syn‐Diastereoselective Homoaldol Reactions: γ‐Hydroxyalkylation of (Z)‐2‐Butenyl Carbamates and Synthesis of β,γ‐cis‐Di‐ and ‐Trisubstituted γ‐Lactones

The preparation of the syn-homoaldol derivatives syn-3 in ≥ 85% stereoselectivity is achieved by reaction of the lithiated (Z)-2-butenylcarbamate 1, X  OC(O)N(iPr)2, M  Li, with aldehydes or ketones. Previously, only access to anti-diastereomers was possible. Mercury(II)acetate-catalyzed methanolysis of syn-3 furnishes a cis-γ-lactol methyl ether: the relative configurations at C-3 and C-4 are retained. Subsequent oxidation leads to lactones, e.g. to (±)-quercus-lactone b.