Page 1. Te!~ah&on Letten, Vol. 33, No. 16. pp. '22232226.1992 Printed in Great Britain ... 6. See eg: lwasaki, M.; Toriyama, K.; Nunome, K. Faraday Discuss. Chem. Sac. 1964, 78, 19; Quin, Xue-Khi.; Williams, F. Tetrahedron 1986, 42, 6301. (Received in UK 20 January 1992)