Natural Product Reports Editorial Board Professor G. Pattenden (Chairman) University of Nottingham Dr C. Abell University of Cambridge Dr J. R. Hanson University of Sussex Dr R. B. Herbert University of Leeds Professor M. I. Page The Polytechnic Huddersfield Professor T. J. Simpson University of Bristol Natural Product Reports is a journal of critical reviews published bimonthly which is intended to foster progress in the study of natural products by providing reviews of the literature that has been published during well-defined periods on the topics of the general chemistry and biosynthesis of alkaloids terpenoids steroids fatty acids and 0-heterocyclic aliphatic aromatic and alicyclic natural products. Occasional reviews provide details of techniques for separation and spectroscopic identification and describe methodologies that are useful to all chemists and biologists who are actively engaged in the study of natural products.Articles in Natural Product Reports are commissioned by members of the Editorial Board or accepted by the Chairman for consideration at meetings of the Board. Natural Product Reports (ISSN 0265-0568) is published bimonthly by The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF England. 1991 Annual Subscription Price E.C. f 198.00 Overseas f228.00 U.S.A. $467.00. Change of address and orders with payment in advance to The Royal Society of Chemistry The Distribution Centre Blackhorse Road Letchworth Herts. SG6 1 HN England. Air Freight and mailing in the U.S.by Publications Expediting Service Inc. 200 Meacham Avenue Elmont NY 11 003. US Postmaster send address changes to Natural Product Reports Publications Expediting Service Inc. 200 Meacham Avenue Elmont NY 11 003. Second-Class postage paid at Jamaica NY 11 431 -9998. All other despatches outside the U.K. are by Bulk Airmail within Europe and Accelerated Surface Post outside Europe. Printed in the U.K. 0 The Royal Society of Chemistry 1991 All Rights Reserved No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photographic recording or otherwise without the prior permission of the publishers. Printed in Great Britain by the University Press Cambridge Subscription rates for 1991 E.C. f198.00 Overseas f228.00 U.S.A. US$467.00 Subscription rates for back issues are U.K. (1986)f 130.00 (1 987) f 142.00 (1 988) f 159.00 (1 989) f 169.00 (1990)f 177.00 Overseas f 143.00 f 159.00 f 183.00 f 194.00 f 204.00 U.S.A. US $252.00 US $280.00 US $342.00 US $388.00 US $398.00 Members of the Royal Society of Chemistry should order the journal from The Membership Manager The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF England