Applications to polymer-supported species

The addition of an amino acid and subsequent conversion to the hydantoin were monitored on single beads using both one-dimensional H and COSY NMR.40 With beads of 400-750 pm holding 10-65 nmol of amino acid per bead, acquisition time of 3 min for ID and 40 min for COSY were sufficient when 1-4 beads were placed in the MAS rotor. 

Overlap from resin signals, mentioned previously, can be overcome in several ways, and is particularly important when following reactions involving aromatic groups attached to polystyrene based resins. Spin echo MAS NMR was applied to follow the progress of a reduction of a resin-bound methyl benzoate to the alcohol.45 One-dimensional H as well as COSY and TOCSY spectra were used to elucidate the reaction products, although transformations were readily apparent in the ID spectra alone. 

The Sharpless asymmetric dehydroxylation of resin-bound olefins was monitored using 3H, 13C and HMQC HRMAS NMR.63 The authors found 13C HRMAS NMR to be particularly suited to evaluating the progress of this reaction and permitted the enantiomeric excesses of the products to be determined before they were cleaved from the support. Most importantly, they were able to evaluate the types of substrates amenable to this reaction on solid supports, showing the ability of HRMAS NMR to contribute to synthetic questions. Transformation of the unnatural amino acid Lys(NH2) on a poly (ethylene glycol)-dimethylacrylamide (PEGA) resin to 6-hydroxynorleucine was confirmed by application of TOCSY HRMAS experiments.64 

This diffusion-filtered approach has been extended in application to macroscopic supports, specifically a Synphase lantern.77 The advantage of the diffusion-filtered experiment was extended to suppression of the macroscopic support signals, not just those of the protonated solvents. Although, the lanterns required some modification to fit in the MAS rotors, it is proposed that manufacturers account for this size restriction in future formats of these supports. 

While much of the application of HRMAS to solid-phase organic chemistry has been via H NMR, 13C HRMAS NMR has been used to determine resin loading.78 Two methods were applied, using both internal standards added to the resin in known amounts, or using features of the resin structure itself as a standard for calibration of the spectral intensities. The first method 

---