Scheme 4.
Reagents: i. OsO4, Me3NO.H2O, acetone 18 78% from 4 ii. TBSCl, imidazole, CH2Cl2, 17 66% from 2b; iii. LiHMDS, THF, -50-25 °C, 48%; iv. nBu4NF, THF; v. Ac2O, pyridine; vi. H2, Pd-C, 21 67% from 18, 22 51% from 19, 23 74% from 5.
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