Organocopper reagents homocuprates

The selective formation of 5-hydroxy-2,2-dimethylch romenes instead of the usual 7-hydroxy isomer has been accomplished with organocopper reagents <79JCS(Pl)201). Olivetol bis(tetrahydropyranyl ether) (61) is- selectively metallated at C-2 and the resulting homocuprate (62) reacts with 3-acetoxy-3-methylbut-l-yne to yield 5-hydroxy-2,2-dimethyl-7-pentylchroipene (63 Scheme 7). 

The combinations of chlorotrimethylsilane-hexamethylphosphoramide (HMPA) or chlorotrimethylsi-lane-4-(dimethylamino)pyridine (DMAP) are also powerful accelerants for copper(I)-catalyzed Grignard conjugate additions,33 and stoichiometric organocopper and homocuprate additions (Scheme 21 ).36 However, these reactions must be performed in tetrahydrofuran instead of ether.37 These procedures are noted for their high yields with stoichiometric quantities of Grignard reagents, excellent chemoselectivity and efficiency with a,3-unsaturated amides and esters and enals.58 Typically, additions to enals proceed via the S-trans conformers to afford stereo-defined silyl enol ethers for example, enals (122) and (124) give the ( )-silyl enol ether (123) and (Z)-silyl enol ether (125), respectively. 

The organocopper reagents used for conjugate additions to enones are homocuprates, heterocuprates, higher-order cuprates, and Grignard reagents in the presence of catalytic amounts of copper salts (CuX). 

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