Aldehydes, conjugated LiAlH4 reduction

The synthesis in Scheme 13.21 starts with a lactone that is available in enantiomer-ically pure form. It was first subjected to an enolate alkylation that was stereocontrolled by the convex shape of the cis ring junction (Step A). A stereospecific Pd-mediated allylic substitution followed by LiAlH4 reduction generated the first key intermediate (Step B). This compound was oxidized with NaI04, converted to the methyl ester, and subjected to a base-catalyzed conjugation. After oxidation of the primary alcohol to an aldehyde, a Wittig-Horner olefination completed the side chain. 

House pointed out that 16 can disproportionate (as mentioned above) or that another mechanism may be operative in this reduction.24 Inverse addition of the hydride (a slurry of LiAlH4 is added to the carbonyl compound), short reaction times, and low reaction temperatures generally give moderate to good yields of the conjugate reduction product.24 Good yields of allylic alcohols can often be obtained from a,P-unsaturated aldehydes, however, when the reaction is done at low temperatures with strict control of the stoichiometry. 

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