Natural Product Reports Editorial Board Professor T. J. Simpson (Chairman) Dr J. R. Hanson Dr R. B. Herbert Professor J. Mann Professor D. J. Robins Dr C. J. Schofield Dr D. A. Whiting Editorial Staff Dr Sheila R. Buxton Managing Editor Dr Roxane M. Owen Deputy Ed itor Miss Nicola P. Coward Production Editor Dr Carmel M. McNamara Technical Editor Ms Dawn J. Webb Miss Karen L. White Editorial Secretaries Editorial Office University of Bristol University of Sussex University of Leeds University of Reading University of Glasgow University of Oxford University of Nottingham The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge UK CB4 4WF Telephone +44 (0)1223 420066 Facsimile +44 (0) 1223 420247 E-mail perkin@ rsc.org RSC Server http://c hem istry. rsc.org/rsc/ Natural Product Reports is a bimonthly journal of critical reviews. It aims to foster progress in the study of bioorganic chemistry by providing regular and comprehensive reviews of the relevant literature published during well-defined periods. Topics include the isolation structure biosynthesis biological activity and chemistry of the major groups of natural products -alkaloids terpenoids and steroids aliphatic aromatic and 0-heterocyclic compounds. This is augmented by frequent reviews of the wider context of bioorganic chemistry incl ud ing developments in enzymology n ucleic acids genetics c hem ica I ecology primary and secondary metabolism and isolation and analytical techniques which will be of general interest to all workers in the area.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 UK CB4 4WF. 1997 Annual subscription rate f355.00; US$640.00. Customers in Canada will be charged the sterling price plus a surcharge to cover GST. Change of address and orders with payment in advance to The Royal Society of Chemistry The Distribution Centre Blackhorse Road Letchworth Herts. UK SG6 IHN. Air Freight and mailing in the USA by Publications Expediting Service Inc.200 Meacham Avenue Elmont NY 11003. US Postmaster send address changes to Natural Product Reports Publications Expediting Service Inc. 200 Meacham Avenue Elmont NY 11003. Second-Class postage paid at Jamaica NY 11431-9998. All other despatches outside the UK are by Bulk Airmail within Europe and Accelerated Surface Post outside Europe. Printed in the UK. 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 UK CB4 4WF. 0 The Royal Society of Chemistry 1997 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