Vinyl acetate reaction with methyl lithium

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

A convenient synthesis of (— )- xo-brevicomin (87) utilizes a radical chain reaction of methyl vinyl ketone with (45, 5R)-4-benzyloxymethyl-5-iodomethyl-2,2-dimethyl-l,3-dioxo-lane (209), prepared by treating the (R,R)-tartaric acid derivative 141 with triphenylpho-sphonium iodide in the presence of imidazole. Adduct 215, after acidic hydrolysis of the isopropylidene protecting group, furnishes the bicyclic acetal 216. Subsequent debenzylation and tosylation followed by methylation with lithium dimethylcuprate provides 87 in an overall yield of 17% from (R,R)-tartaric acid. The optical purity of 87 corresponds to greater than 99% ee (Scheme 50). Carrying out a similar series of transformations with ( S,5)-tartaric acid leads to ( + )-exo-brevicomin (90) [78]. 

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