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(II) A student isolated a natural product and after elucidating the structure using chemical and spectroscopic methods, he faced a problem in establishing the stereocenter at carbon-3. There were two possible isomers: A or B as shown below. Use the provided 'H-NMR and NOE spectra to help the student to find the correct structure. Circle the correct isomer and explain why you select this isomer. (5 Marks) CH2 CH2 H-3 CHz-b 3CH3-b 37H-3 CH3-a CH3-a NOE (Irradiating CH3-a) CH3-a CH3-b H-3 Original 'H-NMR CH English (U.S.) NOE (Irradiating CH-a) CH3-a CH3-b Original 'H NMR H-3 7 25 2.0 1.5 ppm Explain