α‐Chiral Acetylenes Having an All‐Carbon Quaternary Center: Phase Transfer Catalyzed Enantioselective α Alkylation of α‐Alkyl‐α‐alkynyl Esters

Abstract

Owing to the synthetic versatility and modularity of acetylenes, chiral building blocks bearing acetylene moieties are widely used as valuable intermediates. [1] Accordingly, tremendous efforts have been devoted to the development of catalytic asymmetric approaches to enable facile access to these attractive materials. The asymmetric transition metal mediated addition of terminal acetylenes (asymmetric alkynylations) to various prochiral electrophiles (Scheme 1, left), [2] such as aldehydes, [3] ketones, [4] aldimines, [5] and electron-deficient alkenes, [6] as well as asymmetric additions to prochiral alkynyl substrates (Scheme 1, right), [7][8][9][10][11] have been successfully adopted for this purpose. [12,13] However, most of these procedures are only applicable to the synthesis of a-chiral acetylenes having one hydrogen atom at the a position. [14] We envisaged that asymmetric a functionalization of racemic a-alkyl-a-alkynyl esters via prochiral a-alkynyl enolates would offer a distinctive solution in this realm, providing a-chiral acetylenes having a quaternary stereogenic center (Scheme 2). The asymmetric C À C bond formation with these substrates is highly appealing since it is expected to generate an enantioenriched all-carbon quaternary center having both an appended acetylene and ester as useful functionalities; [15] such a substrate is hardly accessible by use of the existing catalytic asymmetric methods.As a suitable benchmark catalytic system to evaluate the viability of such a transformation, we selected phase transfer catalyzed asymmetric alkylation, taking advantage of its wellestablished practicality in analogous transformations, as well as our broad interest in phase-transfer catalysis. [16] The starting materials, a-alkyl-a-alkynyl esters, could be synthesized from a-iodoalkanoates and monosubstituted acetylenes by using the protocol reported by Oshima and co-workers. [17] In an exploratory study, the asymmetric benzylation of the a-silylethynyl esters 3 was investigated under conventional phase transfer catalyzed alkylation reaction conditions using (S)-1 a as the catalyst (Table 1). [18] To our delight, the reaction of a-(tert-butyldimethylsilyl)ethynyl ester cleanly provided the desired compound in 87 % yield with 63 % ee (Table 1, entry 1). Examination of the steric Scheme 1. Common tactics for the catalytic asymmetric synthesis of a-chiral acetylenes.Scheme 2. Phase transfer catalyzed asymmetric functionalization of a-alkyl-a-alkynyl esters. Q = ammonium. Table 1: Screening of the reaction conditions. [a] Entry Cat. Si Solvent T [8C], t [h] Yield [%] [b] ee [%] [c]

Organocatalyzed Isomerization of α‐Substituted Alkynoates into Trisubstituted Allenoates by Dynamic Kinetic Resolution

“…The asymmetric isomerization of α‐methyl substituted alkynoate 2 a 6 was initially examined in the presence of 10 mol % of HB donor catalysts (Table 1). As expected, when benzothiadiazine catalyst 1 a 3c was used, the reaction of 2 a proceeded almost completely to afford the desired allenoate 3 a in good yield in a 92:8 ratio of 3 a and 2 a (entry 1).…”

Catalytic Synthesis of Nonracemic Azaproline Derivatives by Cyclization of β‐Alkynyl Hydrazines under Kinetic Resolution Conditions

“…The main thrust of this project was to develop a kinetic resolution technique to take advantage of the unprecedented nonmetal‐catalyzed cyclic heteroatom addition mentioned above. Nevertheless we briefly explored asymmetric phase‐transfer‐catalyzed additions of azidodicarboxylates to our γ‐silyl allenyl ester system by using a recent precedent by Maruoka and co‐workers involving carbon electrophiles 16. While aqueous/organic biphasic systems resulted in no reaction with allenyl oxyesters, we observed that CsOH⋅H 2 O in toluene led to their successful hydrazination.…”