Diastereoselective Osmylation and Hydroboration of beta,gamma-Unsaturated N,N-Diisopropylamides and Acid-Catalyzed Conversion to delta-Lactones.

The title reactions of β,γ-unsaturated N,N-diisopropylamides occur with useful diastereofacial selectivity. The major diol isomer from osmylation of alkenes 1, 10, 11, and 12 in the presence of TMEDA at −78 °C corresponds to the facial preference shown in transition state model 41 (RZ = H), while the opposite preference for 42 is observed with the Z-alkene 13. (Table 1). Hydroboration with 9-BBN does not show this inversion of diastereofacial selectivity for the Z-alkene. All of the results in Table 2 correspond to the usual preference for a transition state such as 45. Acid-catalyzed lactonization of the alcohols obtained in Tables 1 and 2 can be carried out with overall retention of configuration to afford δ-lactones. Butenolide 5 was prepared with 90% ee from alcohol 2a via osmylation followed by acid-catalyzed lactonization to 3 and elimination using SOCl2/pyridine.