Organocatalytic Enantioselective Decarboxylative Addition of Malonic Half Thioesters to Imines

Abstract

We describe a biomimetic organocatalytic enantioselective decarboxylative addition of malonic acid half thioesters to imines. This simple protocol makes use of readily available Cinchona-derived organocatalysts and nucleophiles at the carboxylate oxidation state. The resulting b-amino thioesters, being attractive precursors for the preparation of optically active b-amino acids, are formed in good yields and in up to 79 % ee. As suggested by several mechanistic insights the desired products are formed via initial formation of a thioester acetate enolate via decarboxylation of the malonic acid half thioester, followed by addition to the imine.Keywords: b-amino acids; asymmetric catalysis; imines; Mannich reaction; organic catalysis Recently, the metal-assisted direct aldol reaction, that is, the in situ formation of enolates and their involvement into a catalytic process, has gained sustained interest.[1a] The majority of this research focuses on the use of specific reactive ketone-or aldehyde-based nucleophiles. As a consequence, subsequent oxidation of the a-functionalized carbonyl compounds is required in order to obtain a-functionalized carboxylate compounds. Therefore it is desirable to develop processes using nucleophiles at the carboxylate oxidation state and as a result of this demand a few examples employing metal-based catalysts or promoters based on such system have been reported.[1b-g] However, the catalytic generation of an ester enolate is not a trivial task, due to the high pK a value of the a-protons in the parent compound.[2] Inspired by naturally occurring enzymatic polyketide biosynthesis, proceeding through the decarboxylative generation of thioester enolates, Shair and co-workers used malonic acid half thioesters as thioester enolate intermediates in a copper-catalyzed decarboxylative aldol reaction. [3] Indeed, these thioesters are very attractive candidates for the generation of an ester enolate equivalent under very mild reaction conditions, due to the intrinsic driving force connected with the loss of CO 2 , and the stabilizing effect of the sulfur moiety on the resulting enolate. [3] In order to avoid the use of metal salts as catalysts in this biomimetic process, an organocatalytic approach would be highly attractive. To the best of our knowledge there are no reports describing the use of chiral organocatalysts in such systems. Herein we would like to present a straightforward organocatalytic enantioselective decarboxylative addition of malonic acid half thioesters to imines (Mannich reaction) resulting in optically active bamino thioesters which are obvious precursors of important b-amino acids. [4] At the outset of this study initial attempts were performed using simple achiral organic bases as catalysts and we were pleased to find that the reaction could be catalyzed by simple organic bases such as DABCO, imidazole and DBU. For example, the reaction of malonic acid half thioester 2a with N-Ts imine 3a in toluene in the presence of 20 mol % DABCO led to good conversion (80 %) ...

Asymmetric organocatalytic decarboxylative Mannich reaction using β-keto acids: A new protocol for the synthesis of chiral β-amino ketones

“…The proposed addition–decarboxylation sequence is consistent with current mechanistic understanding [31,35,37–38]. However, we cannot exclude an alternative decarboxylation–addition pathway at this stage.…”

Highly Enantioselective Decarboxylative Mannich Reaction of Malonic Acid Half Oxyesters with Cyclic Trifluoromethyl Ketimines: Synthesis of β‐Amino Esters and Anti‐HIV Drug DPC 083

“…For example, this process generally employs an ester equivalent, such as a malonic acid half oxyester or thioester, to participate in the reaction under very mild conditions and thereby circumvent the problems with strong bases and self‐condensation. Catalytic asymmetric decarboxylative Mannich reactions with a wide range of aldimine‐based electrophiles have proven to be highly effective 4–6. However, expansion of the electrophilic acceptors to ketimines for the synthesis of β‐amino esters with a quaternary stereogenic center seems to be more challenging, since controlling the ketimine geometry in combination with the π‐facial selectivity of electrophilic attack is critical for the stereochemical outcome.…”

Organocatalytic Asymmetric Mannich Reactions in the Preparation of Enantioenriched β3-Amino Acid Derivatives

“…In contrast, Nature does not rely on metal ions for generation of malonic acid half-thioesters (MAHTs) enolates that are used in the biosynthesis of fatty acids and polyketides in all kingdoms of life [64,65]. Inspired by naturally occurring enzymatic polyketide biosynthesis, proceeding through the decarboxylative generation of thioester enolates, the first straightforward organocatalytic enantioselective decarboxylative addition of MAHTs 11 to imines resulting in optically active 3 -amino thioesters, obvious precursors of important 3 -amino acids, has been recently devised [66]. A screening of chiral catalysts derived from Cinchona alkaloids and of reaction conditions, revealed that the use of catalyst XIV [67] gave the best results in terms of enantioselectivity and yields.…”