Acid-Catalyzed Diels-Alder Reactions of Cycloalkenones

Whereas the Diels-Alder reaction has been in vogue as a facile method of six-membered ring construction for many years, its application to a one-step cis-octalone synthesis was precluded by discouraging, early reports on the conditions required for the interaction of dienes with 2-cyclohexenones and on the yields of the resultant adducts.^1,2 It is quite conceivable that these observations were responsible in part for the minimization of use of an otherwise attractive route toward angularly alkylated six-membered polycycles in the field of steroid total synthesis. The discovery of Lewis acid catalysis of the cycloaddition of α, β-unsaturated carbonyl compounds³ has rekindled interest in the above reactions.⁴ The following discussion illustrates octalone     

Divergent Synthesis of N-Heterocycles by Merging Borane-Mediated Cyclopropane Ring-Opening and Hydride Abstraction

Main observation and conclusionHerein,we report the synthesis of N-heterocycles by borane-catalyzed dehydrogenative cyclization of 2-cyclopropyl-N,N-dimethyl-a...     

Reactions of cannabinoid tosylhydrazones: Stereochemical aspects

Reactions of the epimeric 7-oxo-hexahydrocannabinol tosylhydrazone acetates (2a and4a) are described. Under aprotic basic reaction conditions followed by acetylation both compounds lead to the known Δ1⁷-tetrahydrocannabinol acetate (6); under protic conditions the axial isomer undergoes ring expansion (to7b), the equatorial one gives6.     

Acid-Catalyzed Reactions of 2,3-Epoxy Derivatives of Citral with Alcohols

Acid-catalyzed reactions of 2,3-epoxy derivatives of citral (which is a widely spread naturally occurring unsaturated aldehyde) with alcohols were studied under conditions of heterogeneous catalysis and in fluorosulfonic acid. A number of new products were obtained, and possible mechanisms of their formation were proposed.     

Stereochemistry of the Reactions of Sulfur-Stabilized Carbanions on Cyclopropane and Cyclohexane Rings

Stereochemistry in alkylations and H-D exchange of configurationally defined α-sulfonyl- and α-sulfinyl-carbanions generated from the compounds I-IV, are discussed.     

Cobalt-Catalyzed Enantioselective Ring-Opening Reactions of Oxa- and Aza-bicyclic Alkenes with Alkenylboronic Acids

Catalytic enantioselective ring-opening of oxa-and aza-bicyclic alkenes with readily available accessible alkenylboronic acids,promoted by a chiral phosphine-Co complex,is reported.Such a process represents the unprecedented Co-catalyzed introduction of various alkenyl groups onto the oxa-and aza-bicyclic alkenes,affording a wide range of multisubstituted functionalized cyclohexenes in up to 98%yield and 99.5:0.5 er.     

Stereochemical Aspects in the Asymmetric Michael Addition of Chiral Imines to Substituted Electrophilic Alkenes

The Michael-type addition of chiral imines, derived from racemic alpha-substituted cyclanones and optically active 1-phenylethylamine, to electrophilic alkenes, in neutral conditions, constitutes one of the most efficient methods for the stereocontrolled construction of quaternary carbon centers. In order to create an additional stereogenic center at the alpha- or beta-position to the quaternary one, the behavior of a variety of alpha- and beta-substituted alkenyl acceptors was examined. In general, these additions are highly regioselective, the alkylation taking place predominantly, if not exclusively, at the more substituted alpha-side of the imine function; however, in some cases (electrophilic alkenes 28 and 49), significant amounts (10-15%) of regioisomeric adducts were obtained. With the exception of methyl propiolate 52, a remarkable control of the absolute configuration of the adducts were always observed with these Michael acceptors. According to the general rule we have previously proposed, the alkylation process takes place preferentially on the less hindered pi-face of the more substituted secondary enamine, in tautomeric equilibrium with the starting imine. An excellent diastereocontrol was always obtained by using the present alpha- and beta-substituted alkenes. These stereochemical outcomes can be interpreted by invoking that the reaction proceeds through a compact approach of the reactants, the hydrogen atom at the nitrogen center of the enamine being transferred to the alpha-vinylic carbon atom of the acceptor, concertedly with the creation of the C-C bond. In this respect the "endo-approach" 58, in which the electron-withdrawing group of the acceptor faced to the nitrogen atom of the enamine (case of acceptors 10, methyl methacrylate, 24, 28, 43, 47, and 49) largely prevails over the "exo-approach" 59 (case of acceptor 38). This predominant "endo-preference" can be reasonably interpreted in terms of a cooperative effect between steric and stereoelectronic factors.     

Reaction of 2,3-O-Isopropylidene-D-Glyceraldehyde and Some Dialkyl and Diarylthiomethyl Sulfoxides: Stereochemical Aspects

Abstract

The reaction of 2,3-O-isopropylidene-D-glyceraldehyde with different dialkyl and diarylthiomethyl sulfoxides occurs with a high selectivity and produces the corresponding 1-alkyl(or l-aryl)-sulfynil-l-alkyl(or 1-aryl) thio-3,4-O-isopropylidene-D-tetroses as pure diastereoisomers. We have determined the absolute configuration of the three chiral centers formed in these reactions.     

Phosphoryl and Thiophosphoryl Transfer Reactions: Stereochemical Imperatives for Metaphosphate and Thiometaphosphate

Abstract

The stereochemical courses of phosphoryl transfer reactions in aprotic solvents and thiophosphoryl transfer reactions in protic solvent have been determined. The extensive racemisation observed in both instances is discussed in terms of metaphosphate and thiometaphosphate intermediates of significant life-times.     

Theoretical Aspects of Cycloaddition Reactions of Phosphorylnitrile Oxides to Unsaturated Compounds

Abstract

Ab initio (4–31G?) and semiempirical (MNDO, AMI) quantum-chemical calculations of phosphorus-functionalized and non- phosphorylated nitrile oxides have been done. In approximation of general multielectronic perturbation theory, all nitrile oxides, including trifluoroacetonitrile oxide and dialkoxyphosphorylnitrile oxides, by type of orbital interactions qualify as electron-donor dipoles.     

A Novel Acid-Catalyzed Isomerization of Aib-Containing Thiodipeptides

The use of amino thioacids in the `azirine/oxazolone method' led to completely epimerized Aib-containing endothiodipeptides (Aib=2-aminoisobutyric acid). It could be established that the epimerization occurred during the acidic hydrolysis of the primarily formed dipeptide thioanilides in which the thiocarbonyl group was shifted from the last to the penultimate amino-acid residue. Several conditions for the hydrolysis were tested, and, in some of them, the degree of epimerization could be reduced. By treatment of the Aib-containing dipeptide thioanilides 21 with ZnCl₂ in AcOH followed by HCl in AcOH, the isomeric endothiodipeptide anilides 25 were formed, i.e., the thiocarbonyl group was again shifted from the last to the penultimate amino acid residue. Under optimized reaction conditions, this novel isomerization proceeded in high yields and without any epimerization. Two conceivable mechanisms are proposed in Scheme 12. X-Ray diffraction analyses were performed for Z−Gly−Aib-Ψ(CS)−N(Me)Ph ( 21f ) and the isomeric Z−Gly-Ψ(CS)−Aib−N(Me)Ph ( 25f ).     

Synthetic and Mechanistic Aspects of Inorganic Insertion Reactions. Insertion of Carbon Monoxide

The synthetic potential and the mechanistic aspects of inorganic insertion reactions of carbon monoxide, especially into metal-carbon σ-bonds, are considered. Reactions of this type are encountered among most 3d, 4d, and 5d elements. In one case it has been demonstrated that the CO insertion proceeds by alkyl migration; this is likely to be a general feature of all such reactions. If an alkyl migration takes place, then insertion of CO into the M? C bond is governed kinetically by the cleavage of that bond. CO abstraction from RCO? M bonds most commonly proceeds by rate-determining vacation of a coordination position. Both CO insertion and abstraction are usually highly stereospecific.     

On the Influence of Metal Alkoxides on the Epoxide Ring-Opening and Condensation Reactions of 3-Glycidoxypropyltrimethoxysilane

The extent of the epoxide ring-opening, the formation of ethyl ether groups as one reaction product of the ring-opening and the condensation degree of RSi(O_0.5)₃ units in sols and gels of the system 3-glycidoxypropyltrimethoxysilane (GPTS)-1.5H₂O-0.01/0.1/1.0 metal alkoxide [Si(OEt)₄, Sn(OBu^t)₄, Al(OBu^s)₃, Al(OEtOBu)₃, Ta(OEt)₅, Ti(OEt)₄, Zr(OBuⁿ)₄] in ethanol has been examined by means of liquid- and solid-state ₁₃C and ₂₉Si NMR spectroscopies. The results reveal a strong epoxide ring-opening effect of Al-alkoxides in hybrid sols after 24 h reaction time and of Zr-, Ta-, Al- and Sn-alkoxides in corresponding hybrid gels already at low concentration (1 mole%). The ring-opening rate increases in sols with higher metal alkoxide concentration (10 mole%) but decreases at 50 mole% concentration of Al-, Ti- and Zr-alkoxides. The ring-opening activity of metal alkoxides in 10 mole% hybrid sols increases after 24 h reaction time in the order Si(OEt)₄ < Ti(OEt)₄ < Zr(OBuⁿ)₄ < Ta(OEt)₅, Sn(OBu^t)₄, Al(OBu^s)₃, Al(OEtOBu)₃. The 24 h hybrid sols and gels contain considerable amounts (up to 90%) of ethyl ether groups as reaction product of the ring-opening reaction which lowers the formation of polyether bonds. The condensation degree (c.d.) of RSi(O_0.5)₃ units of GPTS-1.5H₂O sols with 10 mole% of metal alkoxides increases up to 80% after 7 h reaction time in the order: Si(OEt)₄ Sn(OBu^t)₄ < Zr(OBuⁿ)₄ < Al(OBu^s)₃ < Al(OEtOBu)₃ < Ta(OEt)₅ < Ti(OEt)₄. An additional increase in c.d. up to 90% follows after the thermal sol-gel transformation. Generally, maximum activity of metal alkoxides in ring-opening and condensation reactions was found in sols and gels with 10 mole% additives. The effect of water on the epoxide ring-opening and on c.d. is discussed. Furthermore, the activity of metal alkoxides is compared with corresponding nanoscaled metal oxides.