Gel-phase NMR

Interestingly however as early as 1971 Sternlicht et al. [196] realized that cross-linked resins sufficiently swollen with solvent were sufficiently mobile to yield much improved NMR spectra, and from this early work gel-phase NMR has emerged. 

Scheme 1.5 Solid phase synthesis of C-labelled thiazo-lidinone (12) monitored by Gel-phase NMR specto-scopy.

Despite the success with gel-phase NMR spectroscopy the spectra obtained are of limited value because of the broad peaks, and indeed the NMR spectra of unlabelled samples need relatively long acquisition times (because of the low abundance of C) especially with lightly loaded resins such as TentaGel and Argo-Gel. NMR [206, 207] and NMR [208, 209] spectroscopy of gel beads have also been shown to be a convenient NMR technique for monitoring reactions of fluorine or phosphorous-containing molecules, respectively, attached to solvent-swollen polymer supports. 

A frequent complication in the use of an insoluble polymeric support lies in the on-bead characterization of intermediates. Although techniques such as MAS NMR, gel-phase NMR, and single bead IR have had a tremendous effect on the rapid characterization of solid-phase intermediates [27-30], the inherent heterogeneity of solid-phase systems precludes the use of many traditional analytical methods. Liquid-phase synthesis does not suffer from this drawback and permits product characterization on soluble polymer supports by routine analytical methods including UV/visible, IR, and NMR spectroscopies as well as high resolution mass spectrometry. Even traditional synthetic methods such as TLC may be used to monitor reactions without requiring preliminary cleavage from the polymer support [10, 18, 19]. Moreover, aliquots taken for characterization may be returned to the reaction flask upon recovery from these nondestructive... 

D. Stones, D. J. Miller, M. W. Beaton, T. J. Rutherford and D. Gani, A method for the quantification of resin loading using 19F gel phase NMR spectroscopy and a new method for benzyl ether linker cleavage in solid phase chemistry, Tet. Lett, 1998, 39, 4875 1878. 

Standard (gel-phase) NMR spectra of polymers usually show significant line broadening, mainly because of chemical shift anisotropy and dipolar coupling [98], Only nuclei with strong chemical shift dispersion, e.g. 13C [99-106], 15N [107], 19F [108-112], and 31P [113] give sufficiently resolved gel-phase NMR spectra. The resolution of... 

NMR spectra is improved when the mobility of support-bound molecules increases. Hence, gel-phase NMR of PEG-polystyrene graft supports, for instance, will generally give better spectra than if normal cross-linked polystyrene is used as the support. Gel-phase H NMR spectra, even if recorded on well-solvated and flexible supports, are, however, too poorly resolved to be of use for the characterization of support-bound intermediates. 

Natural abundance gel-phase 13C NMR spectroscopy is used for assessing intermediate structures in solid-phase synthesis [20], but requires long acquisition times and so is not useful for following reactions. Affymax scientists [21] have pioneered the use of specifically 13C-labeled starting materials in conjunction with fast 13C-gel phase NMR to follow multi-step reactions [17,22,23],... 

Since its first description in 1971 [35], gel-phase NMR was applied to peptide chemistry by Manatt and coworkers [36, 37], These authors used 13C NMR to determine the extent of chloromethylation of crosslinked polymers and 19F NMR to monitor protection-deprotection reactions. These two nuclei are the most commonly used in these types of studies, mainly because of their significant chemical shift dispersion, which can alleviate in part the resolution loss due to the non ideal linewidth obtained in the gel state. Apart from restricted molecular motion, that shortens T2 because of an efficient transverse relaxation, other sources of line-broadening derive from magnetic susceptibility variations within the sample (due to the physical heterogeneity of the system) and residual dipolar couplings. 

Fig. 2. 13C Gel-phase NMR spectra of modified polystyrene-2%-co-DVB in CDCI3 at 75 MHz. (a) Merrifield resin (b) epoxy alcohol bound polymer at room temperature (c) epoxy alcohol bound polymer at 50°C (d) model compound in solution. The solvent resonance is marked with an asterisk. (Reproduced from Shapiro and Gounarides4 with permission.)...

The use of a C-enriched building block anchored to a resin makes the gel-phase spectrum selective for the appearance of the new C signal, and the enrichment allows much shorter acquisition times (133, 134) a real-time kinetic was reported for the alkylation of amines with C-enriched bromoacetic acid (135). An example from our laboratories (136) shows the formation of a cyanohydrin on SP is monitored by C-enriched gel-phase NMR using C-benzaldehyde. The appearance of the cyanohydrin signal (63.2 ppm) and its increase at different reaction times is easily monitored by comparison with the constant signals of the solvent (deuterated benzene, 133-126 ppm. Fig. 1.19, spectra A-D). A major drawback of this technique is the cost and the limited availability of C-enriched building blocks, which currently severely limits its application. 

While specific applications of gel-phase NMR have been useful for SPS reaction monitoring, the great potential of SPS NMR is in the determination of structure and the measurement of purity and yields, especially through the use of magic angle spinning-high-resolution (MAS-HR) NMR techniques. This important topic will be addressed in Section 1.4.6. 

The use of gel-phase NMR for monitoring SPS reactions has been described previously. The application of this method to the determination of the purity and yield of a product is not recommended for the reasons already discussed in Section 1.3.4. Another SPS NMR technique, MAS-HR NMR, is more suited to this purpose. 

These precedent syntheses have shown that is possible to circumvent one of the main drawbacks usually attributed to the solid-phase mode, which is the lack of control of the reactions taking place on the support. Thus, the / situ monitoring of all reactions in real time, using FT-IR (KBr pellets), x gel-phase NMR and C MAS-NMR, made possible a Ml and an accurate control of the progress in these syntheses <04QSAR6l>. 

Gel-phase NMR spectroscopy is a very convenient technique for the spectroscopic characterization of functionalized solid supports.Optimal conditions for acquiring standard gel-phase NMR spectra from a variety of resins are well-established optimal conditions, i.e. maximum swelling, result in reasonably narrow lines for the pendant groups. This method has the additional advantage of not requiring any special instrumentation besides a conventional FTNMR spectrometer and a conventional probe. In addition to NMR spectroscopy, NMR l and NMRt l spectroscopy can also be used in those cases where fluorine or phosphorous is present in the handle or in the first amino acid. 

Due to strong line-broadening, standard gel-phase NMR spectroscopy cannot be used for the analysis of protons. High resolution magic angle spinning (HRMAS) NMR spectroscopy has recently been developed to cover this gap.t l This method has the limitation that specialized HRMAS probes are required however, the method has been applied successfully to a variety of solid supports. Amazingly narrow lines are observed for the proton resonances and 2-D versions of HRMAS experiments can be very easily implemented. 

Gel phase 13C NMR is an established technique that was first performed in 1971 (7). Gel phase NMR involves taking an NMR spectrum using a standard... 

One of the first applications of gel phase 13C NMR showed that the build up of a peptide chain and deprotection of Boc groups could be readily followed (8). An example of the gel phase 13C NMR of a peptide is shown in Fig. 1. Similarly 13 C gel phase NMR was used to evaluate and develop new side chain deprotection cocktails (9). Liebfritz and Geralt, describing the use of polyethylene and polystyrene supports, showed the general application of 13C gel phase NMR methodology in the solid phase synthesis of peptides (10,11). 

Application of gel phase NMR to organic solid phase synthesis was first reported by Manatt in 1980 (12). Jones and Leznoff extended this work to show that13 C NMR data could be obtained from polystyrene-supported pheromones and a wide variety of organic substrates (13). The application of 13C gel phase NMR to the study of functional group interconversion of polymer-bound steroids has also been reported (14). High-resolution H gel phase NMR has been reported for an octapeptide (15). The NMR data were obtained by use of a deconvolution method to enhance the spectral resolution. 

Figure 1 Typical peptide 13C Gel phase NMR spectrum in DMSO-d6. Product shown is obtained after deprotection reaction by BF 3.

Figure 2 19F gel phase NMR spectrum for macrocyclization SNAr reaction for an... 

Figure 3 375 MHz 19F gel phase NMR spectrum for resin linked butyl amide. Conversion to product is monitored (a) on TentaGel resin and (b) in solution.

Figure 4 Horner-Wadsworth-Emmons reaction that was monitored by 31P gel phase NMR.

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