Enones partial reduction

The third is partial or total reduction of an aromatic ring. Any catalogue lists a vast number of available substituted benzene rings. Saturated compound 8 can obviously be made by total reduction of 9 but it may not be obvious that partial reduction (Birch) allows the enone 11 also to be made from 9. Birch reduction is the only new method here so we shall revise the Robinson and the Diels-Alder and concentrate on Birch. 

Partial Reduction of Enones, Styrenes and Related Systems... 

The synthesis of lupeol starts with the cyclization of 6-methoxy-p-methyl-a-tetralone 28 with 4-7V 7V -dimethylamino-2-butanone methiodide in the presence of potassium /-butanolate to l,9,10,10a-tetrahydro-7-methoxy-3(2//)-phenanthrone 27. Reduction with sodium borohydride and subsequent hydrogenation of the enone CC double bond in the presence of palladium and strontium carbonate as slightly deactivated catalyst gives the oetahydrophenanthrol 26. Partial reduction of the benzenoid ring to the enone is aceomplished by lithium in liquid ammonia. The enone is derivatized to the benzoate 25 in order to protect the hydroxy group prior to the subsequent synthetic steps. 

An interesting example of the synthetic utility is the partial reduction of compound 14 under standard Birch conditions to give the diene 15, and after acid hydrolysis the exo-enone 16, of difficult access by other methodologies (Scheme 13.7) [8]. 

Further examples of reagents and methods for asymmetric reduction are provided by the complexes obtained by partial decomposition of LiAlH4 by AT-methylephedrine(85) and either AT-ethylaniline" or 3,5-dimethylphenol. The first of these reduces open chain enones to allylic alcohols with enantiomeric excesses approaching 100%, whereas the second has been used to reduce conjugated acetylenic ketones to propargylic alcohols en route to optically pure y-lactones. Enantiomeric excesses of up to 90% are also obtained in the reduction of acetylenic ketones and acetophenones with LiAlH4 complexed with several optically active 1,3-amino-alcohols,and the chiral LiAlH4-derivatives (86) reduce ketones in up to 87% optical yield. ... 

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