Synthesis and Application of P-Stereogenic Phosphines as Superior Reagents in the Asymmetric Aza-Wittig Reaction
摘要:
[GRAPHICS]A wide variety of P-stereogenic aryldialkylphosphines were prepared in enantioenriched form by a systematic diversification of the (-)-sparteine-mediated dynamic kinetic resolution of racemic lithiophosphine-boranes reported by Livinghouse. Excellent asymmetric induction was observed provided that the intermediate lithiophosphine/sparteine complex precipitated from solution; more soluble derivatives returned poor ee's or racemic material. The resulting phosphine-boranes were deprotected and used as reagents in the desymmetrizing asymmetric aza-Wittig reaction of 2-alkyl-2-(3-azidopropyl)cyclohexane-1,3-diones, delivering the highest ee's yet observed in this process (up to 84% ee). Phosphines bearing bulky substituents required heating for the aza-Wittig reaction to proceed to completion, which P-31 NMR studies showed to be due to interception of the reaction by the formation of unreactive (E)-phosphazides. This was circumvented by use of methyltrioxorhenium to catalyze the formation of iminophosphoranes from the azide and phosphine, allowing reactions to take place at ambient temperature, although the ee's of the asymmetric reactions were reduced in these examples.
Concise access to indolizidine and pyrroloazepine skeleta via intramolecular Schmidt reactions of azido 1,3-diketones
作者:Duanpen Lertpibulpanya、Stephen P. Marsden
DOI:10.1039/b608801e
日期:——
Readily prepared 2-alkyl-2-azidopropylcycloalkyl-1,3-diones undergo intramolecular Schmidt rearrangement with a range of hard Lewis acids, leading to indolizidinediones and pyrroloazepinediones. Chiral aluminium-based Lewis acids could also be used to mediate this symmetry-breaking transformation, but no significant asymmetric induction was observed.
Overcoming Product Inhibition in Catalysis of the Intramolecular Schmidt Reaction
作者:Hashim F. Motiwala、Charlie Fehl、Sze-Wan Li、Erin Hirt、Patrick Porubsky、Jeffrey Aubé
DOI:10.1021/ja402848c
日期:2013.6.19
carrying out the intramolecularSchmidtreaction of alkylazides and ketones using a substoichiometric amount of catalyst is reported. Following extensive screening, the use of the strong hydrogen-bond-donating solvent hexafluoro-2-propanol was found to be consistent with low catalyst loadings, which ranged from 2.5 mol % for favorable substrates to 25 mol % for more difficult cases. Reaction optimization
Chiral Brønsted Acid-Promoted Enantioselective Desymmetrization in an Intramolecular Schmidt Reaction of Symmetric Azido 1,3-Hexanediones: Asymmetric Synthesis of Azaquaternary Pyrroloazepine Skeletons
The enantioselectivedesymmetrization of 2‐substituted‐2‐azidopropyl 1,3‐hexanediones through an asymmetric intramolecular Schmidt reaction using a chiral Brønsted acid has been developed for the first time. Synthetically interesting pyrroloazepine skeletons with an azaquaternary stereogenic center with up to 59 % ee are accessed effectively (see scheme; R=H, alkyl, or aryl).
首次开发了使用手性布朗斯台德酸通过不对称分子内Schmidt反应对2-取代2-叠氮丙基1,3-己二酮进行对映选择性脱对称。合成有趣的吡咯并ze庚因骨架,其氮杂季铵立体异构中心的ee高达59%(请参阅方案; R = H,烷基或芳基)。