Enantiopure bicyclic piperidinones: stereoselectivity in lactam enolate alkylations

Synthesis of 3-Alkyl Oxazolidines, Derived from 2-Hydroxymethyl Piperidine, as Analytical Standards for the Analysis of Volatile Aldehydes in the Workplace

Hexahydro-3-alkyl-3H-oxazolo[3,4-a]pyridines 4-15 for the quantitative analysis of various aldehydes were obtained in good yield via the condensation reaction of 2-hydroxymethylpiperidine (2-HMP) with aldehydes under mild conditions. When acrolein was used, the bicyclic 17 was obtained. This novel compound has suitable physical characteristics for an analytical standard. The hexahydro-3-vinyl-3H-oxazolo[3,4-a]pyridine 16 can be obtained at higher temperatures using an excess of acrolein (3 equiv). Following the same procedure as for 16, but with an excess of 2-HMP (2 equiv), a diastereomeric mixture of 18/19, which are both bisadducts of 2-HMP with acrolein, was obtained. The latter mixture can be easily converted into pure 18. Mechanistically, a thorough 1H-NMR study did not show any evidence that the condensation reaction proceeded via an enamine. The reaction probably proceeded through an elusive hemiaminal and fleeting iminium ion, which underwent subsequent cyclization to give hexahydro-3-alkyl-3H-oxazolo[3,4-a]pyridines 4-16. The reaction pathways for the preparation of 4-18 are described.

Regio- and Enantioselective Catalytic δ-C-H Amidation of Dioxazolones Enabled by Open-Shell Copper-Nitrenoid Transfer

Controlling regio- and enantioselectivity in C-H functionalization reactions is of paramount importance due to their versatile synthetic utilities. Herein, we describe a new approach for the asymmetric δ-C(sp³)-H amidation catalysis of dioxazolones using a Cu(I) precursor with a chiral bisoxazoline ligand to access six-membered lactams with high to excellent regio- and enantioselectivity (up to >19:1 rr and >99:1 er). Combined experimental and computational mechanistic studies unveiled that the open-shell character of the postulated Cu-nitrenoids enables the regioselective hydrogen atom abstraction and subsequent enantio-determining radical rebound of the resulting carbon radical intermediates. The synthetic utility of this asymmetric cyclization was demonstrated in the diastereoselective introduction of additional functional groups into the chiral δ-lactam skeleton as well as in the rapid access to biorelevant azacyclic compounds.

Spirocyclic systems derived from pyroglutamic acid

The synthesis and likely conformational structure of rigid spirocyclic bislactams and lactam-lactones derived from pyroglutamic acid, and their suitability as lead structures for applications in drug development programmes using cheminformatic analysis, has been investgated.

Chiral Oxazolopiperidone Lactams: Versatile Intermediates for the Enantioselective Synthesis of Piperidine-Containing Natural Products

Phenylglycinol-derived oxazolopiperidone lactams are exceptionally versatile building blocks for the enantioselective construction of structurally diverse piperidine-containing natural products and bioactive compounds. These lactams are readily available in both enantiomeric series by cyclocondensation of the chiral amino alcohol with a delta-oxo acid derivative and allow the substituents to be introduced at the different ring positions in a regio- and stereocontrolled manner, providing access to enantiopure polysubstituted piperidines bearing virtually any type of substitution pattern, and also quinolizidines, indolizidines, perhydroquinolines, hydroisoquinolines, as well as complex indole alkaloids. Of particular interest are cyclocondensation reactions with racemic or prochiral delta-oxo (di)acid derivatives in processes involving dynamic kinetic resolution and/or differentiation of enantiotopic or diastereotopic ester groups, as they directly lead to lactams that already incorporate the carbon substituents on the heterocyclic ring. The use of (S)-3,4-dimethoxyphenylalaninol or (S)-tryptophanol in the above cyclocondensation reactions expands the potential and the scope of the methodology, providing a straightforward route to enantiopure benzo[a]- and indolo[2,3-a]quinolizidines.

On the Origin of the Stereoselectivity in the Alkylation of Oxazolopiperidone Enolates

The origin of the diastereoselective alkylation of enolates of oxazolopiperidones is studied by means of theoretical calculations and experimental assays. For the unsubstituted oxazolopiperidone, the alkylation with methyl chloride is predicted to afford mainly the exo product, a finding further corroborated from the analysis of the experimental outcome obtained in the reaction of the racemic oxazolopiperidone. However, such a preference can be drastically altered by the presence of substituents attached to the fused ring. In particular, when the angular carbon adopts an R configuration in a phenylglycinol-derived oxazolopiperidone, the presence of a phenyl ring at position 3 forces the pseudo-planarity of the bicyclic lactam, and the diastereoselectivity is dictated by the internal torsional strain induced in the enolate. However, when the angular carbon adopts an S configuration, the preference for the exo alkylation stems from the intermolecular steric hindrance between the enolate and the alkylating reagent. Interestingly, the intramolecular hydrogen bond formed between the phenyl ring and the carbonyl oxygen in the enolate largely reduces the difference in stability of the two TSs compared to the unsubstituted oxazolopiperidone.

Alkylation of Phenylglycinol-Derived Oxazolopiperidone Lactams. Enantioselective Synthesis of β-Substituted Piperidines

The stereochemical outcome of the alkylation of a variety of phenylglycinol-derived oxazolopiperidone lactams is studied. The influence of the configuration of the C-8a stereocenter and the effect of the substituents at the C-8 and C-8a positions on the stereoselectivity of the reaction are discussed. The synthetic utility of these alkylation reactions is illustrated with the synthesis of cis and trans 3,5-disubstituted, 2,5-disubstituted, and 2,3,5-trisubstituted enantiopure piperidines, and the indole alkaloids 20R- and 20S-dihydrocleavamine.

Unexpected epimerization at C2in the Horner–Wadsworth–Emmons reaction of chiral 2-substituted-4-oxopiperidines

The observed epimerization at C2 in the Horner-Wadsworth-Emmons (HWE) reaction of chiral 2-substituted-4-oxopiperidines has been investigated and, on the basis of the experimental results, a mechanism for this unexpected process has been proposed.