High-Field 1H NMR Spectroscopic Studies of Prochiral Protons Attached to Chiral Moieties used in Asymmetric Reactions

The synthesis of N-substituted propionyland phenylacetylsubstituted chiral moieties based on the oxazolidin-2-one or 1,3-oxazin-2-one ring systems has been acheived via the use of a variety of different reagents and bases. A 'H NMR spectroscopic study of the prochiral protons Ha and Hb in the methylene group of these systems has been undertaken at 200, 250 and 360 MHz with the aim, of determining the extent, if any, to which the diastereotopicity imparted by the chiral auxiliary in a specific asymmetric transformation is reflected in the NMR parameters. For the N-propionyl derivatives the coupling patterns of the prochiral methylene protons Ha and Hb are almost the same, viz, a doublet of quartets for each proton. The chemical shifts and the coupling constants are very similar and fall in the range 2.63-3.03 ppm, and 16.518.3 Hz for JHaHb and 7.3±0.2 Hz for Ha--IWN1e• The coupling patterns for Ha and Hb of the Nphenylacetyl derivatives of most of these chiral auxiliaries are pairs of doublets with a coupling constant JHaHb in the range 15.5-16.2 Hz, whilst some of them have the same chemical shift and therefore appear as a singlet. In these systems no correlation was observed between the diastereotopicity imparted by the chiral auxiliary as measured by the stereochemical outcome for the aldol reactions (and Diels-Alder reactions for the corresponding acrylate derivatives) in terms of diastereomeric excess (d.e.%) and the chemical shift separation of the diastereotopic protons. Attempts to distinguish between the prochiral methylene protons Ha and Hb in the Npropionyl derivatives of the different auxiliaries by 'H NMR nOe difference spectra failed apart from that based on the horny! moiety. For this purpose, its titanium-derived complex was prepared by treatment of the N-propionyl derivative with titanium tetrachloride in deuteriated chloroform. Compared to the freely rotating system, both 11a and H,, in the complex had distinct chemical shifts and appeared as a doublet of quartets, which were well separated from each other. In addition, the signals were shifted to higher frequency relative to the free propionate, indicative of electron donation to the titanium. A comparison of the chiral efficiency of both the carbohydrateand terpenoid-derived auxiliaries found that the measured levels of asymmetric induction imparted by lithium enolate systems, which are generated via lithium diisopropylamide, were markedly different and in favour of the former presumably due to chelation control by ketal and/or pyranose ring oxygens. On the other hand, much higher levels of induction are measured with boron enolates derived from terpenoid-based auxiliaries with di-n-butylboron triflate (Bu2130Tf), and moreover, can bring about the opposite sense of induction compared to the lithium enolate systems. These aspects of stereocontrol are discussed in terms of different transition states. In the presence of excess Bu2BOTf used to promote anti-selectivity in asymmetric aldol reactions with benzaldehyde, a homochiral terpenoid-derived I ,3-oxazin-2-one resulted unexpectedly in ring cleavage by an intramolecular process to form a N-substituted tetrahydroI ,3-oxazine-2,4-dione in virtually quantitative yield. Mechanistic aspects of this unusual reaction are discussed.