HMBC Lactose

Considering the complexity of lactose, a complex and overlapping HMBC spectrum for lactose is expected. The spectrum found in Figure 5.24 measures up to this expectation. Students are encouraged to analyze this spectrum, which is facilitated with the labeling. 

The common theme so far in our correlation experiments has been to allow spins to evolve during q under the influence of directly coupled nuclear spins. We have seen the power of COSY, HMQC, HMBC, and INADEQUATE to provide us with detailed structural information for ipsenol, caryophyllene oxide, and lactose. In this section, we will develop another method for showing correlations and apply it to molecules with distinct, isolated proton spin systems such as carbohydrates, peptides, and nucleic acids. 

FIGURE 5.24 The HMBC spectrum for lactose. See text for an explanation of the inset. 

The HMQC-TOCSY spectrum for lactose is given in Figure 5.28 with all of the proton and carbon resonances labeled. The overall appearance of this spectrum is reminiscent of an HMBC but the correlations are quite different. It is equally interesting and useful to start on the proton axis (F2) or the carbon axis (FI). If we start on the proton axis at 5.23 ppm, the anomeric proton for the a-anomer of glucose (ad), and proceed downward vertically, we find six correlations to the six carbons of this glucose residue. If we refer back to the simple HMQC spectrum for lactose, we find only one correlation for this proton. Likewise, the anomeric proton of the /3-anomer of glucose at 4.67 ppm also shows six correlations to the carbons of its respective glucose residue. 

Previously, it was reported that the chemical shift of the proton atoms of the C-12 of the fatty acid methyl ester can be found at 3.503 ppm (50). In the HMBC-NMR spectrum, there is a cross peak between this proton of C-12 and the carbon atom with a chemical shift of 95.23 ppm. This is a tertiary carbon atom. The discrimination between quaternary and tertiary C signals has been performed by using an attached proton test (APT) pulse sequence, one each second. The cross peak refers to the three-bond coupling of the H-12 protons with the anomeric center C-1 of lactose. The H-data of C-1 were found with the one-bond coupling constant, which is 150 MHz. With a COSY-NMR spectrum and its corresponding HMBC spectrum, the H and C NMR values of C-2, C-3, could be determined. 

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