenyl-2,2'-bipyridine N,N'-dioxide, exhibited extremely high catalytic activity for the asymmetric allylation of aldehydes with allyl(trichloro)silane. The allylation of aromatic aldehydes proceeded in the presence of 0.01 or 0.1 mol % of the dioxide catalyst to give the corresponding homoallyl alcohols of up to 98% ee.

The Reaction of Nitrones with Organometallic Compounds: Scope, Limitations and Synthetic Applications

Synthetic organic chemists have widely exploited nitrones for the last 50 years as precious substrates for the assembly of complex nitrogen containing frameworks through inter and intramolecular 1,3-dipolar cycloadditions. In the last decade the interest for nitrones was further on increased by a second opportunity, offered by the nucleophilic addition of organometallic reagents to give N,N-disubstituted hydroxylamines. Grignard, lithium reagents and allylic zinc derivatives react very efficiently with nitrones in a wide temperature range, both the reaction rate and the stereochemical outcome of the process can be significantly modified by the addition of nitrone-chelating Lewis acids. The allylation of nitrones is also carried out under Sakurai conditions in the presence of trimethylsilyltriflate, and different results are obtained using allylic silanes or stannanes. Homoallylic hydroxylamines, generated by the allylation of nitrones, are exploited in 5-exo-trig iodocyclizations to give 5-iodomethyl isoxazolidines, useful precursors of hydroxylated acyclic and cyclic amino compounds. Enolates and silyl enol ethers under Mukayama conditions are also investigated. In particular, 2-trimethylsilyloxyfuran reacts with nitrones, in the presence of nitrone activators, to give 3-substituted tetrahydrofuro[2,3-d]isoxazol-5(2H)-ones, easily transformed into a variety of polyhydroxylated piperidine and indolizidine azasugars. The most recent solutions to achieve the control of stereochemistry in nucleophilic additions to nitrones will be examined.