Stereoselectivity Wittig directed

Protic solvents or the addition of proton-active compounds after oxaphosphetane formation shift the stereoselectivity of the reaction in the direction of the ( )-form. If the Wittig reaction is carried out in C2HsOD or if the oxaphosphetane solution, prepared at —75 °C in an aprotic solvent, is treated with deuterated ethanol, then deuterium is incorporated in high yield into the ( )-olefm formed, and the degree of deuterium labelling of the coexisting (Z)-olefin is lower. On the basis of these findings the mechanism discussed below has been established (Scheme 5). 

Directed aldol condensation (2, 249). Biichi and Wiiesl used Wittig s method of aldol condensation for a highly stereoselective synthesis of the racemic sesquiterpene aldehyde nuciferal(3)by reaction of thealdehyde( I) with lithiopropylidene-t-butyiimine (2). The product was obtained in 83 % yield. 

The idea was that the lactone 227 could come from 228 by the sequence (i) stereo- and regioselective a-hydroxylation 229, (ii) Wittig style reaction on the ketone to add the extra two carbon atoms 230, (iii) stereoselective conjugate reduction of the double bond with the OH group directing the reagent to the bottom face of the alkene, and (iv) lactonisation. 

The Wittig-Horner olefination of the aldehyde 28 provided alkenes 29 which were subjected to radical cyclization leading to benzofused tricyclic j6-lactams 30, obtained as single diastereomers (Scheme 8) [36]. A convenient, direct regio- and stereoselective route to optically pure unusually fused or bridged tricyclic /3-lactams has been developed by the use of intramolecular nitrone-alkene cycloaddition reactions. For example, the aldehyde 21 can be transformed into nitrone 31 which subsequently was used for a variety... 

Reinhard W. Hoffmann was bom in 1933 in Wurzburg. He studied chemistry at the Universitat Bonn and received his Dr. rer. nat. in 1958 under the direction of Prof. B. Helferich. He spent two years as postdoctoral fellow at The Pennsylvania State University with Prof. G. W. Brinkley and moved then to the Universitat Heidelberg for another postdoctoral year with Prof. G. Wittig. In 1964 he obtained his habilitation and in 1967 he moved as lecturer to the Technische Hochschule Darmstadt and in 1970 to his present position as Professor of Organic Chemistry at the Universitat Marburg. His research interests are new methods in stereoselective C-C-bond formation, their application in natural products synthesis and the stereochemistry of reactive organometallic compounds. 

With ortHo-phosphane as a removable directing group high regioselectivity combined with good stereoselectivity was achieved (see 114/115) [52, 53], and joined by a Wittig reaction and subsequent hydrogenation hydroformylations lead to the saturated ketones 116 [54]. 

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