Tandem Inter [4 + 2]/Intra [3 + 2] Cycloadditions. 17. The Spiro Mode. Efficient and Highly Selective Synthesis of Azapropellanes
摘要:
A new variant of the tandem inter [4 + 2]/intra [3 + 2] nitroalkene cycloaddition has been developed. Intermolecular [4 + 2] cycloaddition of a 2-nitroalkene (bearing an unsaturated ester moiety, the dipolarophile) with a vinyl ether produces a cyclic nitronate substituted at C(3) wherein the only stereogenic center is the anomeric carbon C(6). Since the dipolarophile is attached to a tether extending from the C(3) of the nitronate (a-carbon of the nitroalkene), the intramolecular [3 + 2] cycloaddition affords a spiro tricyclic nitroso acetal. The cycloaddition proceeds smoothly for three- and four-atom tethers to afford five- and six-membered rings. Both E- and Z-unsaturated esters serve well as dipolarophiles, but the E-isomers react more selectively. Hydrogenolysis of the nitroso acetals affords the spiro tricyclic alpha-hydroxy lactams in good yield. Remarkably high levels of asymmetric induction are observed with the use of a chiral vinyl ether derived from (1R,2S)-2-phenylcyclohexanol. The origin of asymmetric induction is a combination of the established face selectivity of the enol ether and the preference for a distal fold of the tether away from the: substituent on the anomeric carbon. The scope and limitations of this transformation and an analysis of the origin of stereoselectivity are provided.
Organocatalytic Access to a <i>cis</i>-Cyclopentyl-γ-amino Acid: An Intriguing Model of Selectivity and Formation of a Stable 10/12-Helix from the Corresponding γ/α-Peptide
作者:Rossana Fanelli、Dénes Berta、Tamás Földes、Edina Rosta、Robert Andrew Atkinson、Hans-Jörg Hofmann、Kenneth Shankland、Alexander J. A. Cobb
DOI:10.1021/jacs.9b10861
日期:2020.1.22
In this study, we have developed a highly enantioselective organocatalytic route to the (1S,2R)-2-(aminomethyl)cyclopentane-1-carboxylic acid monomer precursor, which has a cis-configuration between the C- and N-termini around the cyclopentane core. Kinetic measurements show that the product distribution changes over time due to epimerization of the C1 center. Computations suggest the cis-selectivity is a result of selective C-C bond formation, while subsequent steps appear to influence the selectivity at higher temperature. The resulting gamma-amino acid residue was incorporated into a novel gamma/alpha-peptide, which forms a well-ordered 10/12-helix with alternate H-bond directionality in spite of the smallest value of the zeta-angle yet observed for a helix of this type. This highly defined structure is also a result of the narrow range of potential zeta-angles in our monomer. In contrast, the larger range of potential zeta-values observed for the corresponding trans-system can be fulfilled by several competing helical structures.
Electroorganic chemistry. VII. Anodic oxidation of cyclopropanes