A stereospecific synthesis of oxazolinyloxiranes

Borylation and rearrangement of alkynyloxiranes: a stereospecific route to substituted α-enynes

1,3-Enynes are important building blocks in organic synthesis and also constitute the key motif in various bioactive natural products and functional materials. However, synthetic approaches to stereodefined substituted 1,3-enynes remain a challenge, as they are limited to Wittig and cross-coupling reactions. Herein, stereodefined 1,3-enynes, including tetrasubstituted ones, were straightforwardly synthesized from cis or trans-alkynylated oxiranes in good to excellent yields by a one-pot cascade process. The procedure relies on oxirane deprotonation, borylation and a stereospecific rearrangement of the so-formed alkynyloxiranyl borates. This stereospecific process overall transfers the cis or trans-stereochemistry of the starting alkynyloxiranes to the resulting 1,3-enynes.

Lithiated oxazolinyloxiranes and oxazolinylaziridines: key players in organic synthesis

This paper focuses on α-lithiated oxazolinyloxiranes and oxazolinylaziridines, their generation, reactions, and synthetic applications. The ability of the oxazoline ring in providing stabilization to such α-heterosubstituted carbanions either through electronic effects and coordinative action has been stressed as well as the contribution to the configurational stability or instability of such species. IR spectroscopic data, multinuclear NMR investigations, and ab initio calculations planned to get insights on chemical properties and structural features have been carried out. A number of new reactions including alkylations, addition reactions, hydroxyalkylations, cyclopropanations, lactonizations, and rearrangements have been discovered, giving access to a variety of substances including: aziridinolactones, epoxylactones, aryl alkanones, polysubstituted cyclopropanes, cyclopropanefused lactones, dihydro-oxazoloisoquinolines, diversely functionalized oxazolines, and products that can be derived from them by synthetic elaboration.

Rearrangement reactions of lithiated oxiranes

The first computational study of the rearrangement reactions of oxiranes initiated by lithium dialkylamides is presented. Aside from the well-known carbenoid insertion pathways, both β-elimination and α-lithiation have been suggested as the exclusive mechanism by which oxiranes react in the presence of organolithium bases. The products of the former are allyl alcohols (and, in some cases, dienes) and are ketones in the case of the latter. The computational studies reported in this work indicate that both mechanisms could be simultaneously operational. In particular, our work shows that the allyl alcohols from β-elimination are unlikely to undergo 1,3-hydrogen transfer to the vinyl alcohols and thus to the ketones, suggesting that ketones are formed through the opening of the oxirane ring after α-substitution. Elimination of LiOH from the lithiated allyl alcohol is found to result in the diene product. Low activation barriers for β-elimination are offered as the explanation for the few special cases where the allyl alcohol is the dominant or exclusive product. These findings are consistent with the product distributions observed in several experiments.

A computational study of mixed aggregates of chloromethyllithium with lithium dialkylamides

DFT calculations were performed to examine the possible formation of mixed aggregates between chloromethyllithium carbenoids and lithium dimethylamide (LiDMA). In the gas phase mixed aggregates were readily formed and consisted of mixed dimers, mixed trimers, and mixed tetramers. THF solvation disfavored the formation of mixed tetramers and resulted in less exergonic free energies of mixed dimer and mixed trimer formation.

A Computational Study of Oxiranyllithium

[reaction: see text] Computational methods were used to determine the structure, bonding, and aggregation states of oxiranyllithium in the gas phase and in THF solution, at 200 and 298 K. THF solvation was modeled by microsolvation with explicit THF ligands, forming a supermolecule that includes the oxiranyllithium aggregate and its first solvation shell. Because oxiranyllithium has a chiral center, two diastereomeric dimers were formed, the RR and the RS, along with their enantiomers. Similarly, three diastereomers of the tetramer were formed, the RRRR, RRRS, and RRSS and their enantiomers. Oxiranyllithium was found to exist predominantly as the tetramer in the gas phase, while the dimer was the dominant species in THF solution. The relative concentrations of the different stereoisomers were calculated from equilibrium constants.

Lithiation and Reactions of Stilbene Oxides: Synthetic Utility

The lithiation of trans- and cis-stilbene oxides (+/-)-1 and 8 has been investigated. While with 8, lithiation occurred exclusively at the benzylic position, with the trans isomer (+/-)-1, ortho-lithiation competed with alpha-lithiation depending upon the experimental conditions. The configurational stability of the alpha-lithiated cis- and trans-stilbene oxides (+/-)-2 and (+/-)-9, respectively, was proved as well as that of scalemic stilbene oxide (R,R)-2.

Lithium 2,2,6,6-Tetramethylpiperidide-Mediated α- and β-Lithiations of Epoxides: Solvent-Dependent Mechanisms

Lithium 2,2,6,6-tetramethylpiperidide (LiTMP)-mediated alpha- and beta-lithiations of epoxides are described. LiTMP displays a markedly higher reactivity than does lithium diisopropylamide, consistent with literature reports. Detailed rate studies of LiTMP/THF and LiTMP/Me(2)NEt mixtures reveal similar rates but significant mechanistic differences. LiTMP-mediated alpha-lithiation of cis-cyclooctene oxide with subsequent oxacarbenoid formation and transannular C-H insertion proceeds via monosolvated dimers in both THF and Me(2)NEt. LiTMP-mediated beta-lithiation of 2,3-dimethyl-2-butene oxide affords the corresponding allylic alcohol via a monosolvated monomer in THF and a monosolvated dimer in Me(2)NEt. We discuss how the solvent-dependent aggregation of LiTMP markedly influences the rate profile. The reaction transition structures are examined with density functional computations.

Metalation of 2-chloromethyl-2-oxazolines: synthesis of 1,2,3-tris(oxazolinyl)cyclopropanes and derivatives

2-Chloromethyl-2-oxazoline converts cleanly into trans-1,2,3-tris(oxazolinyl)cyclopropane upon treatment with strong bases such as LDA or KN(SiMe(3))(2). Deprotonation of the above cyclopropane followed by the addition of electrophiles allows the preparation of more functionalized tris(oxazolinyl)cyclopropanes.

The first generation and stereospecific alkylation of alpha-trifluoromethyl oxiranyl anion

[reaction: see text] Generation and reactions of oxiranyl anion 2 stabilized by a trifluoromethyl group are described. Treatment of (S)-2,3-epoxy-1,1,1-trifluoropropane (75% ee) with n-BuLi followed by electrophiles gave corresponding 2-alkylated epoxide 3 with retention of stereochemistry in moderate to good yields. The reaction is applicable for a general synthesis of optically active trifluoromethylated tertiary alcohols.