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

The utility of intramolecular cycloaddition- and 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 2-methyl-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 SO₂-extrusion 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 regio- and 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 (+)-α-allokainic acid as well as its (-)-antipode have been prepared enantioselectively from β-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 diastereo- and 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.