Mevalonic acid lactone is incorporated into ct-cyclopiazonic acid with retention of the 4-pro-R-hydrogen atom; [1,2-13C2]acetate has been used as a probe to show that the cyclization of P-cyclopiazonic acid into acyclopiazonic acid involves a syn addition to the double bond.

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Abstract-The application of selective population inversion (SPI) to the assignment of 13C resonances and to the determination of the relative sign of W-lH coupling constants is demonstrated for l-phenyl-l,2-dibromo-2-nitroethane. A negative sign for the geminal C-243-1 coupling constant has been determined from the I3C spectra. The relationship between SPI and TSI is discussed.A PULSED double resonance method, selective population inversion (SPI), which should give the same information as INDORl and which can easily be performed on modern FT NMR spectrometers, has recently been described.2 In 13C-(1H) SPI a selective r.f. 7~ pulse applied at a proton transition inverts the populations of the upper and lower energy levels. A strong nonselective r.f. 7r/2 pulse is immediately applied in the usual way to the 13C nuclei and Fourier transformation of the resulting free induction decay gives the frequency spectrum. This communication describes how the method has been used to assign I3C resonances and to determine the relative sign of a long range l3C--IH coupling constant in I-phenyl-l,2-dibromo-2-nitroethane (1).C,H,CHBr-CHBrNO,(1) RESULTS lH NMR spectrumThe 100 MHz lH spectrum of 1 in deuterochloroform shows a five proton phenyl resonance at 6 = 7.38 and an AM spin system for the aliphatic protons with JAM = 10.9 Hz. Irradiation of the phenyl resonance sharpened the low frequency AM doublet giving the assignment d a .~ = 5-50 and 6~~ = 6.37. L i 13C NMR spectrumThe 25.2 MHz single resonance FT 13C spectrum of 1 in deuterochloroform (Fig. 1) shows a complex pattern for the aromatic carbons, but the aliphatic resonances are simpler, exhibiting clearly the directly bonded and long range 13C--lH couplings. The chemical shifts of the aliphatic carbons (C-1 and C-2) and the relevant 13C--lH coupling constants for the assignment proved below are given in Table 1. Decoupling of the aromatic protons removed the fine structure from the C-1 doublet and left the C-2 quartet unchanged. C-2, therefore, shows a geminal coupling to H-1, but no similar geminal interaction between C-1 and H-2 is observed. 13C-(1R} SPI EXPERIMENTS 13C AssignmentThe I3C resonances of the aliphatic carbon atoms are basically X parts of AMX spin systems. A stick spectrum of the C-1 isotopomer AMX system (JMx > JAM > 0; JAx c11 0) is shown in Fig. 2 with the transitions numbered according to the AMX energy level diagram (Fig. 3). A T pulse at v M -&JMx with sufficient power will invert the populations of the energy levels E2 and E,, as well as the populations of E6 and E3, because both M1 and M3 are irradiated. At maximum, the intensities of the progressively connected transitions X4 and X2 will increase fivefold, while the regressive transitions X3 and X1 will appear as negative peaks with three times their normal intensity. The result of the experiment is shown in the insert of Fig. 1 and provides