1. For a review see:G. M. Sanders, J. Pot, E. Havinga, Fortschr. Chem. org. NatStoffe27, 131 (1969).
2. P. B. Braun, J. Hornstra, C. Knobler, E. W. M. Rutten andC. Romers, Acta crystallogr.B29, 463 (1973).
3. C. Altona, personal communication.
4. A. J. De Kok andC. Romers, to be published.
5. Determined byM. R. Daha at our laboratory (1970).