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crosspeaks (three equivalent protons in a singlet) and must be attached to a quaternary carbon (singlet pattern in *H spectrum). For example, h9 (CH3) shows HMBC crosspeaks to c7 (Cq), c12 (CH2), c14 (CH2), and c18 (CH) (Table 11.1). This CH3 group (*H singlet) must be connected to c7 (quaternary carbon) and this carbon must be attached to c12, c14, and c18 (Fig. 11.59). Note that the correlation to c7 (Cq) is a two-bond relationship and the correlations to c12, c14, and c18 are three-bond relationships. Because we know the methyl group has to be attached to a quaternary carbon, the ambiguity (2 versus 3 bonds)

Figure 11.58

does not cause any confusion. In a similar way, the bonding networks can be constructed for h1, h2, h3, h10, and h13 (Fig. 11.59). In the case of h1 and h2 there is clearly a shared carbon since both CH3 proton signals are correlated to c15 and c19. Since both c15 and c19 are quaternary, however, it is not possible to determine which one is connected to CH3 (h1) and which is connected to CH3 (h2). Note that c18 is shared by the h1 network and the h9 network, and c28 is shared by the h2 network and the h13 network. In the case of h3, there are only three HMBC correlations: CH3 (h3) is attached to c23 (Cq), which is attached to c27 and c32 and an oxygen atom. This is the same quaternary carbon that is connected to the OCH3 (methyl ether) group (see above). The h3 methyl group could also have been connected to either c27 or c32, since these are also quaternary, but other HMBC evidence shows that the structural fragment in Figure 11.59 is correct. Other ambiguities

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