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The utility of intramolecular cycloadditionand ene—reactions in terms of entropy effects, regio— and stereo— selectivity is illustrated by the following recent syntheses of natural products 1) Optically pure (+)-estradiol has been synthesized from 2methyl-2-cyclopentenone by a sequence of 9 steps in high overall yield. The key steps are a regioselective deprotonation/alkylation of 5(1, 3-dihydroisothianaphthen-2 ,2-dioxide) -carbonitrile with an easily accessible, functionalized, enantiomerically pure ring D-unit and a subsequent thermolysis which involves SO2extrusion and intramolecular trapping of the transient orthoquinodimethane giving the trans-anti-trans steroid skeleton in 66% overall yield. 2) Starting from indol-4-carboxaldehyde the otherwise not easily accessible ergot alkaloids (±)-chanoclavine I and for the first time its even scarcer isomer (±)-isochanoclavine I have been prepared by a sequence of 11 synthetic operations in overall yields of 14% and 2.4%. The key step is a regioand stereoselective intramolecular addition of a transient nitrone to a 4-indolylacrylic ester. 3) (-) (S)-2-Cyclopenten-2-carboxylic acid has been converted to the optically pure sesquiterpenes (+)-longifolene and (+)-sativene via a common tricyclic 1,5-diketone intermediate in overall yields of 24% and 9%. These syntheses are centered on an efficient regio-selective intramolecular (2+2) -photocycloaddition followed by a retrolaldolisation. 4) The neurophysiologically interesting cyclic amino diacid (+)-a-allokainic acid as well as its (-)-antipode have been prepared enantioselectively from -chloroacrylates by a series of 4 synthetic operations in over 15% overall yield. The crucial step is an intramolecular ene reaction of a l,6-diene. Treatment of the latter with a mild Lewis-acid promotes a dramatically accelerated, highly diastereoand enantiostereoselective cyclization. This is the first examples of an "enetype" reaction proceeding with high asymmetric induction followed by regeneration of the auxiliary chirality directing group. INTRODUCTION Over the last 12 years a major part of our research has been focused on intramolecular cycloaddition and ene reactions. By now the utility of these reactions for the efficient synthesis of complex cyclic molecules has become generally recognized, as is apparent by the rapid development of this field (Ref. 1 , 2). Today I would like to outline some recent applications of intramolecular versions of Diels-Alder and nitrone additions, photo-additions and ene reactions to the synthesis of natural products.

Regio- and stereo-selective syntheses of cyclic natural products by intramolecular cycloaddition- and ene-reactions