Using only 10% of CrCl 2 as catalyst, manganese-powder as reducing agent and TMSCl as scavenger, 2-methylene-a,w-dialdehydes and -ketones can be coupled to form cyclic diols diastereoselectively. The diastereomeric excess strongly depends on the ring size and the substituents of the w-carbonyl group. The greater the ring size the higher diastereoselectivities are observed. In all cases cis-diols are preferentially formed.

A chromium‐catalyzed pinacol‐type cross‐coupling reaction between α,β‐unsaturated carbonyl compounds and aldehydes is reported. Even sterically demanding substrates could be coupled to afford the corresponding pinacols in good yields. Systematic studies concerning the origin of the diastereoselectivities led to the proposal of a mechanism for this synthetically useful reaction. Acroleins with α‐branched alkyl side chains were coupled to give the corresponding syn pinacols, while on the other hand, acroleins with less bulky substituents furnished the anti derivatives. The effects of both the substrates and the reagents on the diastereo‐ and enantioselectivities were investigated. An unexpected catalytic formation of cyclopropanols was found.

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1C o 2 ( C O ) 8 C a t a l y z e d P a u s o n -K h a n d R e a c t i o n u n d e r M i c r o w a v e I r r a d i a t i o n Stefan Fischer, Ulrich Groth,* Marc Jung, Andreas Schneider

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