Lithium dimethylcuprate reaction with epoxides

The C2-symmetric epoxide 23 (Scheme 7) reacts smoothly with carbon nucleophiles. For example, treatment of 23 with lithium dimethylcuprate proceeds with inversion of configuration, resulting in the formation of alcohol 28. An important consequence of the C2 symmetry of 23 is that the attack of the organometallic reagent upon either one of the two epoxide carbons produces the same product. After simultaneous hydrogenolysis of the two benzyl ethers in 28, protection of the 1,2-diol as an acetonide ring can be easily achieved by the use of 2,2-dimethoxypropane and camphor-sulfonic acid (CSA). It is necessary to briefly expose the crude product from the latter reaction to methanol and CSA so that the mixed acyclic ketal can be cleaved (see 29—>30). Oxidation of alcohol 30 with pyridinium chlorochromate (PCC) provides alde-... 

Selective opening of allylic epoxides (9, 329-330). The reaction of the trimethyl-silyl enol ether (I) of a, /Tepoxycyclohexanone with lithium dimethylcuprate (or di-n-butylcuprate) in THF proceeds as expected to give 2.25,26 However when the reaction is conducted in ether, 2 and 3 are formed in about equal amounts.26... 

Indeed, in diethyl ether, lithium dimethylcuprate usually reacts with the a-enone group to give a methyl-substituted bromo ketone. Addition of hexamethylphosphoric triamide (HMPT), however, slows down this reaction to such an extent that displacement of the bromo substituent takes place [698], Another remarkable example of the influence of HMPT on chemoselectivity is the reaction of an arsonium ylide, Ph3As= CH-CH=CH-Ph, with benzaldehyde in tetrahydrofuran solution, yielding either an epoxide (in THE) or an alkene (in THF/HMPT) [699],... 

A A 24-methyl-25,26-dihydroxysteroidal side-chain was found in a highly polyhydroxysteroid (161) from the extracts of the New Caledonian starfish Arc/iasfer typicus (126). Simplified side-chain models were synthesized by epoxidation of ( )-2-methyl-2-pentenol followed by reaction with lithium dimethylcuprate to give the (2R,3R)/(2S,3S)-2, 3-dimethylpentane-1,2-diol enantiomeric pair (197a, b) (Fig. 22). This was converted into the (2S,3R)/(2R,3S)-enantiomeric pair (198a, b) by... 

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