5-Pentenylmagnesium bromide

In contrast to this, the introduction of longer alkyl chains with the aid of copper-promoted 1,6-addition reactions to A" -steroids normally proceeds with unsatisfactory aiyS ratios [15b, 16], In some cases, improvement of the diastereoselectivity by fine tuning of the reaction conditions has been possible. The ratio of the epimeric products 15 and 16 in the copper-catalyzed 1,6-addition of 4-pentenylmagnesium bromide to dienone 14, for example, was improved from 58 42 to 82 18 by adjustments to the quantity of nucleophile and the solvent composition (Eq. 4.6) [16f ]. 

Diastereoselective hydride addition is quite versatile, and it provides facile synthetic access to ( —)-pinidine (661), an alkaloid isolated from several species of Pinus, as well as its unnatural isomer (+ )-pinidine (660b). The unstable aldehyde 655, prepared in four steps from 624 [202], undergoes Grignard addition with 4-pentenylmagnesium bromide followed by Swem oxidation to afford ketone 656 in 90% yield for the two steps. Stereoselective hydride addition with L-Selectride provides the -yn-alcohol 657 (91 9), while zinc borohydride reduction provides almost exclusively the anri-alcohol 658 (>99 1) (Scheme 144). 

However, such addition to /3-substituted a-styrylmagnesium bromides like 1-phenyl-1-pentenylmagnesium bromide does not proceed in THF, even under forcing conditions [117]. 

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