Qu. 1 . A key step in the synthesis of the natural product Lycopodine is shown below. Suggest a mechanism for this reaction, indicate clearly whether each step in the mechanism is a pericyclic or non-pericyclic process, and classify each pericyclic process according to its type.

Qu 2 . In the sequence shown below, treatment of the reactant with sodium thiophenoxide (a base and also a thiophile) produces two transient isomers X and Y prior to the final product Z. The product Z has the following spectral characteristics. n_max 3610 cm ^-1, ¹H nmr d 2.0 (1H, broad s), 2.26 (3H, s), 2.8 (1H, m, J 13.6, 7.8Hz), 2.9 (1H, J 13.6, 5.8),4.40 (1H, J 7.8, 7.0, 5.8 Hz), 5.5 (1H, J 15.6, 7.0), 6.33 (1H, J 15.6), 7.4-7.6 (5H, m). Suggest a structure for Z, and a mechanism for its formation. Indicate clearly whether each step in the mechanism is a pericyclic or non-pericyclic process, and classify each pericyclic process according to its type. Pay particular attention to the stereochemistry of Z

Qu.3. The ¹H NMR spectrum of the compound shown below has some very peculiar properties. At -50C, four peaks are observed at d 5.73 and 6.25 (assigned to the olefinic protons) and at 6.95 and 7.04 (assigned to the aromatic protons). As the temperature is increased to about 20C, the two peaks due to the olefinic protons coalesce to a single peak, as do the two aromatic peaks. Further heating to 120C results in each now single olefinic and aromatic peak further coalescing to a single peak, as do the two singlets due to the Bu ^t groups. Suggest pericyclic mechanisms to explain the behaviour observed at both 20C and 120C.

Qu 1.

For further details, see P. A. Grieco and Y. J. Dai,J. Am. Chem. Soc ., 1998, 120, 5128.

Qu 2 . Thiophenoxide acting as a base deprotonates to form an allylic anion stabilised by the adjacent sulfur groups. Reprotonation allows a [2,3] sigmatropic rearrangement to occur with stereospecific formation of the E ( trans) alkene. Thiophenoxide now acts as a thiophile to form the allylic alcohol (IR evidence). The two benzylic protons are non equivalent in the NMR because they are adjacent to a chiral centre. The large value of the ³J olefinic coupling indicates trans stereochemistry, induced by minimising steric repulsions between the methyl groups in the transition state:

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For further details, see B. F. Bonini, M. C. Franchini, G. Mazzanti, J-W. Slief, M. A. Wegman and B. Zwanenburg, ChemComm, 1997, 1011.

Qu 3 . The coalescence of NMR peaks on heating normally indicates a pericyclic process that renders protons equivalent. A [1,5] sigmatropic migration of the oxygen atom (with implied retention of configuration at this atom) will exchange the environments of the protons labelled Ha and Hb, causing their NMR signals to coalesce. In fact, the process also makes Hc and Hd equivalent. The transition state for the [1,5] migration has a plane of symmetry bisecting Ha/Hb and Hc/Hd.

To exchange the environments of the olefinic and aromatic protons requires a different transition state, corresponding to a [3,3] sigmatropic rearrangement. This allows e.g Ha and Hc to exchange, resulting in the olefinic and aromatic NMR peaks becoming a single peak, and also allowing the two tert-butyl groups to become equivalent. This transition state has C _2h symmetry, adopts a chair conformation and is about 12 kJ/mol higher in energy than the [1,5] one, in agreement with the higher coalescence temperature. For further details, see T. Yamato, J. Matsumoto and K. Fujita, Perkin Trans 1, 1998, 123).

Transition state for 1,5 migration		Transition state for 3,3 migration
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