Kinetic resolution of amines with enantiopure 3-N,N-diacylaminoquinazolin-4(3H)-ones

Abstract

The title compounds (DAQs) are chiral when the two N-acyl groups are different because of the absence of rotation around the N–N bond (a chiral axis). Enantiopure DAQs have been obtained by incorporation of a chiral centre in enantiopure form either into the substituent at the Q2-position or into one of the N-acyl groups, or into both, followed by separation of diastereoisomers. This separation is unnecessary in one case because conversion of the N-monoacylaminoquinazolinone (MAQ) into the DAQ is completely diastereoselective. Neither is separation of diastereoisomers necessary with 3-[N,N-di-(S)-2-acetoxypropanoylamino]-2-diphenylmethylquinazolin-4(3H)-one 37a: this DAQ 37a has its N–N bond rendered a chiral axis by the bias in its imide moiety wholly in favour of one exo/endo conformation.

The high chemoselectivity exhibited by N,N-diacetyl- or N,N-dibenzoylaminoquinazolinones in reaction with the less hindered of two secondary amines (pyrrolidine in the presence of 1 eq. of piperidine) has a stereoselective counterpart: reaction of the above enantiopure DAQs enantioselectively with racemic amines leading to kinetic resolution. Using 1 eq. of DAQ and 2 eq. of amine, both the derivatised and unreacted amine enantiomers are recovered with high enantiomeric excess (ee) (better than 90% ee in some cases). Some of the higher ees are found in the recovered amides where non-chemoselective attack on both N-acyl groups of the DAQ has occurred: from the opposite configurations of the amine component in the two amides and from the low enantiopurity of the recovered unreacted amine, reaction of each of the N-acyl groups with complementary enantiomers of the amine is occurring (parallel kinetic resolution).

Although higher ees are, in general, obtained using secondary amines, high ees are obtained in some cases using 1-phenylethylamine and, in particular, amino acid esters (valine and alanine).

The sense of enantioselectivity in the reactions of these DAQs with amines is controlled by the configuration of the N–N axis: replacing the Q group in an N-(S)-2-acetoxypropanoyl-N-acetyl-bearing DAQ by phthalimide, thus eliminating the N–N chiral axis, drastically reduces the level of kinetic resolution.