Chiral discrimination of cryptochiral saturated quaternary and tertiary hydrocarbons by asymmetric autocatalysis

Chiral discrimination of saturated hydrocarbons has been very difficult to establish, or has not been possible at all. The first chiral discrimination of cryptochiral 5-ethyl-5-propylundecane 1, that is, (n-butyl)ethyl(n-hexyl)(n-propyl)methane, a chiral saturated quaternary hydrocarbon, which is known to exhibit practically no detectable value of optical rotation between 280 and 580 nm, has been accomplished by asymmetric autocatalysis of pyrimidyl alkanol. The absolute configuration of 1 has been determined. In the presence of (R)- or (S)-1, the reaction between pyrimidine-5-carbaldehyde and diisopropylzinc affords (S)- and (R)-pyrimidyl alkanol with 91-97% ee, respectively. Thus, asymmetric autocatalysis serves as a powerful tool for the chiral discrimination of saturated hydrocarbons.     

Practically Perfect Asymmetric Autocatalysis with (2‐Alkynyl‐5‐pyrimidyl)alkanols

Extremely high enantioselectivity (>99.5% ee) and chemical yield (>99%) are achieved in an asymmetric autocatalytic reaction. A (5‐pyrimidyl)alkanol with a tert‐butylethynyl group at its 2‐position ( 1 ) is a very efficient asymmetric autocatalyst in the enantioselective alkylation in Equation (1).     

Enantiodivergent formation of a chiral cytosine crystal by removal of crystal water from an achiral monohydrate crystal under reduced pressure

An achiral nucleobase cytosine forms an achiral monohydrate crystal (space group: P2₁/c) by crystallization from a water solution. It was found that the removal of crystal water under reduced pressure at room temperature afforded a chiral crystal of anhydrous cytosine (P2₁2₁2₁). The crystal chirality of anhydrous cytosine corresponds to the enantiotopic crystal face of the achiral monohydrate; therefore, when the enantiotopic b¹-face is exposed to the reduced pressure, dehydration occurred in the direction from the b¹-face to provide [CD(+)310_KBr]-cytosine crystal. In contrast, dehydration from the b²-face gave the opposite enantiomorphous [CD(?)310_KBr]-cytosine crystal. The correlation between enantiotopic faces and the formed crystal chirality is opposite to that from dehydration by heating. The formed chiral cytosine crystals act as a chiral trigger for asymmetric autocatalysis with enantioenrichment amplification of pyrimidylalkanol.     

Asymmetric autocatalysis: triggered by chiral isotopomer arising from oxygen isotope substitution

Trigger happy: Chiral oxygen isotopomers of hydrobenzoin ([(18) O](R)-1 and [(18) O](S)-1) acted as chiral triggers to induce the enantioselective addition of iPr(2) Zn to pyrimidine-5-carbaldehyde. An extremely small chiral influence arising from the presence of the oxygen isotope ((18) O) is amplified through asymmetric autocatalysis to enantioenrich the 5-pyrimidyl alkanol product.     

A reversal phenomenon of enantioface selectivity by the cooperative operation of two chiral catalysts

The reversal phenomenon of enantioface selectivity in the asymmetric addition of diisopropylzinc to aldehydes was observed by mixing two chiral β-amino alcohol catalysts. The opposite enantiomeric product was formed on using a mixture of two chiral catalysts that possess individual enantioselectivity.