1. Carruthers, W. Cycloaddition Reactions in Organic Synthesis; Pergamon Press: Oxford, 1990.
2. For a review, see: Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49–92.
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4. For leading references, see: Brunner, H.; Reimer, A. Bull. Chem. Soc. Fr. 1997, 134, 307–314; Lautens, M.; Tam, W.; Lautens, J. C.; Edwards, L. G.; Crudden, C. M.; Smith, A. C. J. Am. Chem. Soc. 1995, 117, 6863–6879.
5. Representative procedure for the homo Diels–Alder reaction: In a 50 mL round-bottomed flask under nitrogen, CoBr2(dppe) (40 mg, 64.8 μmol), and ZnI2 (63. mg, 197.4 μmol) were suspended in dry dichloromethane (2.0 mL) and 2,3-dimethyl-1,3-butadiene (0.5 mL, 363 mg, 4.42 mmol) and phenylacetylene (0.5 mL, 465 mg, 4.55 mmol) were added. To this green mixture 15–17 mg of Bu4NBH4 were added until the colour changed to a light brown and the reaction mixture was stirred at ambient temperature for 12 h. After addition of pentane (5 mL), the mixture was filtered through a small amount of silica gel and the solvents evaporated. The product (684 mg, 3.71 mmol, 84%) was obtained as a colourless oil after column chromatography on silica gel with pentane as eluent.